CLASSIFICATION AND TOPOLOGY OF HYDROGEN ENVIRONMENTS IN
HYDROUS MINERALS
by
Madison Camille Barkley
_____________________
A Dissertation Submitted to the Faculty of the
Department of Geosciences
In Partial Fulfillment of the Requirements
For the Degree of
DOCTOR OF PHILOSOPHY
In the Graduate College
THE UNIVERSITY OF ARIZONA
2011
2
THE UNIVERSITY OF ARIZONA
GRADUATE COLLEGE
As members of the Dissertation Committee, we certify that we have read the dissertation
prepared by Madison Camille Barkley
entitled CLASSIFICATION AND TOPOLOGY OF HYDROGEN ENVIRONMENTS
IN HYDROUS MINERALS
and recommend that it be accepted as fulfilling the dissertation requirement for the
Degree of Doctor of Philosophy
_______________________________________________________________________
Date:
Robert T. Downs
_______________________________________________________________________
Date:
Charles T. Pewitt
_______________________________________________________________________
Date:
M. Bonner Denton
_______________________________________________________________________
Date:
Roy A. Johnson
_______________________________________________________________________
Date:
Przemyslaw Dera
Final approval and acceptance of this dissertation is contingent upon the candidate’s
submission of the final copies of the dissertation to the Graduate College.
I hereby certify that I have read this dissertation prepared under my direction and
recommend that it be accepted as fulfilling the dissertation requirement.
________________________________________________ Date:
Dissertation Director: Robert T. Downs
3
STATEMENT BY AUTHOR
This dissertation has been submitted in partial fulfillment of requirements for an
advanced degree at the University of Arizona and is deposited in the University Library
to be made available to borrowers under rules of the Library.
Brief quotations from this dissertation are allowable without special permission, provided
that accurate acknowledgment of source is made. Requests for permission for extended
quotation from or reproduction of this manuscript in whole or in part may be granted by
the copyright holder.
SIGNED: Madison Camille Barkley
4
ACKNOWLEDGEMENTS
This project was supported by the Carnegie-DOE Allicance Center (CDAC) under
cooperative agreement DEFC52-08NA28554, the RRUFF project, the Arizona Historical
Society, and by other scholarship sources (University of Arizona Graduate Diversity
Fellowship 2007, American Federation of Mineralogical Societies Scholarship 20082009, ChevronTexaco Geology Fellowship 2009-2010, Galileo Circle Scholar Award
2011, Tucson Gem and Mineral Society Scholarship 2011, BP Geology Fellowship
2011).
Portions of this work were performed at GeoSoilEnviroCARS (Sector 13)
Advanced Photon Source (APS), Argonne National Laboratory. GeoSoilEnviroCARS is
supported by the National Science Foundation – Earth Sciences (EAR-0622171) and
Department of Energy – Geosciences (DE-FG02-94ER14466). Use of the Advanced
Photon Source was supported by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences, under contract No. DE-AC02-06CH11357.
I would like to thank my advisor, Prof. Robert Downs for his guidance, patience,
and inspiration. Many thanks go to my committee members, Dr. Charles T. Prewitt, Dr.
Roy A. Johnson, Dr. M. Bonner Denton, for their encouragement and practical advice,
and Dr. Przemyslaw Dera, for his support while conducting research at Argonne National
Laboratory and for holding me to a high research standard. I would also like to thank Dr.
Hexiong Yang who was always there to listen and give advice. I am grateful to him for
helping me sort out the technical details of my work. Special Thanks go to my co-authors
who helped make this dissertation possible.
I would like to express my heart-felt gratitude to my parents, Dolores Hill and William
Barkley for a lifetime of support and encouragement.
5
DEDICATION
This dissertation is dedicated to and in loving memory of my great-aunt
Natalie Vernet Berry-Wallace
"Bignat"
June 12, 1932 – March 11, 2011
for her unwavering love, support, and encouragement.
6
TABLE OF CONTENTS
ABSTRACT........................................................................................................................ 7
INTRODUCTION.............................................................................................................. 8
Explanation of the Problem and Its Context...................................................................... 8
Literature Review............................................................................................................... 9
Explanation of Dissertation Format.................................................................................. 12
PRESENT STUDY........................................................................................................... 14
REFERENCES................................................................................................................. 22
APPENDIX A – CLASSIFICATION OF HYDROGEN ENVIRONMENTS IN
HYDROUS MINERALS ...................................................................................... 30
APPENDIX B – THE STRUCTURE DETERMINATION OF THE HIGH-PRESSURE
POLYMORPH OF BEHOITE, Be(OH)2 ............................................................... 50
APPENDIX C – NEW INSIGHTS INTO THE HIGH-PRESSURE POLYMORPHISM
OF SiO2 CRISTOBALITE .................................................................................. 76
APPENDIX D – KÔZULITE, AN Mn-RICH ALKALI AMPHIBOLE ....................... 91
APPENDIX E – DESPUJOLSITE, Ca3Mn4+(SO4)2(OH)6•3H2O ...................................
101
APPENDIX F – BOBDOWNSITE, A NEW MEMBER OF THE WHITLOCKITEGROUP FROM BIG FISH RIVER, YUKON, CANADA …........................... 114
APPENDIX G – STRUCTURE OF WALSTROMITE, BaCa2Si3O9, AND ITS
RELATIONSHIP TO CaSiO3-WALSTROMITE AND
WOLLASTONITE-II ………………………………………………………. 152
7
ABSTRACT
The inspiration for my topic was the use of hydrogen as a fuel source. Currently,
the use of hydrogen is limited by the need to find a safe storage solution. I wanted to
know how Nature stores hydrogen in minerals. The studies in my dissertation examine
the nature of materials that contain hydrogen as OH groups through a variety of
techniques. First, I examine the 450 known mineral species that contain isolated OH
groups and have crystal structure determinations that include the location of the hydrogen
atoms. I identify nine unique classes of OH hydrogen environments.
The hydrogen environment exemplified by the mineral behoite is of particular
interest because of behoite's structural relationship with SiO2 cristobalite. I conducted
two high-pressure studies exploring the similarities and differences in the behaviors of
behoite and cristobalite as a function of pressure.
In the process of categorizing the OH hydrogen environments in minerals I
encountered minerals who's structures needed to be refined. Refinements for two
minerals, despujolsite, a member of the fleischerite group of minerals, and kôzulite, an
Mn rich amphibole, are presented and discussed.
Two manuscripts, one on the new mineral bobdownsite, and the other on the
mineral walstromite, are appended to this dissertation to highlight additional original
research conducted throughout my graduate career.
8
INTRODUCTION
Explanation of the Problem and Its Context
One of the challenges facing society is alternative energy sources. The federal
government has identified hydrogen fuel as a potentially clean and cheap solution to our
national dependence on foreign sources of fossil fuels. However, the practical aspects of
hydrogen fuel use are hindered by the simple problem of how to store hydrogen safely
and in concentration (Ross 2006). This project will identify the ways that hydrogen is
stored in minerals in Nature.
Elements are the fundamental building block of minerals. Hydrogen is by far the
most abundant element in the universe: roughly 93% of all atoms or 75% of all matter by
weight (Dyar & Gunter 2008). However, more than 90% of the Earth is composed of
only four elements. In order of abundance these elements are oxygen, oxygen, silicon,
and magnesium. Despite its smaller relative abundance on Earth, more than half of all
minerals contain the element hydrogen.
There are currently 4532 mineral species recognized by the International
Mineralogical Association, 2580 of which contain hydrogen (57%). The American
Mineralogist Crystal Structure Database currently contains about 14,000 crystal
structures of minerals representing almost all of the known crystal structures. This study
examines the hydrated mineral structures contained within the database to identify all
variations of structural hydrogen storage in these minerals. For instance, we have
observed that the hydrogen environment, which is characterized by the structural and
atomic interactions of the hydrogen atom, in brucite is vastly different than the hydrogen
9
environment in hydrogarnet. How many different types of hydrogen environments exist
in minerals? How do we recognize them? Do these environments change with pressure?
To answer these questions, we establish a classification system based upon the analysis of
known crystal structures of hydrated minerals contained within the American
Mineralogist Crystal Structure Database.
Literature Review
Hydrogen and the Hydrogen Bond
The following describes the classic model of hydrogen bonding. When hydrogen
bonds to a highly electronegative element like oxygen, the lone electron of the hydrogen
is drawn to one side of its nucleus. This leaves the proton (nucleus) exposed creating an
apparent positive charge on the opposite side of the atom. This allows the hydrogen to
form another (weak) bond with the electron of another "acceptor" element (D—H···A).
This second bond is called the hydrogen bond (Jeffrey 1997). Not all O-H bonds will
result in a hydrogen bonds, however hydrogen bonds are common in many minerals.
Pauling (1931) was the first to use the term "hydrogen bond" in his paper detailing the
nature of the chemical bond in the H-F-H ion. The concept of the hydrogen bond is
attributed to Huggins (1922) and independently to Latimer and Rodebush (1920). 1 Fuller
(1959) classified hydrogen bond lengths and angles in crystals. He found that the type of
donor and acceptor groups controls bond lengths and that the angle of the bond has little
1
In 1969 Huggins presented a paper to the Royal Swedish Academy of Sciences
claiming the he invented the concept of the hydrogen bond before Latimer and Rodebush
in a 1919 thesis for an advanced inorganic chemistry course at the University of
California. Huggins' 1922 paper is his first published mention of the concept of the
hydrogen bond. (Jeffrey 1997)
10
effect on the lengths. However, he noted that bonds are generally not present when the OH…O angle is less than 160°. Brown (1976a,b) discusses the structure, geometry, and
energies of O-H…O hydrogen bonds. He suggests strong hydrogen bonds with O-O
distances less than 2.7 Å are linear, while weaker hydrogen bonds with O-O distances
greater than 2.7 Å are generally bent. The strength and occurrence of hydrogen bonds
depends on factors such as bond valence and O-O distance. The nature of the hydrogen
bond in minerals has been studied for many rock-forming minerals (c.f. Armbruster et al.
2001; Baur 1973; Burns and Hawthorne 1994; Catti et al. 1977; Cioslowski and Mixon
1992; Ferraris et al. 1986; Frost et al. 2003; Furukawa et al. 2008; Hadzi 1965; Jacobs et
al. 2005; Jacobsen et al. 2000; Kleppe et al. 2003; Wan and Ghose 1977). The hydrogen
environments in minerals, however, have not been systematically examined and
categorized in the way presented in this study.
Locating Hydrogen in a Structure
Before modern X-ray and neutron technologies, it was difficult to resolve and
locate the hydrogen atom within the structure of a mineral and to distinguish hydroxyl
groups from water molecules. Hydrogen atom positions can be predicted by observing
which oxygen atoms (or other donor anions) in the structure appear to be under-bonded.
Donnay (1970) describes a method of locating hydrogen atoms using Pauling's principle
of local neutralization of charge. Liang and Hawthorne (1998) calculate the hydrogen
positions in micas and clay minerals using the Static Structure-Energy Minimization
method for periodically repeating structures.
11
Currently, X-ray diffraction is the most common method for locating hydrogen in
crystal structures. However, the intensity of scattered X-rays is a function of atomic
number and the scattering of hydrogen with Z=1 is often overwhelmed by the scattering
of other elements. Few high-pressure determinations of the hydrogen positions within
mineral structures have been made due to the absorption and scattering from the
diamonds and beryllium plates in diamond anvil cells (Parise et al. 1994; Chakoumakos
et al. 1997; Prewitt and Parise 2000; Winkler 2010).
Neutron diffraction is an alternative method for locating the hydrogen in a crystal
structure (Knorr and Depmeier 2006). Hydrogen is a strong scatterer of neutrons, but the
requirement of a large sample size limits the achievable pressures in high-pressure
studies. Recent successful determinations of hydrogen positions in minerals by neutron
diffraction were completed for the crystal structure of hydrogarnets (Lager et al. 1987,
1989; Lager and von Dreele 1996), hydroxyl-clinohumite (Berry and James 2001),
hydroxylapatite (Leventouri et al. 2001), topaz (Komatsu et al. 2008), epididymite and
eudidymite (Gatta et al. 2008), and lawsonite (Kolesov et al. 2008).
Hydrogen Storage In Minerals
The United States Department of Energy has set criteria for volumetric and
gravimetric density of stored hydrogen (Zhou 2005). In addition to density requirements,
hydrogen storage systems must be reversible (Zuttel 2003). Currently, there are 6
methods for hydrogen storage: High pressure gas cylinders (up to 800 bar), liquid
hydrogen in cryogenic tanks, adsorbtion on materials with large surface areas, absorbed
on interstitial sites in host metals, chemically bonded in covalent and ionic compounds,
12
and through oxidation of reactive metals (Zuttel 2003).
The high-pressure gas cylinders are the most common storage system (Zuttel 2004).
Synthetic complex hydrides such as alanates, amides, and borohydrides are being studied
as potential hydrogen storage systems (Zeybek and Akin 2005). However, complications
with kinetics of dehydrogenation and rehydrogenation (use and refueling) limit feasibility
(Orimo et al. 2007; Zeybek and Akin 2005).
Previous work on hydrogen storage in minerals has been conducted on nominally
anhydrous minerals in the upper mantle (c.f. Bai and Kohlstedt 1992; Baikov et al. 1992;
Bell and Rossman 1992; Bolfan-Casanova 2005; Hirschmann et al. 2005; Holl et al.
2008; Ingrin and Skogby 2000; Inoue et al. 1995; Jacobsen et al. 2005; Kleppe et al.
2006; Litasov et al. 2003; Ross et al. 2003; Smyth 1987, 2006; Swope et al. 1995) and in
zeolites (c.f. Langmi et al. 2005; Langmi et al. 2003; Weitkamp et al. 1995). In anhydrous
minerals, hydrogen can be incorporated as point defects within the crystal (Prewitt and
Parise 2000; Kohn et al. 2002). In zeolites, hydrogen can be stored in channels and
"cages" in the structure.
Explanation of Dissertation Format
This dissertation follows the University of Arizona Graduate College required
format for dissertations composed primarily of published or publishable manuscripts. An
introductory chapter and a research summary chapter serve to acquaint the reader with
the nature of the research and major findings, which are presented in detail in seven
appended manuscripts. My studies are appended either in the form of published
manuscripts (APPENDICES C, D, E, G), manuscripts submitted for publication
13
(APPENDICIES F), or prepared manuscripts for future publication (APPENDICES A,
B). Appendices A, B, E, F, and G are prepared by me as a first author. Appendix C is
included as a complimentary manuscript to Appendix B and is prepared with significant
experimental contributions from me as fourth author. Appendix F is prepared with
significant experimental and manuscript preparation contributions from me as second
author. My advisor and co-author, Dr. Robert Downs guided me with the direction of my
research, provided assistance with my presentation of ideas, and helped me with the
polishing of the manuscripts for publication. My co-authors Dr. Hexiong Yang, Dr.
Przemyslaw Dera, Dr. Kimberly Tait, Dr. Richard Thompson, Dr. Stanley Evans, Dr.
Marcus Origlieri, Dr. Jakub Plasil, and Dr. Ronald Miletich provided valuable assistance
and feedback at many steps in the writing process.
14
PRESENT STUDY
The methods, results, and conclusions of this study are presented in seven
manuscript appended to this dissertation. The following is a summary of the major
findings in these papers.
Appendix A: Classification of Hydrogen Environments in Hydrous Minerals
In this study we examine the 2580 known mineral species that contain hydrogen
using crystal structure data from the American Mineralogist Crystal Structure Database
(Downs and Hall-Wallace 2003). Excluding those that contain ammonia or hydronium,
2487 contain OH, either as H2O (water) or as isolated OH groups. Only 17 mineral
species contain H and no O. Of the 2487 that contain OH, 1592 have experimentally
determined crystal structures, with 600 that contain only OH without water, while the
other 992 contain water with or without additional OH. This study focuses on the
structures that contain isolated OH groups. There are 450 mineral species that contain OH
without water and have crystal structure determinations that include the location of the
hydrogen. We have identified nine unique classes of OH environments. The class that
contains the largest number of minerals is exemplified by brucite, where the hydrogen is
bonded to the oxygen atom that is common to three edge-sharing octahedra. This
structural environment is also found in the micas, clays, and amphiboles.
Appendices B & C
Of the nine hydrogen environments that we identified, the one that includes behoite
15
Be(OH)2 (Ehlmann and Mitchell 1969) seemed particularly interesting because of its
structural similarity with SiO2 cristobalite. At ambient conditions, behoite is isostructural
with SiO2 cristobalite but with each bridging oxygen atom replaced by an OH group.
Silica is one of the most studied systems, and behoite will provide a model for the effect of
hydrogen on the silica system. Appendices B and C are complimentary and discuss the
high-pressure behavior of the mineral behoite, Be(OH)2, and its relationship to the silica
polymorph cristobalite.
Appendix B: The Structure Determination of the High-Pressure Polymorph of Behoite,
Be(OH)2
This high-pressure experiment focuses on a metal hydroxide, specifically
beryllium hydroxide. Several studies have been conducted on other divalent metal
hydroxides, however, the high pressure behavior of β-Be(OH)2, the mineral behoite, has
not been previously explored. Single crystal X-ray diffraction techniques are used to
investigate the high-pressure phase transition of behoite. Natural behoite crystals from
Mont St. Hilaire, Quebec, Canada are examined under three distinct experimental
conditions. The structure is characterized in air, and at low pressures to 1.74 GPa at a
small laboratory facility at the University of Arizona and two experiments at the
Advanced Photon Source, Argonne National Laboratory; one at a beamline with an
insertion device to 10.44 GPa and the other on a bending magnet to 37.09 GPa. The
single crystal refinement of the natural behoite sample yields a unit cell of a = 4.5362(8),
b = 4.6204(7), c = 7.0531(7) Å, V = 147.84(2) Å3 with space group P212121. A phase
16
transition in the interval of 1.73 – 3.64 GPa was indicated by a distinct change in the
diffraction pattern. We refined the unit cell and solved the structure for the high-pressure
polymorph at 5.98 GPa with a = 5.738(2), b = 6.260(3), c = 7.200(4) Å, V = 258.7(2) Å3
and space group Fdd2. The high-pressure polymorph persisted up to 37 GPa. The
transition of behoite to the high-pressure polymorph is both displacive and reversible
with the mechanism being the rotation of the BeO4 tetrahedra.
Appendix C: New Insights into the High-Pressure Polymorphism of SiO2 Cristobalite
Single-crystal X-ray diffraction experiments with cristobalite from Ellora Caves,
India were performed. X-ray diffraction data was collected for five pressure points from
0.68 to 5.31 GPa with methanol-ethanol as the pressure medium. The sample was then
loaded with helium gas as the pressure medium and additional pressure points were
collected to 18.09 GPa. We observed the monoclinic high-pressure phase II, a =
8.011(3), b = 4.544(3), c = 8.890(2) Å, β = 121.0(2)°, in agreement with earlier in situ
studies (Dove et a. 2000), and its crystal structure was unambiguously determined in
space group P21/c. Additional experiments using the same material reveal that the well
known reversible displacive phase transition to cristobalite-II, which occurs at
approximately 1.8 GPa can be suppressed by rapid pressure increase, leading to an
overpressurized metastable state, persisting to pressure as high as 10 GPa. Single-crystal
data have been used to refine the structure models of both cristobalite and the monoclinic
high-pressure phase II over the range of pressure up to the threshold of formation of
cristobalite XI (13 GPa), providing important constraints to assess the feasibility of the
17
two competing silica densification models proposed, based on quantum mechanical
calculations.
Appendices D through G
To learn X-ray diffraction techniques, a variety of crystal structures were studied,
including kôzulite, despujolsite, bobdownsite, and walstromite.
Appendix D: Kôzulite, an Mn-Rich Alkali Amphibole
Kôzulite is a member of the brucite type OH environment. The crystal structure of
kôzulite, an Mn-rich alkali amphibole with the ideal formula NaNa2[Mn2+4
(Fe3+,Al)]Si8O22(OH)2, was refined from a natural specimen with composition (K0. 20Na0.
80)(Na1. 60
Ca0. 18Mn2+0. 22)(Mn2+2 . 14Mn3+0. 25Mg2. 20-
Fe3+0.27Al0.14)(Si7.92Al0.06Ti0.02)O22[(OH)1.86F0.14]. The site occupancies determined from
the refinements are M1 = 0.453(1) Mn + 0.547(1) Mg, M2 = 0.766(1) Mn + 0.234(1) Mg,
and M3 = 0.257(1) Mn + 0.743(1) Mg, where Mn and Mg represent (Mn+Fe) and
(Mg+Al), respectively. The average M—O bond lengths are 2.064(1), 2.139(1), and
2.060(1) Å for the M1, M2, and M3 sites, respectively, indicating the preference of large
Mn2+ for the M2 site. Four partially occupied amphibole A sites were revealed from the
refinement, with A(m) = 0.101(4) K, A(m)0 = 0.187(14) Na, A(2) = 0.073(6) Na, and
A(1) = 0.056(18) Na, in accord with the result derived from microprobe analysis (0.20 K
+ 0.80 Na), considering experimental uncertainties.
18
Appendix E: Despujolsite, Ca3Mn4+(SO4)2(OH)6•3H2O
In our efforts to understand the hydrogen bonding environments and the
relationships in the hydrogen bonding schemes in the minerals of the fleischerite group,
we noted that the structural information of despujolsite needed to be improved. The
crystal structure of despujolsite, the Ca-Mn member of the fleischerite group, ideally
Ca3Mn4+(SO4)2(OH)6•3H2O, was redetermined from a natural specimen. All non H atoms
were refined with anisotropic displacement parameters and with the H atoms located. The
structure of this compound is characterized by layers of CaO8 polyhedra connected by
Mn(OH)6 octahedra and SO4 tetrahedra. The average Ca—O, Mn—O, and S—O bond
lengths are 2.489, 1.915, and 1.472 Å respectively. There are two distinct hydrogen
bonds: OH3—H1···O2 and OW4—H2···O1, with O—O bond distances of 2.819(2) and
2.789(2) Å respectively. This work represents the first description of hydrogen in the
fleischerite group of minerals. Because despujolsite contains both OH and H2O groups it
does not fit into my proposed classification scheme.
Appendix F: Bobdownsite, a New Member of the Whitlockite-Group from Big Fish River,
Yukon, Canada
Many minerals with OH groups are also found with F substituting for OH. A new
mineral, bobdownsite, the F-analogue of whitlockite with the ideal formula
Ca9Mg(PO3F)(PO4)6, Ca9(Mg,Fe2+) (PO3OH)(PO4)6 has been found on a sample from Big
Fish River, Yukon, Canada. It is found in the Lower Cretaceous bedded ironstones and
shales exposed on a high ridge on the west side of the river, upstream from phosphatic
19
nodule slopes. The paragenesis appears to be kulanite followed by bobdownsite with
accessory arrojadite and minor quartz. It has also been found at the Tip Top mine, Custer
County, South Dakota, USA, and in Martian meteorites.
Bobdownsite is colourless, transparent with white streak and vitreous luster. It is brittle,
with a Mohs hardness of 5.0; no cleavage or parting is observed. Fracture is irregular,
uneven and subconchoidal and no twinning is observed macroscopically. The measured
and calculated densities are 3.14 and 3.16 g/cm3 respectively. Bobdownsite is insoluble in
water, acetone, and hydrochloric acid. Optically, the new mineral is uniaxial (-), ω =
1.625(2), ε = 1.622(2), and it does not fluoresce under long- or short-wave ultraviolet
light. The compatibility index (1-KP/KC) is 0.005 (superior). The electron microprobe
analysis yielded an empirical formula of (Ca8.76Na0.24)Σ=9(Mg0.72Fe3+0.13Al0.11Fe2+0.04)Σ=1
(P1.00O3F1.00)(P1.00O4)6. Bobdownsite, isostructural with whitlockite, is trigonal with space
group R3c and unit-cell parameters a = 10.3224(3), c = 37.070(2) Å, and V = 3420.7(6)
Å3. Its structure is characterized by the [Mg(PO4)6]16- ligand, referred to as the “Mgpinwheel.” The Mg-pinwheels are flattened in the c direction and can be thought of as
defining oblate ellipsoids whose short axes are parallel to c and that form a regular
packing arrangement. The isolated, non-polymerized pinwheels form layers
perpendicular to c, which are held together by high-coordination number intralayer Ca
cations. The interlayer cations are also Ca atoms. The Raman spectra of bobdownsite
resemble those of phosphate minerals in the whitlockite group. However, the welldefined doublet of terrestrial whitlockite and low-REE merrilite in the 950-975 cm-1
band, corresponding to the distinct phosphate groups P(B1) and P(B2), is not seen in
20
bobdownsite.
Appendix G: Structure of Walstromite, BaCa2Si3O9, And Its Relationship to CaSiO3Walstromite and Wollastonite-II
In an attempt to understand transformations between settings, I conducted a stud
on the structural relationship between the important calcium silicate minerals
walstromite, CaSiO3-walstromite, and wollastonite-II. This study highlights the
importance of correctly identifying structural relationships in minerals. The crystal
structure of walstromite, ideally BaCa2Si3O9, was refined with data from single-X-ray
diffraction on a natural specimen from the type locality Esquire No. 8 clam, Big Creek,
Fresno County, California, USA. It is triclinic, with space group P 1 and unit cell
parameters a = 6.7335(2), b = 9.6142(3), c = 6.6859(2) Å, α = 69.638(2)°, β =
102.281(2)°, γ = 96.855(2)°, V = 396.01(2) Å3. The only previously published structure
for walstromite was based on photographic film intensity data collected from synthetic
BaCa2Si3O9 (Dent Glasser and Glasser 1968). Due to uncertainty in oxygen positions, the
reported final R-factor was 0.16. The current refinement yielded an R factor of 0.030
with the inclusion of anisotropic displacement parameters.
Walstromite is a Ba-Ca cyclosilicate characterized by Si3O9 three-membered rings.
It is related to the important calcium silicate group of minerals, especially to CaSiO3walstromite, through the substitution of Ba into one of the three distinct Ca sites. Joswig
et al. (2003) suggested that the structural changes caused by the replacement of Ba2+ by
Ca2+ are minimal and that walstromite is isomorphic with CaSiO3-walstromite, but
21
topologically different from high-pressure wollastonite-II (Ca3Si3O9). Our study
demonstrates that wollastonite-II and CaSiO3-walstromite are identical phases, and are
isostructural with walstromite. This isomorphism implies that the high-pressure CaSiO3
phase may be a potential host for large cations in deep Earth environments.
22
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24
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30
APPENDIX A:
CLASSIFICATION AND TOPOLOGY OF HYDROGEN ENVIRONMENTS IN
HYDROUS MINERALS
Madison C. Barkley1,2 and Robert T. Downs2
1
2
Arizona Historical Society, Phoenix, Arizona 85007, U.S.A.
Department of Geosciences, University of Arizona, Tucson, Arizona 85721, U.S.A.
31
CLASSIFICATION AND TOPOLOGY OF HYDROGEN ENVIRONMENTS IN
HYDROUS MINERALS
Madison C. Barkley1,2 and Robert T. Downs2
1
Arizona Historical Society, Phoenix, Arizona 85007
2
Department of Geosciences, University of Arizona, Tucson, Arizona 85721, U.S.A.
ABSTRACT
There are currently 4532 minerals recognized by the International Mineralogical
Associaion, 2580 of which contain the element hydrogen (57%). Aside from those
minerals that contain ammonia or hydronium, 2487 minerals contain OH either as H2O or
isolated OH groups. Of these, 1592 have experimentally determined crystal structures
listed in the American Mineralogist Crystal Structure Database, with about 600 reported
structure that contain only OH without H2O. The other 992 contain H2O with or without
additional OH. There are 450 known mineral species that contain OH without H2O and
have crystal structure determinations that include the location of the H atom. The crystal
structures of these 450 minerals have been examined, and their OH sites categorized. We
found nine unique classes of OH sites: Zoisite Type, Brucite Type, Epidote Type,
Alunite-Jarosite Type, Datolite Type, Apatite Type, Behoite Type, Vayrynenite Type,
and Euclase Type.
Keywords: hydrogen, hydroxyl anion, crystal structure, hydrogen storage
INTRODUCTION
One of the challenges facing society is alternative energy sources. The federal
government has identified hydrogen fuel as a potentially clean and cheap solution.
However, the practical aspects are hindered by the simple problem of how to concentrate
32
and store hydrogen safely. We are studying the hydrogen environments, which are
characterized by the structural and electronic interactions of the hydrogen atom, in
minerals in order to create a body of knowledge on how Nature stores hydrogen in solids.
For instance, the hydrogen environment in brucite is vastly different than the
environment in hydroxylapatite. This creates obvious questions, such as how many
different environments exist in minerals and how do we recognize them? To answer these
questions, we are establishing a classification based upon the analysis of the known
crystal structures. We will not, however, focus on the H environment in nominally
anhydrous materials.
Hydrogen is by far the most abundant element in the universe: roughly 93% of all
atoms or 75% of all matter by weight (Dyer and Gunter 2008). However, more than 90%
of the Earth is composed of only four elements. In order of abundance these elements are
oxygen, iron, silicon, and magnesium. Despite its smaller relative abundance on Earth,
more than half of all minerals contain the element hydrogen. Hawthorne (1992) suggests
that hydrogen is the significant factor in the distribution of mineral species within the
Earth and Evans (1964) suggests that hydrogen plays an important role in the
polymerization process. There are currently 4532 mineral species recognized by the
International Mineralogical Association, 2580 of which contain hydrogen (57%). This
study will examine these hydrated minerals to determine the crystallographic
environment of their hydrogen atoms, and create a classification of natural hydrogen
environments within these hydrated minerals.
33
When the hydrogen atom bonds to a highly electronegative element like oxygen,
the one electron of the hydrogen is drawn to the bonded side of the atom. This leaves the
proton (nucleus) exposed creating an apparent positive charge on the opposite side of the
atom. This allows the hydrogen to form another (weak) bond with the electron of another
element, usually oxygen. This second bond is called a hydrogen bond (Jeffrey 1997). Not
all O-H bonds are associated with hydrogen bonds. The nature of the hydrogen bond in
minerals has been studied for many rock-forming minerals (c.f. Armbruster et al. 2001;
Baur 1973; Burns and Hawthorne 1994; Catti et al. 1977; Cioslowski and Mixon 1992;
Ferraris et al. 1986; Frost et al. 2003; Furukawa et al. 2008; Hadzi 1965; Jacobs et al.
2005; Jacobsen et al. 2000; Kleppe et al. 2003; Wan and Ghose 1977).
Hawthorne (1992) investigated the roles of the principle H bearing species in
minerals (OH, H2O, H3O, NH, and H5O2). Of particular interest is his treatment of OH
and H2O. Hawthorne explains the different roles that hydrogen plays in the structures of
minerals based on the presence of an OH group or H2O molecule as a component of the
structural unit or a component of an interstitial complex.
To date, only hydrogen environments involving H2O molecules in minerals have
been systematically examined and categorized. Classification of water molecules based
on coordination of the lone pair orbitals was proposed by Chidambaram et al. (1964). The
model was further improved by Hamilton and Ibers (1968) and by Ferraris and FranchiniAngela (1972). Using data from 40 neutron diffraction studies, Ferraris and FranchiniAngela created 5 classes of water molecule environments in minerals based on the
coordination number of the oxygen in the H2O group. In the Class 1 environment, water
34
is coordinated to one cation, which is aligned along the bisector of the lone-pair orbitals.
Class 1' is different than Class 1 in that the angle between the oxygen, the cation, and the
plane of the water molecule is around 45°. In Class 2 and 3 environments water is
coordinated by 2 and 3 cations respectively. Class 4 involves cations that are not
specifically along lone-pair orbitals or their bisectors. In this study we will categorize the
hydrogen environments of OH groups based on a similar classification scheme:
coordination of the oxygen atom in the OH group.
METHODOLOGY
The American Mineralogist Crystal Structure Database (Downs and Hall-Wallace
2003) currently contains about 14,000 crystal structures of minerals representing almost
all of the known crystal structures. We examine the 2580 known mineral species that
contain hydrogen using crystal structure data from the American Mineralogist Crystal
Structure Database. Excluding those that contain ammonia or hydronium, 2487 contain
OH, either as H2O (water molecules) or as isolated OH groups. Only 17 mineral species
contain H and no O. Of the 2487 that contain O-H, 1592 have experimentally determined
crystal structures, with 600 that contain only OH without water, while the other 992
contain water with or without additional OH. We focus on the structures that contain
isolated OH groups. There are 450 known mineral species that contain OH without H2O
and have crystal structure determinations that include the location of the H atom. The
crystal structures of these 450 minerals have been visually examined using the structure
visualization program XtalDraw (Downs 2004), and their OH sites categorized. We
35
found nine unique classes of OH sites based on the coordination of the oxygen atom in
the OH group.
DESCRIPTION OF HYDROGEN ENVIRONMENTS
We have identified nine unique classes of OH. This classification will be presented
below.
Zoisite Type: In the zoisite type environment the OH is bonded to two edge-sharing
octahedra. There are 13 mineral species found in this group including some micas.
Brucite Type: In the brucite type environment the OH is bonded to three edge-sharing
octahedra. There are 155 mineral species found with this OH environment. Notable
groups include amphibole, axinite, diaspore, humite, lazulite, tourmaline, and some
micas.
Epidote Type: In the epidote type environment the OH is bonded to two edge sharing
octahedra and a third cation. The coordination number of the third cation in epidote is 8.
However, there are variations of this environment where the coordination of the third
cation is 5 (althausite), 7 (cechite), and 9 (calderonite). There are forty-three minerals
found with OH in this environment.
Alunite-Jarosite Type: In the alunite-jarosite type environment the OH is bonded to two
octahedra and a large low bond strength cation such as K or Pb. Eight minerals have been
found with OH in this environment. This environment is different from the epidote type
in that the octahedra are no longer sharing edges.
Datolite Type: In the datolite type environment the OH is bonded to two 8 coordinated
36
cations (edge sharing) and one 4 coordinated coordinated cation (corner sharing). Nine
minerals have been found with OH in this environment.
Apatite Type: In the apatite type environment the OH is bonded to three corner-sharing 8
coordinated cations. There are 8 minerals found with this type of environment.
Behoite Type: In the behoite type environment the OH is the single shared O between two
tetrahedra. Wulfengite is also a member of this group.
Vayrynenite Type: The hydrogen environment in vayrynenite is characterized by two 4
coordinated cations (square planar) and one 6 coordinated cation.
Euclase Type: In the euclase type environment the OH is the single shared O between an
octahedron and a tetrahedron. Eleven minerals have been found with OH in this
environment.
AKNOWLEDGEMENTS
The authors gratefully acknowledge the support of this study from the RRUFF Project,
Chevron Texaco, BP p. l. c., Tucson Gem and Mineral Society, University of Arizona
Galileo Circle, and Carnegie-DOE Alliance Center under cooperative agreement DE
FC52-08NA28554.
REFERENCES
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Medenbach, O., Gutzmer, J. (2001) Structure, compressibility, hydrogen bonding,
and dehydration of the tetragonal Mn3+ hydrogarnet, henritermierite. American
37
Mineralogist, 86, 147-158.
Baur, W.H. (1973) Reconstruction of local atomic environments in the disordered
hydrogen-bonded crystal structure of paraelectric ammonium dihydrogen phosphate
and potassium dihydrogen phosphate. Acta Crystallographica, B29, 2726-2731.
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32, 895-902.
Catti, M., Ferraris, G., Filhol, A. (1977) Hydrogen bonding in the crystalline state.
CaHPO4 (monetite), P-1; or P1? A novel diffraction study. Acta Crystallographica,
B33, 1223-1229.
Chidambaram, R., Sequeira, A., Sikka, S.K. (1964) Journal of Chemical Physics, 41,
3616.
Cioslowski, J., Mixon, S.T. (1992) Universality among topological properties of electron
density associated with the hydrogen-hydrogen nonbonding interactions. Canadian
Journal of Chemistry, 70, 443-449.
Downs, R.T. (2004) XtalDraw. University of Arizona, Tucson.
Downs, R.T., Hall-Wallace, M. (2008) The American Mineralogist Crystal Structure
Database. American Mineralogist, 88, 247-250.
Dyar, M.D., Gunter, M.E. (2008) Mineralogy and Optical Mineralogy. Mineralogical
Society of America: Chantilly, Virginia.
Evans, R.C. (1964) An Introduction to Crystal Chemistry. Cambridge University Press:
Cambridge.
Ferraris, G., Franchini-Angela, M. (1972) Survey of the geometry and environment of
38
water molecules in crystalline hydrates studied by neutron diffraction. Acta
Crystallographica, B28, 3572.
Ferraris, G., Fuess, H., Joswig, W. (1986) Neutron diffraction study of MgNH4PO4•6H2O
(struvite) and survey of water molecules donating short hydrogen bonds. Acta
Crystallographica, B42, 253-258.
Frost, R.L., Kloprogge, J.T., Williams, P.A. (2003) Raman spectroscopy of lead sulphatecarbonate minerals - implications for hydrogen bonding. Neues Jahrbuch für
Mineralogie, Monatshefte, 2003, 529-542.
Furukawa, A.S., Komatsu, K., Vanpeteghem, C.B., Ohtani, E. (2008) Neutron diffraction
study of δ-AlOOD at high pressure and its implication for symmetrization of the
hydrogen bond. American Mineralogist, 93, 1558-1567.
Hadzi, D. (1965) Infrared spectra of strongly hydrogen-bonded systems. Pure and
Applied Chemistry, 11, 435-453.
Hamilton, W.C., Ibers, J.A. (1968) Hydrogen Bonding in Solids. Benjamin: New York.
Hawthorne, F.C. (1992) The role of OH and H2O in oxide and oxysalt minerals.
Zeitschrift für Kristallographie, 201, 183-206.
Jacobs, H., Niemann, A., Kockelmann, W. (2005) Low temperature investigations of
hydrogen bridge bonds in the hydroxides β–Be(OH)2 and ε–Zn(OH)2 by Raman–
spectroscopy as well as X–ray and neutron diffraction. Zeitschrift für Anorganische
und Allgemeine Chemie, 631, 1247-1254.
Jacobsen, S.D., Smyth, J.R., Swope, R.J., Sheldon, R.I. (2000) Two proton positions in
the very strong hydrogen bond of serandite, NaMn2[Si3O8(OH)]. American
39
Mineralogist, 85, 745-752.
Jeffrey, G.A. (1997) An Introduction to Hydrogen Bonding. Oxford University Press:
Ney York.
Kleppe, A.K., Jephcoat, A.P., Welch, M.D. (2003) The effect of pressure upon hydrogen
bonding in chlorite: A Raman spectroscopic study of clinochlore to 26.5 GPa.
American Mineralogist, 88, 567-573.
FIGURE CAPTIONS
Figure 1: Zoisite Type hydrogen environment as seen in aluminoceledonite (6
coordinated Al and 6 coordinated Mg). (a) Polygon view; (b) Alternate polygon view; (c)
View with atoms as spheres; (d) Alternate view with spheres.
Figure 2: Brucite Type hydrogen environment as seen in actinolite (6 coordinated Mg).
(a) Polygon view; (b) Alternate polygon view; (c) View with atoms as spheres; (d)
Alternate view with spheres.
Figure 3: Epidote Type hydrogen environment as seen in cobaltaustinite (8 coordinated
Ca and 6 coordinated Co). (a) Polygon view; (b) Alternate polygon view; (c) View with
atoms as spheres; (d) Alternate view with spheres.
Figure 4: Alunite-jarosite Type hydrogen environment as seen in alunite (12 coordinated
K and 6 coordinated Al). (a) Polygon view; (b) Alternate polygon view; (c) View with
atoms as spheres; (d) Alternate view with spheres.
Figure 5: Datolite Type hydrogen environment as seen in datolite (8 coordinated Ca and
4 coordinated B). (a) Polygon view; (b) Alternate polygon view; (c) View with atoms as
spheres; (d) Alternate view with spheres.
Figure 6: Apatite Type hydrogen environment as seen in apatite (CaOH) (8 coordinated
Ca). (a) Polygon view; (b) Alternate polygon view; (c) View with atoms as spheres; (d)
Alternate view with spheres.
Figure 7: Behoite Type hydrogen environment as seen in behoite (4 coordinated Be). (a)
Polygon view; (b) Alternate polygon view; (c) View with atoms as spheres; (d) Alternate
view with spheres.
40
Figure 8: Vayrynenite Type hydrogen environment as seen in vayrynenite (4 coordinated
Be, 4 coordinated P, and 6 coordinated Mn) coordinated. (a) Polygon view; (b) View
with atoms as spheres.
Figure 9: Euclase Type hydrogen environment as seen in cascandite (6 coordinated Ca
and 4 coordinated Si). (a) Polygon view; (b) View with atoms as spheres.
41
42
43
44
45
46
47
48
49
50
APPENDIX B:
THE STRUCTURE DETERMINATION OF THE HIGH-PRESSURE
POLYMORPH OF BEHOITE, Be(OH)2
Madison C. Barkley1,2, Przemyslaw Dera3, Robert T. Downs2, and Ronald Miletich4
1
Arizona Historical Society, Phoenix, Arizona 85007, U.S.A.
Department of Geosciences, University of Arizona, Tucson, Arizona 85721, U.S.A
3
Center for Advanced Radiation Sources, Argonne National Laboratory, University of
Chicago, Argonne, Illinois, U.S.A.
4
Mineralogisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld
236, 69120 Heidelberg, Germany
2
51
THE STRUCTURE DETERMINATION OF THE HIGH-PRESSURE
POLYMORPH OF BEHOITE, Be(OH)2
Madison C. Barkley1,2, Przemyslaw Dera3, Robert T. Downs2, and Ronald Miletich4
1
Arizona Historical Society, Phoenix, Arizona 85007
Department of Geosciences, University of Arizona, Tucson, Arizona 85721, U.S.A
3
Center for Advanced Radiation Sources, Argonne National Laboratory, University of
Chicago, Argonne, Illinois, U.S.A.
4
Mineralogisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld
236, 69120 Heidelberg, Germany
2
ABSTRACT
Single crystal X-ray diffraction techniques were used to investigate the highpressure phase transition of behoite, Be(OH)2. Behoite is isostructural with SiO2
cristobalite at ambient conditions. Natural behoite crystals from Mont St. Hilaire,
Quebec, Canada (RRUFF No. R060659; http://rruff.info) were examined under three
distinct experimental conditions. The structure was characterized in air, and at low
pressures to 1.74 GPa at a small laboratory facility and two experiments were run at APS,
Argonne National Laboratory; one at a beamline with an insertion device to 10.44 GPa
and the other on a bending magnet to 37.09 GPa. The single crystal refinement of the
natural behoite sample yielded a unit cell of a = 4.5362(8), b = 4.6204(7), c = 7.0531(7)
Å, V = 147.84(2) Å3 with space group P212121. A phase transition in the interval of 1.74
– 3.64 GPa was indicated by a distinct change in the diffraction pattern. We refined the
unit cell and solved the structure for the high-pressure polymorph at 5.98 GPa with a =
5.738(2), b = 6.260(3), c = 7.200(4) Å, V = 258.7(2) Å3 and space group Fdd2. The highpressure polymorph persisted up to 37 GPa. The transition of behoite to the high-pressure
52
polymorph is both displacive and reversible with the mechanism related to the rotation of
the BeO4 tetrahedra.
Keywords: behoite, beryllium hydroxide, hydrogen, high-pressure
INTRODUCTION
Beryllium hydroxide is currently known to exhibit four phases at ambient
conditions. Alkali addition to a beryllium salt solution produces a gelatinous beryllium
hydroxide (Bear and Turnbull 1965). Upon aging this amorphous product transforms to a
metastable tetragonal crystalline polymorph (α), which eventually transforms into a
stable orthorhombic crystalline polymorph (β) (Bear and Turnbull 1965; Eagleson 1993)
that is found naturally as the mineral behoite (Ehlmann and Mitchell, 1970). The fourth
polymorph is also natural and is known as the monoclinic mineral clinobehoite
(Nadezhina et al. 1989).
The structure of many dihydroxides with divalent metals is based on octahedral
coordination of the metal ions in layered structures [e.g. brucite, Mg(OH)2; portlandite
Ca(OH)2; Sr(OH)2; Ba(OH)2]. These compounds commonly crystallize with hexagonal
symmetry in the CdI2 structure. In contrast, β-Be(OH)2 (behoite), and zinc hydroxide, εZn(OH)2 (wulfingite), exhibit a three dimensional tetrahedral framework that. Stahl et
al. (1998) and Kusaba et al. (2007) recognized as a modified SiO2 cristobalite-type
structure, in itself a derivative of the diamond structure, with OH groups replacing the
bridging O atoms of cristobalite.
The crystal structure of β-Be(OH)2, was first solved by Seitz et al. (1950) from a
53
synthetic sample. It was shown to have an orthorhombic cell of a = 4.611(5) Å, b =
7.025(8) Å, c = 4.526(5) Å, in which Be is tetrahedrally coordinated to OH groups. Joe
Williams of San Saba, Texas recognized the first natural crystals of behoite in an
examination of samples from the Rode Ranch pegmatite (Ehlmann and Mitchell 1969)
associated with a suite of rare earth minerals. Other notable localities, where behoite
occurs in miarolitic cavities, nepheline syenite, and nepheline-syenite pegmatites, include
Mont Saint-Hilaire, Quebec, Canada (Horváth and Gault 1990), and Tvedalen and
Lågendalen, Oslo region, Norway (Anderson 1993; Engvoldsen et al. 1991). Using X-ray
studies, Ehlmann and Mitchell (1969) confirmed the orthorhombic nature of the mineral
found at the Rode Ranch pegmatite and showed that it has the same structure as the
synthetic β-Be(OH)2, of Seitz et al. (1950) and isotypic to ε-Zn(OH)2.
At ambient conditions behoite is also isotypic to SiO2 α-cristobalite, but with each
bridging O atom replaced by an OH group. The cubic β-cristobalite is the stable phase in
the silica phase diagram, found at room pressure, between 1743 K and 1900 K (Downs
and Palmer 1994). However, even though the atoms are displaced and the symmetry
changes, the Si-O bonds are not broken when the temperature is decreased to <1743K,
and α-cristobalite represents this topologically equivalent low-temperature phase. A
variety of tectosilicates, including α-cristobalite, undergo displacive transformations
associated with anion-anion repulsion and the consequent rotation of tetrahedra. Downs
and Palmer (1994) reported the phase transformation of α-cristobalite at room
temperature and critical pressure between 1.18 – 1.6 GPa. Recently Dera et al (2011)
showed that the critical pressure for the transformation can be raised as high as 10 GPa
54
by rapid pressure increase. Cristobalite transforms to the monoclinic high-pressure phaseII at ~1.8 GPa and upon further compression (~12 GPa) to monoclinic cristobalite X-I
(Dera et al. 2011).
Like SiO2 α-cristobalite, behoite and Zn(OH)2 undergo a phase transition between
1.3 and 2.0 GPa. There have been several studies on ε-Zn(OH)2 that have led to insights
on the high-pressure phases. Kusaba et al. (2007) examined X-ray powder diffraction
profiles at pressure and observed a transformation from the orthorhombic modified
cristobalite structure to a tetragonal phase at ~1.1 GPa. They also observed a previously
unknown transition at 2.1 GPa. When a single crystal was subsequently examined, they
observed that the transition from orthorhombic to tetragonal occurred with significant
distortion to the external crystal form, however still preserving the single crystal.
Structure refinement showed that the tetragonal phase exhibits the α-cristobalite
structure. Kusaba et al. (2008) did not provide more details on the phase that was created
at 2.1 GPa because the diffraction peaks were too broad to even permit indexing (Kusaba
et al (2010), but they did find that at 14.5 GPa, a new orthorhombic phase was formed.
With temperature increased to 400 ºC, Zn(OH)2 transformed to the brucite structure, as
also previously observed by Baneyeva and Popova (1969) , which remained metastable
when quenched to room conditions.
Previous high-pressure experiments on behoite were conducted using synthetic
powders. Miletich (2006) studied the transformation in behoite up to 10.9 GPa with
powder methods. Profiles were measured using synchrotron radiation and the MAR345
image-plate system (ID27) at the European Synchrotron Radiation Facility (ESRF). He
55
used a 4:1 methanol-ethanol pressure medium and observed the transformation between
1.79 and 3.11 GPa but did not report the high-pressure structure. In his study, the
morphology of the crystal did not change suggesting that the transition is not
reconstructive.
In this study, a natural crystal of behoite from Mont St. Hilaire, Quebec, Canada
is examined by single crystal X-ray diffraction and Raman spectroscopy in order to
explore its high-pressure behavior and to compare its transformation to that found for
Zn(OH)2 and SiO2 cristobalite.
EXPERIMENTAL
All samples used in this study are from Mont St. Hilaire, Quebec, Canada and are
in the collection of the RRUFF project (deposition No. R060659; http://rruff.info). The
sample appears as tan colored reticulated groups. The crystals were examined under three
distinct experimental conditions. At the University of Arizona laboratory, the structure
was characterized in air, and at low pressures to 1.74 GPa. Two experiments were run at
APS, Argonne National Laboratory; one at a beamline with an insertion device to 10.44
GPa and the other on a bending magnet to 37.09 GPa.
Experiments at the University of Arizona,
In air, UA1
The structure of behoite from a single-crystal study has never before been
reported. Therefore, an equant, euhedral crystal, approximately 0.10 × 0.08 × 0.05 mm,
was selected and mounted on a Bruker X8 APEX2 CCD X-ray diffractometer equipped
56
with graphite-monochromatized MoKa radiation. X-ray diffraction data were collected to
2θ ≤ 80° with frame widths of 0.5° in ω and 90 s counting time per frame. Cell
refinement and data reduction were completed using SAINT (Bruker 2005). The structure
was determined and refined with SHELX97 (Sheldrick 2008) to an R1 factor of 0.039.
The O atom site locations were found using structure-invariant direct methods while Be
and H site locations were found using difference Fourier maps. A summary of the data
collection, crystallographic data, and refinement statistics is given in Table 1. Unit cell
parameters are given in Table 2. Final coordinates and isotropic displacement parameters
of all atoms are listed in Table 3. Anisotropic displacement parameters are recorded in
Table 4, and selected bond distances in Table 5. The unit cell matches those given by
Seitz et al. (1950).
At pressure, UA2 - UA4
The crystal from UA1 was loaded into a four-post diamond anvil cell with a
15:4:1 methanol:ethanol:water pressure medium. Pressure in the cell was determined by
the fluorescence spectra of included ruby chips. Single crystal intensities were collected
on a Picker four-circle diffractometer equipped with MoKα radiation at 45 kV, 40 mA.
All accessible data from a sphere of reciprocal space with 0° ≤2θ ≤ 60° were collected
using scans of 1° width in step increments of 0.025° and 3 sec/step counting time. Two
standard reflections, (-101) and (-112), were measured every 6 hours; no significant
variations in the intensities of the standard reflections were observed. We collected 3 sets
of intensities and refined crystal structures below the transition at 0.53 GPa (UA2), 1.03
GPa (UA3), and 1.74 GPa (UA4). The pressure was increased to 2.78 GPa and the crystal
57
appeared to undergo a phase transition because no peaks could be located. However, the
morphology of the crystal did not change. The pressure was further increased to 3.09
GPa. The only peak located at this pressure point was (-112), one of the standards.
However, the intensity profile of the peak was significantly degraded. The pressure was
increased in the DAC to 4.56 GPa in an effort to stabilize the high-pressure polymorph.
No peaks were found after the final increase in pressure. Since the Raman data from
Miletich (2006) suggests that the transition from the behoite to the high-pressure
polymorph is reversible, we decreased the pressure in the cell. Peaks were absent at 2.02
GPa (decompression), however, they returned at 1.13 GPa. The crystal was then
transferred from the diamond cell to a fiber and run with the same parameters as in air
(UA1) on the Bruker X8 to confirm the reappearance of the peaks. The structure of the
crystal was refined and appeared to be the same as before the transition. A summary of
the data collection, crystallographic data, and refinement statistics is given in Table 1.
Unit cell parameters are given in Table 2. Final coordinates and isotropic displacement
parameters of all atoms are listed in Table 3. Selected bond distances are given in Table 5
IDD1 – IDD5
Additional crystals were prepared for high-pressure data collection with the
insertion device at 13ID-D of the GSECARS facility, Advanced Photon Source, Argonne
National Laboratory. A behoite crystal approximately 0.01 × 0.015 × 0.005 mm was
loaded into a symmetric piston-cylinder Princeton-type diamond anvil cell. Diamond
anvils with culets of 0.36 mm were mounted on asymmetric backing plates (cubic boron
nitride towards the X-ray source and tungsten carbide towards the detector). Re metal
58
gaskets pre-indented to 0.031 mm were used for sample containment. A 16:4:1
methanol:ethanol:water mixture was used as the pressure medium. Pressure in the cell
was determined by the fluorescence spectra of ruby spheres included in the cell collected
at each pressure point both before and after the X-ray data collection. A monochromatic
beam with incident energy of 37 KeV was focused with a pair of Kirkpatrick-Baez
mirrors to a spot of 0.003 × 0.005 mm. Diffraction images were collected using a
MAR165 Charge Coupled Device (CCD) detector, placed at a sample-to-detector
distance of approximately 200 mm. During the exposure the sample was rotated about the
vertical axis of the instrument (ω) in the range of ±28° with a typical exposure time of 28
seconds for the full 56° rotation. Diffraction images were collected at three detector
positions, differing by a translation of 70 mm perpendicular to the incident beam
(translated 70mm to the left, centered, and 70mm to the right). Single crystal X-ray
diffraction data were collected at five pressures: 0.34 GPa (IDD1), 7.68 GPa (IDD2),
10.44 GPa (IDD3), 8.43 GPa (IDD4) (decompression), and 5.98 GPa
(IDD5)(decompression). Diffraction images for were analyzed using the GSE_ADA/RSV
software package (Dera 2007). The number of peaks observed in measurements other
than IDD5 did not allow reliable structure refinement. Integrated peak intensities for
IDD5 were fit using the GSE_ADA program, and corrected for DAC absorption, Lorenz
and polarization effects. Because of the high incident energy and negligible sample
thickness and absorption coefficient, absorption effects were ignored. Peaks from
exposures at the three different detector positions were scaled and merged together. The
resulting lists of peaks with corresponding squares of structure factors |F|2 and their
59
standard deviations were used for the structure refinement, carried out with the SHELXL
software (Sheldrick 2008). The structure of the high-pressure polymorph at 5.98 GPa
(IDD5) was solved by means of the ab initio simulated annealing approach, as
implemented in the software Endeavour (Putz et al. 1999; Brandenburg and Putz 2009).
The structure model was then refined at 5.98 GPa (IDD5) using the SHELXL program. A
summary of the data collection, crystallographic data, and refinement statistics for IDD5
is given in Table 1. Unit cell parameters for IDD1-IDD5 are given in Table 2. Final
coordinates and isotropic displacement parameters of all atoms are listed for IDD5 in
Table 3 and selected bond distances are given in Table 5.
BMD1 – BMD8
Two diamond anvil cells were prepared for high-pressure single crystal X-ray
diffraction data collection with the bending magnet at the 13BM-D station at GSECARS,
APS, Argonne National Laboratory. Single crystals of behoite approximately 0.066 ×
0.041 × 0.022 mm (BMD1) and 0.03 × 0.03 × 0.01 mm (BMD2-8) were loaded into a
symmetric piston-cylinder Princeton-type diamond anvil cell. Diamond anvils with culets
of 0.48 mm (BMD1) and 0.25 mm (BMD2-8) were mounted on asymmetric backing
plates. Re metal gaskets pre-indented to 0.047 mm (BMD1) and 0.043 (BMD2-8) were
used for sample containment. Both cells were gas loaded with Ne as a pressure medium
using the GSECARS/COMPRES gas loading system and pressures were obtained by
ruby fluorescence. Data was collected at 8 pressure points as indicated in Table 2. It
appears that the sample was crushed and no data was collected when the pressure was
raised above 37 GPa. Diffraction images were analyzed using the GSE_ADA/RSV
60
software package (Dera 2007). Integrated peak intensities were fit using the GSE_ADA
program, and corrected for DAC absorption, Lorenz and polarization effects. We
succeeded in refining the unit cells for BMD1-BMD6, however, the incident intensity in
the BM station was insufficient to measure precise structure factor amplitude data from
the very small (5 micron thickness) and very weakly scattering sample, rendering
structural refinements impossible. Experiments BMD7 and BMD8 had too few
reflections (16 and 12 respectively) to accurately constrain the unit cell parameters at
those pressures, but they did not indicate that a transformation had occurred. Unit cell
parameters for BMD1-BMD6 are given in Table 2
RESULTS AND DISCUSSION
The single crystal refinement of the natural behoite sample at ambient conditions
yielded a unit cell of a = 4.5362(8) Å, b = 4.6204(7) Å, c = 7.0531(7) Å with a space
group of P212121 which is in agreement with the cell (in a different setting) of Ehlmann
and Mitchell (1969) of a = 4.64(2) Å, b = 7.05(2) Å, c = 4.55(3) Å which was determined
using a natural powder sample. The setting was changed in this report to match that of
cristobalite. The structure is characterized by a three dimensional tetrahedral framework
of corner sharing Be(OH)4 groups. There are two unique OH groups, which is consistent
with the findings of Lutz et al. (1998) who observed four Raman active libration modes
(two for each OH- ion) for β-Be(OH)2. Hydrogen bonds exist between O1—H1···O2 and
O2—H2···O1. The donor-acceptor distance and <DHA are 2.942 Å and 149.86o for
O1—H1···O2, and 2.911 Å and 159.73o for O2—H2···O1. At ambient conditions the
61
length of the hydrogen bonds (H1···O2 and H2···O1) are both ~2.11 Å. Upon
compression, (at 1.7 GPa) the H1···O2 bond stretches slightly to 2.164 Å and the
H2···O1 distance stretches significantly to 2.33 Å. Based on this difference, it is
anticipated that on further compression the H2···O1 bond will be broken.
We refined the unit cell and solved the structure for the high-pressure polymorph
at 5.98 GPa. Structural refinements of the high-pressure polymorph suggest an
orthorhombic phase different than that proposed by Miletich (2006). Miletich proposed
an orthorhombic unit cell of a = 6.5977 Å, b = 7.3374 Å, c = 5.9774 Å, V = 289.37 Å3, Z
= 8 with an F centered lattice with a space group of Fmmm at 3.92 GPa. The unit cell
parameters from this study (Table 2, Figure 1) for 5.98 GPa are a = 5.738(2) Å, b =
6.260(3) Å, c = 7.200(4) Å, V = 258.7(2) Å3 with space group Fdd2. In contrast to
behoite, the high-pressure polymorph only has one symmetrically independent OH group.
A hydrogen bond exists with an O—H distance of 0.723 Å and an O—H···O distance is
2.95 Å (5.98 GPa). The observed short O-H distance is due to the effect of the electron
density in the bond.
Behoite and the high-pressure polymorph can be related according to
T[hkl]B=[hkl]HPP where [hkl]B and [hkl]HPP represent (hkl) of behoite and the highpressure polymorph respectively, and
1 −1 0


T = 1 1 0
0 0 1
There appears to be a supergroup-subgroup relationship between behoite and the high-
€
pressure polymorph. Behoite with the space group P212121 has point group symmetry 222
62
and the high-pressure polymorph with the space group Fdd2 has point group symmetry
2mm. Both 222 and 2mm are subgroups of the supergroup 2/m 2/m 2/m. The diffraction
images before and after the transition are shown in Figure 2.
The high-pressure polymorph persisted up to 37 GPa. Unlike the Zn(OH)2
pressure induced transition (Kusaba et al. 2007), no changes in the morphology of the
behoite crystals were observed up to the highest pressure reached in our experiments. The
transition of behoite to the high-pressure polymorph is both displacive and reversible
with the mechanism being the rotation of the beryllium tetrahedra. This is consistent with
the mechanism for the pressure induced transformations in the Zn(OH)2 system (Kusaba
et al. 2007) and related tectosilicates (Hazen and Finger 1979; Dove et al. 1993; Downs
and Palmer 1994). However, the second pressure induced phase transition between 2.13.4 GPa (Kusaba 2010) and twinning observed on decompression in the Zn(OH)2 system
was not observed in Be(OH)2.
The unit cell volume of behoite and the high-pressure polymorph as a function of
pressure is shown in Figure 3. Miletich (2006) proposed equations of state for the two
polymorphs. For the high-pressure polymorph he proposed a 3rd-order BM-EOS with K0
= 45.6 GPa and K' = 3.4 is used. Equation of state data for behoite suggests a large
difference from the established values for α-cristobalite. Behoite with K' constrained to 4
has a bulk modulus around 40 GPa, compared with 20 GPa (K' constrained to 4) for αcristobalite (Dera et al. 2011). Downs and Palmer (1994) suggest that the compressibility
of α-cristobalite is directly related to the bending of Si-O-Si angles and the shortening of
Si-Si distances only. The average Be-O-Be angle in behoite ranges from 126.4° at 0.53
63
GPa and 124.8° at 1.74 GPa. Over a 1.05 GPa range the Si-O-Si angle in cristobalite
decreases by 6° (Downs and Palmer 1994) and the Be-O-Be angle in behoite decreases by
only 2° over a range of 1.2 GPa. The rotation of the tetrahedra in behoite can easily be
seen in Figure 4. The Be-O-Be angles are less compressible than the Si-O-Si angles due
to the bridging hydroxyl groups in behoite. This suggests the inclusion of hydrogen in the
behoite structure makes the Be-O-Be angle rigid.
The fact that the tetrahedrally coordinated Be(OH)2 persisted to pressures as high
as 37 GPa is very interesting, since in SiO2 and other chemical systems forming silicalike structures, transformations to octahedrally coordinated phases or decomposition to
amorphous phases occur at pressures lower than 30 GPa (Hemley 1887; Baneyeva and
Popova 1969; Hemley et al. 1988; Sankaran et al. 1988; Zhang and Org 1993; Kusaba
and Kikegawa 2008). We initially thought that the transformation sequence in behoite
would mimic the transformation sequence of alpha cristobalite, but it appears the
inclusion of hydrogen in the structure inhibits the rotation of the BeO4 tetrahedra and thus
the pressure induced displacive transitions.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the support of this study from Chevron
Corporation, BP p. l. c., the University of Arizona Galileo Circle, the Tucson Gem and
Mineral Society, and Carnegie-DOE Alliance Center under cooperative agreement DE
FC52-08NA28554. Portions of this work were performed at GeoSoilEnviroCARS (Sector
13), Advanced Photon Source (APS), Argonne National Laboratory.
64
GeoSoilEnviroCARS is supported by the National Science Foundation - Earth Sciences
(EAR-0622171) and Department of Energy - Geosciences (DE-FG02-94ER14466). Use
of the Advanced Photon Source was supported by the U. S. Department of Energy, Office
of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.
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FIGURE CAPTIONS
Figure 1. The unit cell parameters as a function of pressure for behoite and the highpressure polymorph. The diamonds, circle, and squares represent the a, b, and c unit cell
dimensions respectively. The black symbols represent the unit cell dimensions of behoite
in air (UA1). The solid line is a quadratic fit to the high-pressure data. IDD1 has been
excluded due to the calibration error.
Figure 2. Diffraction images of behoite before (IDD1) and the high-pressure phase after
(IDD2) the transition.
Figure 3. The evolution of the unit cell volume as a function of pressure. The black
diamond represents the volume in air (UA1). The gray diamonds and gray squares
represent the volume of behoite and the high-pressure phase respectively. The solid line
is a quadratic fit to the high-pressure data. IDD1 has been excluded due to the calibration
error.
Figure 4. The structures of behoite (UA2) and the high-pressure polymorph (IDD5)
looking down (00 ) for behoite (001) for the high-pressure polymorph.
69
TABLES
Table 1. Data Collection and Refinement Details
=====================================================================
UA1 (0 GPa)
R[F2 > 4σ(F2)]
0.046
No. of reflections collected
2665
wR(F2)
0.116
No. of independent reflections
651
Goodness-of-fit
1.26
No. of parameters refined
22
No. of reflections with F>4σ(F) 614
Rint
0.0391
UA4 (1.74 GPa)
R[F2 > 4σ(F2)]
0.0417
No. of reflections collected
330
wR(F2)
0.1043
No. of independent reflections
150
Goodness-of-fit
1.12
No. of reflections with I>2σ(I)
146
No. of parameters refined
37
Rint
0.047
UA2 (0.53 GPa)
R[F2 > 4σ(F2)]
0.064
No. of reflections collected
317
wR(F2)
0.173
No. of independent reflections
141
Goodness-of-fit
1.43
No. of parameters refined
22
No. of reflections with I>2σ(I)
141
Rint
0.064
IDD5 (5.98 GPa)
R[F2 > 4σ(F2)]
0.044
No. of reflections collected
92
wR(F2)
0.096
No. of independent reflections
92
Goodness-of-fit
1.14
No. of reflections with F2 > 4σ(F2) 89
No. of parameters refined
22
Rint
0.0904
R[F2 > 4σ(F2)]
0.0579
UA3 (1.03 GPa)
wR(F2)
0.1348
No. of reflections collected
319
Goodness-of-fit
1.031
No. of independent reflections
139
No. of parameters refined
20
No. of reflections with I>2σ(I)
139
Rint
0.058
=====================================================================
Table 2. Unit cell parameters as a function of pressure. Name identifies the experiment. Behoite (0 - 1.74
GPa) has space group P212121. The high-pressure polymorph (8.43 - 30.11 GPa) has space group Fdd2.
=====================================================================
P (GPa)
Name
a(A)
b(A)
c(A)
V(A3)
--------------------------------------------------------------------------------------------------------------------0
UA1
4.5362(8)
4.6204(7)
7.0531(7)
147.84(2)
0.34
IDD1*
4.506(2)
4.599(6)
7.049(3)
146.1(2)
0.53
UA2
4.5288(5)
4.620(1)
7.0547(6)
147.63(4)
1.03
UA3
4.5033(3)
4.5974(8)
7.0442(4)
145.83(3)
1.74
UA4
4.4748(2)
4.5700(6)
7.0355(3)
143.87(2)
3.64
BMD1
5.819(4)
6.349(4)
7.237(6)
267.4(3)
5.98
IDD5
5.738(2)
6.260(3)
7.200(4)
258.7(2)
7.68
IDD2
5.660(3)
6.160(3)
7.151(4)
249.3(2)
8.43
IDD4
5.642(4)
6.132(4)
7.139(5)
246.9(3)
10.44
1DD3
5.610(4)
6.084(3)
7.107(4)
242.6(3)
15.18
BMD2
5.559(12)
5.961(11)
7.002(18)
232.0(9)
19.31
BMD3
5.518(10)
5.878(10)
6.949(10)
225.5(6)
23.67
BMD4
5.480(8)
5.815(8)
6.863(10)
218.8(5)
70
27.49
BMD5
5.442(10)
5.763(10)
6.797(10)
213.2(6)
30.11
BMD6
5.420(10)
5.728(10)
6.766(10)
210.1(6)
============================================================================
* There is an irresolvable calibration error with the unit cell parameters.
============================================================================
Table 3. Atomic Coordinates
============================================================================
Name
Atom
x
y
z
Ueq
--------------------------------------------------------------------------------------------------------------------UA1
Be
0.0303(3)
0.7083(3)
0.6264(2)
0.0107(3)
(0 GPa)
O1
0.1922(2)
0.1052(2)
0.0426(1)
0.0118(2)
O2
0.1568(2)
0.4217(2)
0.7312(1)
0.0116(2)
H1
0.279(6)
0.069(5)
0.556(3)
0.025(6)
H2
0.326(5)
0.442(5)
0.774(3)
0.018(5)
UA2
(0.53 GPa)
Be
O1
O2
H1
H2
0.9692(14)
0.8073 (8)
0.8430 (6)
−0.730 (11)
−0.69 (2)
0.710(2)
0.108 (2)
0.4265 (15)
0.05 (3)
0.41 (4)
0.6267(7)
0.0429 (4)
0.7316 (4)
0.546 (6)
0.750 (12)
0.0117(13)
0.0168 (10)
0.0139 (10)
0.008 (18)
0.10 (3)
UA3
(1.03 GPa)
Be
O1
O2
H1
H2
0.9683 (15)
0.8067 (12)
0.8405 (8)
0.725 (14)
0.69 (3)
0.707 (3)
0.108 (3)
0.425 (2)
0.00 (3)
0.48 (5)
0.6277 (7)
0.0432 (6)
0.7324 (4)
0.536 (9)
0.758 (12)
0.0111 (14)
0.0161 (13)
0.0138 (12)
0.00 (3)
0.10 (4)
UA4
(1.74 GPa)
Be
O1
O2
H1
H2
0.971 (2)
0.8042 (19)
0.8379 (14)
0.72 (2)
0.73 (4)
0.714 (6)
0.104 (5)
0.418 (4)
0.06 (6)
0.49 (8)
0.6285 (13)
0.0423 (8)
0.7319 (7)
0.546 (11)
0.74 (2)
0.012 (3)
0.016 (2)
0.0129 (19)
0.00 (4)
0.11 (9)
IDD5
(5.98 GPa)
Be
0.00000
0.00000
0.1884(8)
0.009(2)
O
0.0171(3)
0.2090(6)
0.3160(8)
0.0109(6)
H
-0.040(11)
0.301(9)
0.276(10)
0.003(13)
============================================================================
Table 4. Anisotropic Displacement parameters for UA1
============================================================================
U11
U22
U33
U23
U13
U12
Be
0.0113(6)
0.0110(6)
0.0097(6)
-0.0002(5)
-0.0005(5)
-0.0004(5)
O1
0.0128(4)
0.0093(4)
0.0133(4)
0.0014(3)
-0.0039(3)
-0.0007(3)
O2
0.0100(4)
0.0116(3)
0.0133(4)
0.0026(3)
-0.0002(3)
-0.0002(3)
============================================================================
71
Table 5. Geometric Parameters
=====================================================================
UA1
Be
— O1 1.635(2)
— O1 1.637(2)
— O2 1.623(2)
— O2 1.645(2)
Avg.
1.635
UA2
Be
UA3
Be
UA4
Be
IDD5
Be1
— O1
— O1
— O2
— O2
Avg.
1.620(9)
1.636(7)
1.609(11)
1.651(10)
1.629
— O1
— O1
— O2
— O2
Avg.
1.616(12)
1.638(8)
1.599(15)
1.648(13)
1.625
—O1
—O2
—O2
—O1
Avg.
1.604(19)
1.65(3)
1.60(2)
1.648(15)
1.63
—O
—O
Avg.
1.601(5)
1.622(5)
1.612
======================================================================
Figure 1
FIGURES
72
Figure 2
0.34 GPa (IDD1)
7.68 GPa (IDD2)
73
Figure 3
74
Figure 4
0.53 GPa (UA2)
6 GPa (IDD5)
75
76
APPENDIX C:
NEW INSIGHTS INTO THE HIGH-PRESSURE POLYMORPHISM OF SIO2
CRISTOBALITE
Przemyslaw Dera, John Lazarz, Vitali B. Prakapenka
Center for Advanced Radiation Sources, The University of Chicago, Argonne National
Laboratory, Building 434A, 9700 South Cass Ave, Argonne, IL 60439, U.S.A.
Madison Barkley, and Robert T. Downs
Department of Geosciences, University of Arizona, 209 Gould-Simpson Building, Tucson,
Arizona 85721-0077 Tucson, Arizona 85721-0077, U.S.A.
Published in Physics and Chemistry of Minerals - Online First, 31 March 2011
Paper is given as the scanned reprint
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
APPENDIX D:
KÔZULITE, AN MN-RICH ALKALI AMPHIBOLE
Madison C. Barkley, Hexiong Yang and Robert T. Downs
Department of Geosciences, University of Arizona, Tucson, Arizona 85721-0077 Tucson,
Arizona 85721-0077, U.S.A.
Published in Acta Crystallographica E66, i83
Paper is given as the scanned reprint
92
93
94
95
96
97
98
99
100
101
APPENDIX E:
DESPUJOLSITE, Ca3Mn4+(SO4)2(OH)6•3H2O
Madison C. Barkley1,2, Hexiong Yang2, Stanley H. Evans2, Robert T. Downs2, and
Marcus J. Origlieri2
1
2
Arizona Historical Society, Phoenix, Arizona 85007, U.S.A.
Department of Geosciences, University of Arizona, 209 Gould-Simpson Building,
Tucson, Arizona 85721, U.S.A
Published in Acta Crystallographica E67, i47-i48
Paper is given as the scanned reprint
102
103
104
105
106
107
108
109
110
111
112
113
114
APPENDIX F:
BOBDOWNSITE, A NEW MEMBER OF THE WHITLOCKITE-GROUP,
FROM BIG FISH RIVER, YUKON, CANADA
Kimberly T. Tait1,2, Madison C. Barkley1, Richard M. Thompson1, Marcus Origlieri1,
Stanley H. Evans1, Charles T. Prewitt1, and Hexiong Yang1
1
Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona
85721, U.S.A.
2
Department of Natural History, Royal Ontario Museum, 100 Queen’s Park, Toronto,
Ontario, Canada M5S 2C6
Accepted for publication in The Canadian Mineralogist
115
Bobdownsite, a new member of the whitlockite-group,
from Big Fish River, Yukon, Canada
Kimberly T. Tait1*,2, Madison C. Barkley1, Richard M. Thompson1, Marcus J. Origlieri1,
Stanley H. Evans1, Charles T. Prewitt1, and Hexiong Yang1
1
Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721-0077, U.S.A.
2
Department of Natural History, Royal Ontario Museum, 100 Queen’s Park, Toronto, Ontario, Canada M5S 2C6
* Corresponding author: ktait@rom.on.ca
Abstract
A new mineral species, bobdownsite, the F-analogue of whitlockite, ideally
Ca9Mg(PO4)6(PO3F), has been found in Lower Cretaceous bedded ironstones and shales exposed
on a high ridge on the west side of Big Fish River, Yukon, Canada. The associated minerals
include siderite, lazulite, an arrojadite-group mineral, kulanite, gormanite, quartz, and collinsite.
Bobdownsite from the Yukon is tabular, colorless, and transparent, with a white streak and
vitreous luster. It is brittle, with a Mohs hardness of ~5; no cleavage, parting, or macroscopic
twinning is observed. Fracture is uneven and subconchoidal. The measured and calculated
densities are 3.14 and 3.16 g/cm3, respectively. Bobdownsite is insoluble in water, acetone, or
hydrochloric acid. Optically, it is uniaxial (-), ! = 1.625(2), " = 1.622(2). The electron
microprobe analysis yielded an empirical formula of
(Ca8.76Na0.24)#=9.00(Mg0.72Fe3+0.13Al0.11Fe2+0.04)#=1.00)(P1.00O4)6(P1.00O3F1.07).
Bobdownsite was examined with single-crystal X-ray diffraction and found to be
isostructural with whitlockite, trigonal with space group R3c and unit-cell parameters a =
10.3224(3) Å, c = 37.070(2) Å and V = 3420.7(6) Å3. The structure was refined to an R1 factor of
0.031. Its structure is characterized by the [Mg(PO4)6]16- ligand, or the so-called “Mg-pinwheel”.
The isolated pinwheels are held together by intralayer Ca cations to form layers parallel to (001),
116
which are linked together by interlayer Ca cations along [001]. The Raman spectra of
bobdownsite strongly resemble those of whitlockite and merrillite. Bobdownsite represents the
first naturally-formed phosphate known to contain a P-F bond.
It has subsequently been found in the Tip Top mine, Custer County, South Dakota, USA.
Based on our study, we conclude that the “fluor whitlockite” found in the Martian meteorite SaU
094B (Gnos et al. 2002) is also bobdownsite.
Keywords: bobdownsite, whitlockite, phosphate, crystal-structure, X-ray diffraction, Raman
spectra
Introduction
A new member of the whitlockite group, bobdownsite, ideally Ca9Mg(PO4)6(PO3F), has
been found in Big Fish River, Yukon, Canada. The new mineral is named after Dr. Robert T.
Downs, a professor of mineralogy in the Department of Geosciences at the University of
Arizona, for his outstanding contributions to the field of mineralogy, particularly his work on the
American Mineralogical Crystal Structure Database (AMCSD: http://rruff.geo.arizona.edu/
AMS/amcsd.php) and his dedication on the development of the RRUFF project, an Internetbased, internally consistent, and integrated database of Raman spectra, X-ray diffraction, and
chemical data for minerals (http://rruff.info). Bob Downs lived and worked in the Yukon
Territory in the 1970’s, where this mineral was found. The new mineral and its name have been
approved by the Commission on New Minerals and Nomenclature and Classification (CNMNC)
of the International Mineralogical Association (IMA2008-037). Part of the co-type sample has
been deposited at the University of Arizona Mineral Museum (Catalogue # 18820) and the
RRUFF project (deposition # R050109).
117
Whitlockite, Ca9Mg(PO4)6(PO3OH), crystallizing in trigonal symmetry (space group R3c),
is one of the primary phosphate minerals, along with apatite, that occur in lunar rocks, Martian
meteorites, and in many other groups of meteorites. Under terrestrial conditions, whitlockite is
mainly found as an uncommon secondary mineral in complex zoned granite pegmatites and in
phosphate rock deposits. It is also known as a biogenic mineral (pathogenic mineralization in
organisms) and a cave mineral of the secondary biogenic origin (e.g. Anthony et al., 1990;
Lagier and Baud 2003; Jolliff et al. 2006; Hughes et al. 2008; Orlova et al. 2009). Recently,
Ionov et al. (2006) reported the discovery of whitlockite in mantle xenoliths from Siberia,
suggesting that this mineral may be an important host for some lithophile trace elements in the
Earth’s upper mantle. Furthermore, whitlockite-type phosphates are holding great promise as
potential storages for radioactive wastes because of their high resistance to environmental
corrosions (e.g., Fong et al 2007; Orlova et al. 2009). They have also served as the base for the
crystallo-chemical design of compounds with nonlinear-optical, catalytic, ferroelectric, ionconductive properties, and luminescent properties (Belik et al. 2001; Guo et al. 2010; Du et al.
2011).
Phosphate minerals belonging to the whitlockite group include whitlockite
Ca9Mg(PO4)6(PO3OH), strontiowhitlockite Sr9Mg(PO4)6(PO3OH), merrillite Ca9NaMg(PO4)7,
ferromerrillite (IMA2006-039) Ca9NaFe(PO4)7, as well as bobdownsite Ca9Mg(PO4)6(PO3F).
These minerals are characterized by the [M(PO4)6]16- ligands, or the so-called “M-pinwheels” (M
= Mg, Fe), which are held together by intralayer Ca2+ cations to form layers parallel to (001).
These layers are further linked together by interlayer Ca2+/Na+/Sr2+ cations along [001] (Gopal &
Calvo 1972; Calvo & Gopal 1975; Schroeder et al. 1977; Hughes et al. 2006, 2008; Nestola et al.
2009). This paper describes the physical and chemical properties of bobdownsite and its
118
structural relationships with other related phosphates based on single-crystal X-ray diffraction
and Raman spectroscopic data. To our knowledge, bobdownsite represents the first naturallyformed phosphate known to contain a P-F bond, although several synthetic phosphate
compounds have been reported to contain PO3F groups, such as NaK3(PO3F) (Durand et al.,
1975) and LiNH4(PO3F) (Durand et al., 1978).
Description
Occurrence, physical and chemical properties
Bobdownsite was first found on a sample, misidentified as whitlockite, from Big Fish
River, Yukon, Canada (at about Latitude 68º 28$N and Longitude 136º 30$W). The sample was
recovered from the Lower Cretaceous bedded ironstones and shales exposed on a high ridge on
the west side of the river, upstream from phosphatic nodule slopes in the summer of 2001. At the
exposure, bobdownsite occurs in an east-west trending faulted vein. The vein is largely choked
with fault gouge composed of anhedral bobdownsite, but euhedral crystals occur as
well. Dilation in the vein ranges from 0 to 4 cm maximum. South facing undulations in the vein
create the largest openings where the best specimens are exposed. Bobdownsite occurs with
siderite, lazulite, an arrojadite-group mineral, kulanite, gormanite, quartz, and collinsite. Crystals
range from microscopic to 2.7 cm wide and 0.4 cm thick (Fig. 1). The mineral has subsequently
been found in samples from the Tip Top mine, Custer County, South Dakota, USA, and on a
Martian meteorite. Big Fish River is the type locality.
Bobdownsite from the Yukon is tabular, colorless in transmitted light, transparent with
white streak and vitreous luster. The Tip Top mine material is found as druses of pale purple,
yellow or transparent colorless rhombohedral crystals. It is brittle, with a Mohs hardness of ~5;
no cleavage, parting or macroscopic twinning is observed. Fracture is uneven and subconchoidal.
119
The measured and calculated densities are 3.14 and 3.16 g/cm3 respectively. Bobdownsite is
insoluble in water, acetone, or hydrochloric acid. The new mineral is uniaxial (-), ! = 1.625(2), "
= 1.622(2), and it does not fluoresce under long- or short-wave ultraviolet light. The
compatibility index (1-KP/KC) is 0.005 (Superior).
Chemical analyses were conducted with a Cameca SX-50 electron microprobe at 15 kV
and 10 nA. For comparison, we examined three samples: bobdownsite from Big Fish River,
bobdownsite from the Tip Top Mine, and the type whitlockite from the Palermo #1 quarry,
Grafton County, New Hampshire (Harvard University ISGN:HRV000XO6 95030), which are
designated as R050109, R070654, and R080052, respectively, hereafter for discussion. Standards
used were anorthite (Al), MgF2 (F), albite (Na), diopside (Mg, Ca), apatite (P), and fayalite (Fe).
Based on 28 O atoms, an empirical formula of
(Ca8.76Na0.24)#=9(Mg0.72Fe3+0.13Al0.11Fe2+0.04)#=1(P1.00O4)6(P1.00O3F1.07) is obtained for R050109
bobdownsite, and (Ca8.99Na0.05)#=9.04(Mg0.96Al0.04)#=1.00(P1.00O4)6(P1.00O3(F0.85(OH)0.15)#=1) for
R070654 bobdownsite, leading to a simplified formula of Ca9Mg(PO4)6(PO3F). The chemical
formula for the type whitlockite was determined to be
(Ca8.82Na0.18Sr0.01)#=9.00(Mg0.62Fe2+0.18Fe3+0.17Mn0.02Al0.02)#=1.01(P1.00O4)6(P1.00O3OH) (Table 1).
The OH content of bobdownsite was examined by the thermal gravimetric analysis (TGA).
The data of R050109 bobdownsite, along with that of R080052 whitlockite for comparison, are
presented in Figure 2. All data were collected on a Mettler Toledo TGA/SDTA 851
Thermogravimetric Analyzer, using a heating rate of 15 ºC/min. Some weight loss for
whitlockite below 300 ºC may be related to absorbed or non-essential water. The weight loss at
~700 ºC for whitlockite is interpreted as dehydration. In contrast, bobdownsite does not display
any significant weight loss up to 1100 ºC, consistent with the lack of OH in this mineral. Note
120
that Gopal et al. (1974) measured the thermal dehydration curves of both synthetic and natural
whitlockite and observed two major dehydration events, one at ~720 ºC and the other at ~1000
ºC. However, they did not offer an interpretation for the dehydration event at ~1000 ºC. We did
not observe the dehydration event for the type whitlockite at 1100 ºC.
The Raman spectra of R050109 and R070654 bobdownsite, as well as R080052
whitlockite, were collected on randomly oriented crystals from 9 scans at 30 s and 100% power
per scan on a Thermo Almega microRaman system, using a solid-state laser (100% power) with
a wavelength of 532 nm and a thermoelectrically cooled CCD detector. The laser is partially
polarized with 4 cm-1 resolution and a spot size of 1 µm.
X-ray crystallography
Powder X-ray diffraction data of the type bobdownsite were collected on a Bruker D8
Advance diffractometer with CuK! radiation (Table 2). Based on trigonal symmetry, the unitcell parameters determined from these data using the CrystalSleuth software (Laetsch and
Downs, 2006) are a = 10.3440(2) Å, c = 37.0998(8) Å, and V = 3437.8(1) Å3.
Single-crystal X-ray diffraction data of R050109, along with R070654 bobdownsite and
R080052 whitlockite, were collected on a Bruker X8 APEX2 CCD X-ray diffractometer
equipped with graphite-monochromatized MoK% radiation. All data were collected from nearly
equi-dimensional crystals cut from each sample with a frame width of 0.5° in ! and 30 s
counting time per frame. All reflections were indexed on the basis of a trigonal unit-cell (Table
3). The intensity data were corrected for X-ray absorption using the Bruker program SADABS.
The systematic absences of reflections indicate possible space group R3c (#161) or R-3c (#167).
All crystal structures were refined using SHELX97 (Sheldrick 2008) in space group R3c. The
121
positions of all atoms were refined with anisotropic displacement parameters. Final coordinates
and displacement parameters of atoms for the type bobdownsite are listed in Table 4, and
selected bond-distances are listed in Table 5. Final coordinates and displacement parameters of
atoms for R070654 bobdownsite and R080052 whitlockite are recorded in Table 6 and Table 7,
respectively. R070654 bobdownsite from the Tip Tip mine contains [F0.85(OH)0.15], which,
together with the fully hydrated whitlockite sample, provides complementary structural
information as a function of F/OH content.
Discussion
Crystal structure
Bobdownsite is isotypic with whitlockite, whose structure and relationships with other
phosphate compounds have been studied extensively (Gopal & Calvo 1972; Gopal et al. 1974;
Calvo & Gopal 1975; Schroeder et al. 1977; Hughes et al. 2008). The principal structural
component of bobdownsite is the [Mg(PO4)6]16- ligand, referred to as the “Mg-pinwheel” after
Hughes et al. (2006). The Mg-pinwheels are flat in the c direction and can be thought of as
defining oblate ellipsoids whose short axes are parallel to c and that form a regular packing
arrangement. The isolated pinwheels, held together by intralayer Ca1 cations, form layers
parallel to (001) (Fig. 3). These layers are linked together by interlayer Ca2 and Ca3 cations.
Figure 4 shows that the anionic units in each pinwheel layer form a perfectly eutactic, i.e.
closest-packed, planar arrangement. The term “eutaxy” (O’Keeffe & Hyde 1996) is preferred to
“closest-packing” because neither the atoms nor the ligands can be considered to be in contact
with each other. The arrangement of anionic units is distorted eutactic in three dimensions. The
122
term “cubic eutaxy” will be used because the ligands form a distorted cubic-closest packed
arrangement (O’Keeffe & Hyde 1996).
Eutactic arrangements create tetrahedral and octahedral interstitial sites. The octahedral
interstitial sites in bobdownsite contain the second structural component, namely isolated PO3F
tetrahedral groups (Fig. 4). The three oxygen atoms that form the base of a PO3F tetrahedron are
located approximately in the midplane between the two pinwheel layers and are each coordinated
to the closest two interlayer Ca cations (Fig. 4), in addition to P5+. The P5+ position in the PO3F
group is split — it is randomly located either just above or just below the midplane between the
two pinwheel layers, leading the apical F- anion of the PO3F tetrahedron to point either along +c
or –c.
The whitlockite-type compounds contain a metal atom (M = Mg, Fe, Mn) that is slightly
offset from the origin along c. This atom is octahedrally coordinated and resides at the center of
an M-pinwheel. For any compound isostructural with whitlockite, an atom M at the origin will
result in a perfect cubic eutactic arrangement of M atoms when c = 2"6a, and the M-pinwheel
layers are constrained to be in perfect planar eutaxy for any value of the axial ratio. Hence,
distortion from perfect cubic eutaxy depends only on the distortion from the ideal c / a ratio of
2"6.
Table 8 compares selected crystal chemical data and a quantitative measure of packing
efficiency for bobdownsite and some isostructural compounds, together with packing efficiency
for a hypothetical ideal eutactic whitlockite-type structure and several model whitlockite-type
structures with a = 10 Å and c / a varying from 3.6 to 4.75. The measure of packing efficiency,
Ucp, described by Thompson and Downs (2001), is calculated by fitting an arrangement of 675
ideal cubic closest-packed spheres to their observed equivalents such that the mean square
123
distance between observed and ideal atoms is minimized. Minimization is accomplished by
allowing the ideal arrangement to scale, rotate, and translate. Ucp is the minimal mean square
displacement, which is zero for a perfectly eutactic arrangement of spheres, or in this case
pinwheel ligands. The Ucp value increases as an arrangement becomes more distorted from
perfect eutaxy.
Values of Ucp for observed whitlockite-type compounds range from 23.7 to 28.3 Å2. These
values represent extreme distortion from perfect eutaxy and result from the fact that the items
being packed together are far from spherical, more closely resembling disks as their height to
diameter ratio equals 0.51, which drastically reduces the c / a axial ratio from its ideal eutactic
value of 2"6. The distortion from eutaxy represented by non-zero Ucp values can result from
distortion of the cell parameters from ideal, the distortion of atomic positions from ideal, or a
combination of both. The whitlockite structure is an example of the unusual case where the
atomic positions are ideal so that all distortion results from non-ideal cell parameter ratios.
Figure 5 is a plot of Ucp vs. c / a for the observed whitlockite-type structures in Table 8
(solid circles) and some model structures (hollow circles). The model structures all have a = 10
Å and are included to add additional insight into the whitlockite structure type. A quadratic
trendline fits the model data points exactly. The values of Ucp for the observed structures do not
fall exactly on the trendline because Ucp is a measure of Euclidean distance and is therefore
affected to a small degree by differences in unit cell volume in all cases except perfect eutaxy,
which always has Ucp = 0 for any cell size. These results suggest that there is little difference in
principle between the processes that take place when oxygen anions with charge –2 are forced
into close proximity and those that occur when much larger ligands with charge –16 get close to
each other.
124
Figure 6 shows the structure of another PO3F-bearing, synthetic compound NaK3(PO3F)2
(Durand et al. 1975). The fundamental structural unit of this material is the Na-centered pinwheel
polymerized by sharing tetrahedra to form a planar polymer. In this case, the PO3F tetrahedra are
not interstitial, but are part of the pinwheels. Like Mg in bobdownsite, the Na atoms form
perfectly eutactic planar arrangements. In three dimensions, however, eutaxy is not maintained
because the Na-pinwheel planes are stacked directly over each other, AAA… These planes are
held together by K+. Also like bobdownsite, the apical anion is F- pointing along the stacking
vector of the planes and is bonded only to P5+ in the PO3F tetrahedron and the interplanar K+
cations.
The synthetic compound LiNH4PO3F (Durand et al. 1978) is also a layer structure (Fig. 7),
with F- at the apices of the PO3F tetrahedra pointing into the interlayer space and NH4+ playing
the role of the interlayer cation. However, the layers themselves are very different, and while
they stack in ABAB… fashion along c, there is no obvious eutaxy. These layers can be described
as chains of corner-sharing LiO4 tetrahedra cross-connected by PO3F tetrahedra (Durand et al.
1978).
All of the structures examined above share the common feature that PO3F groups point
into interlayer spaces with F- at the apices either dangling or weakly bonded to the interlayer
cations that bind the layers together. In bobdownsite, F- is 3.7 Å!away!from the nearest cation
(Ca1) outside its tetrahedron, but only 2.7 Å from the nearest oxygen atom (O1) outside the
PO3F group, suggesting that it is bonded only to P5+. In NaK3(PO3F)2, F- is part of the K2
coordination shell at a distance of 2.6 Å from K2, along with six oxygen atoms at 2.9Å. In
LiNH4PO3F, F- is 2.1 Å from one of the hydrogen atoms belonging to NH4+. In these phases, Fdoes not form strong bonds with more than one cation.!
125
Series Trends
It would be useful if bobdownsite could be distinguished from whitlockite on the basis of
unit-cell parameters. Figure 8 shows a plot of c / a for the three members of the solid solution
analyzed in this paper: R050109 bobdownsite with F% [= F/(F+OH)] = 100, R070654
intermediate F% = 84, and R080052 whitlockite F% = 0. This plot appears to show a correlation
between the c / a ratio and fluorine content. Table 9 provides the numerical data for these
analyses and for prior refinements of end-member whitlockite. All of the previous data have F%
= 0, but their c / a values cover a greater range than our three samples. Analysis is complicated
by the variable composition of the M-site and the F-site. Besides OH-, O2- may substitute at the
F-site, as in merrillite. This necessitates the addition of a new partially occupied cation site for
charge balance. Additional study might verify a relationship between fluorine content and cell
parameters given chemical constraints on other sites, but these constraints themselves will limit
the value of any such discovery.
Relationship to Merrillite
Merrillite, like whitlockite, is another main phosphate mineral found in lunar rocks (Albee
et al. 1970; Griffin et al. 1972; Lindstrom et al. 1985; Jolliff et al. 1993, 2006). It is also found in
Martian and various other types of meteorites (Delaney et al. 1984; Dais & Olsen 1991; Prewitt
& Rothbard 1975). In the laboratory, merrillite can be readily obtained by dehydrating
whitlockite (Gopal et al. 1974; Hughes et al. 2008). Merrillite crystals with considerable amounts
of F and traces of Cl are found in several Martian meteorites, including LEW 88516 (Gleason et
al. 1997; Treiman et al. 1994; Lodders 1998), EETA79001A (Zipfel et al. 2000; Folco et al.
2000; Wadhwa et al. 2001; Mikouchi et al. 2001), SaU 005 and 008 (Zipfel 2000), and SaU 094
126
(average F = 1.1 wt%) (Gnos et al. 2002). Gnos et al. (2002) suggested that the F content in the
Martian meteorites are the result of a solid-solution with a structurally analogous "fluor
whitlockite." Based on our study on bobdownsite, we conclude that a whitlockite with F content
greater than ~ 0.9 wt%, including the F-bearing "merrillite" in SaU 094B (Gnos et al. 2002), is
bobdownsite. Fully fluorinated bobdownsite contains ~1.8 wt% F.
Raman spectra
The Raman spectra of bobdownsite and whitlockite are plotted in Figure 9. Obviously, they
are remarkably similar to each other. Moreover, the Raman spectrum of our R070654
bobdownsite from the Tip Top mine strongly resembles that of whitlockite examined by Jolliff et
al. (2006), which was also from the Tip Top mine. Unfortunately, no detailed chemical
composition, especially in terms of F content, was given by Jolliff et al (2006). Based on
previous Raman spectroscopic studies on whitlockite and merrillite (Cooney et al. 1999; Jolliff et
al. 1996, 2006; Xie et al. 2002; Ionov et al. 2006), we made a tentative assignment of major
Raman bands for bobdownsite (Table 10). The strongest band in the 950-975 cm-1 region is
attributed to the symmetric stretching (#1) mode of the PO4 groups. Interestingly, this band is a
well-resolved doublet in our R070654 bobdownsite and the whitlockite studied by Jolliff et al.
(2006), but a very poorly resolved doublet in R050109 bobdownsite and R080052 whitlockite.
This observation, according to Jolliff et al. (2006), may be related to the complex cation
substitutions in the Ca and Mg sites, resulting in a more uniform bonding environments for all
PO4 groups. Indeed, the chemical compositions of R050109 bobdownsite,
(Ca8.76Na0.24)#=9(Mg0.72Fe3+0.13Al0.11Fe2+0.04)#=1(PO4)6(PO3F1.07), and R080052 whitlockite,
(Ca8.82Na0.18Sr0.01)#=9(Mg0.62Fe2+0.18Fe3+0.17Mn0.02Al0.02)#=1,01 (PO4)6(PO3OH), are much more
127
complicated than the chemistry of R070654 bobdownsite,
(Ca8.99Na0.05)#=9.04(Mg0.96Al0.04)#=1(PO4)6(PO3(F0.85(OH)0.15)#=1) (Table 1).
There is a relatively strong and sharp peak at ~923 cm-1 in both bobdownsite and
whitlockite, but it is not observed in anhydrous merrillite (Cooney et al. 1999; Xie et al. 2002;
Jolliff et al. 2006; Ionov et al. 2006). This band is, therefore, assigned to the symmetric
stretching (#1) mode of the PO3F group in bobdownsite or the PO3OH group in whitlockite. It
should be pointed out that the Raman-active O-H stretch of the weakly acidic OH in the PO3OH
group in whitlockite may give rise to a weak band in the 2100-2700 cm-1 region (Jolliff et al.
2006). Nonetheless, this band is not observed in our R080052 whitlockite or the whitlockite
specimen examined by Jolliff et al. (2006), who ascribed this to the strong background
fluorescence in this region.
ACKNOWLEDGEMENTS
This study was funded by the Science Foundation Arizona. We are indebted to Rock S.
Currier for providing the two bobdownsite samples and Rod Tyson for information on the
occurrence of R050109 bobdownsite. Tom Loomis of Dakota Matrix Minerals provided
additional Tip Top mine material. We are grateful to Dr Carl Francis and the Mineralogical
Museum of Harvard University for the type whitlockite sample.
128
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132
List of Tables
Table 1. Chemical compositions of bobdownsite and whitlockite
Table 2. Powder X-ray diffraction data for R050109 bobdownsite. Powder lines with relative
intensities above 10 are marked in bold.
Table 3. Summary of crystallographic data and refinement results for bobdownsite and
whitlockite.
Table 4. Coordinates and displacement parameters of atoms in R050109 bobdownsite.
Table 5. Selected bond distances (Å) in R050109 bobdownsite.
Table 6. Coordinates and displacement parameters of atoms in R070654 bobdownsite.
Table 7. Coordinates and displacement parameters of atoms in R080052 whitlockite.
Table 8. A comparison of some crystallographic parameters for bobdownsite with isostructural
compounds and some model structures.
Table 9. The relationship between composition and cell parameters for the bobdownsitewhitlockite solid solution.
Table 10. Tentative assignment of major Raman bands of bobdownsite.
List of Figure Captions
Figure 1. Tabular, colourless, euhedral crystals of bobdownsite from Big Fish River, Yukon,
Canada.
Figure 2. Comparison of thermal gravimetric analyses (TGA) of R050109 bobdownsite and
R080052 whitlockite.
Figure 3. A portion of a layer of [Mg(PO4)6]16- “pinwheels” (Hughes et al. 2006), viewed along
c. The anionic units form a perfectly closest-packed planar arrangement. The spheres
represent intralayer Ca1 cations.
Figure 4. A portion of two layers from a cubic closest-packed arrangement of Mg octahedra
viewer down c, showing an isolated PO3F tetrahedron in the large octahedral void formed
by six Mg octahedra. The “arms” of the Mg-pinwheels, the six PO4 groups for every Mg
octahedron, have been left off for clarity (see Figure 3), as have the intralayer calcium
cations. The gray octahedra are in the lower layer (zMg = 0.3331) and the white in the
upper (zMg = 0.4997). The spheres represent interlayer Ca2+ cations, with gray ones (Ca2)
at z = 0.3985 and white (Ca3) at z = 0.4343. The base of the tetrahedron is in the
midplane between the two octahedral layers. In this illustration, P5+ has adopted the upper
possibility of its split position, just above the midplane (instead of just below), and so the
tetrahedron points along +c, instead of –c.
Figure 5. A plot of packing efficiency, Ucp, vs. axial ratio (c / a) for some observed whitlockitetype structures (solid circles) and several model whitlockite-type structures with a fixed c
= 10 Å and variable c / a (hollow circles). A value of zero for Ucp represents perfect
eutaxy, while values for the observed structures represent extreme distortion.
Figure 6. Structure of NaK3(PO3F)2 (Durand et al. 1975), viewed down c.
Figure 7. Structure of LiNH4PO3F (Durand et al. 1978), viewed down b.
Figure 8. The relationship between fluorine content and cell parameters for the bobdownsitewhitlockite solid solution.
Figure 9. Raman spectra of (a) R050109 bobdownsite, (b) R070654 bobdownsite, and (c)
R080052 whitlockite.
133
Table 1. Electron microprobe analysis results for R050109 bobdownsite from Big Fish River, R070654
bobdownsite from Tip Top mine, and R080052 whitlockite from Palermo #1 Quarry, New Hampshire.
===============================================================
Oxide
R050109
R070654
R080052
(25 points average) (11 points average) (12 points average)
===============================================================
46.4(3)
46.4(2)
46.9(3)
P2O5
CaO
45.9(2)
47.0(6)
46.6(2)
MgO
2.70(5)
3.6(2)
2.37(5)
Na2O
0.70(5)
0.13(4)
0.53(5)
Al2O3
0.51(13)
0.2(2)
0.12(2)
FeO
0.28(2)*
n.d.
2.38(8)
Fe2O3
0.93(8)*
n.d.
MnO
n.d.
n.d.
0.13(3)
SrO
n.d.
n.d.
0.14(6)
F
1.9(3)
1.5(2)
n.d.
Subtotal
99.3
98.8
99.2
F2=-O
0.80
0.6
Total
98.5(7)
98.2(7)
99.2(3)
Cation numbers normalized to:
28 O
28 O
27.5 O
P
6.99
7.02
7.02
Ca
8.76
8.99
8.83
Na
0.24
0.05
0.18
Mg
0.72
0.96
0.62
Al
0.11
0.04
0.02
Fe2+
0.04
0.18
0.13
0.17
Fe3+
Mn
0.02
Sr
0.01
F
1.07
0.85
===============================================================
Note: n.d. – not detected
$ R050109 bobdownsite: (Ca8.76Na0.24)#=9.00(Mg0.72Fe3+0.13Al0.11Fe2+0.04)#=1.00)(P1.00O4)6(P1.00O3F1.07);
FeO and Fe2O3 readjusted after charge balance as FeO*=(0.25xFe2O3 tot)/1.11145;
Fe2O3*=0.75xFe2O3 tot, where Fe2O3 tot = 1.24(11).
$ R070654 bobdownsite: (Ca8.99Na0.05)#=9.04(Mg0.96Al0.04)#=1.00(P1.00O4)6(P1.00O3(F0.85(OH)0.15)#=1);
OH estimated by difference and charge balance.
$ R080052 whitlockite:
(Ca8.82Na0.18Sr0.01)#=9.00(Mg0.62Fe2+0.18Fe3+0.17Mn0.02Al0.02)#=1.01(P1.00O4)6(P1.00O3OH);
Fe2+ and Fe3+ split to balance charge.
134
Table 2. Powder X-ray diffraction data for R050109 bobdownsite. Powder lines with
relative intensities above 10 are in bold.
===================================================
dcalc
h
k l
Int
dobs
-------------------------------------------------------------------------------------6
8.0740
8.0670
0
1 2
8
6.4544
6.4436
1
0 4
16
5.1759
5.1722
1
1 0
8
4.0354
4.0335
0
2 4
18
3.4279
3.4277
1
0 10
5
3.3716
3.3720
2
1 1
4
3.3315
3.3310
1
2 2
3
3.2231
3.2236
1
1 9
100
3.1815
3.1807
2
1 4
19
2.9857
2.9862
3
0 0
58
2.8578
2.8572
0
2 10
16
2.7344
2.7347
1
2 8
5
2.6890
2.6890
3
0 6
2
2.6543
2.6537
1
1 12
13
2.5864
2.5861
2
2 0
6
2.5008
2.5010
2
1 10
2
2.4779
2.4791
1
3 1
9
2.2446
2.2448
1
0 16
5
2.2311
2.2313
1
1 15
4
2.1769
2.1771
4
0 4
7
2.1477
2.1479
3
0 12
2
2.0866
2.0869
1
2 14
7
2.0169
2.0168
0
4 8
8
1.9170
1.9174
4
0 10
6
1.8790
1.8790
2
3 8
6
1.8642
1.8640
1
4 6
1
1.8166
1.8165
0
1 20
1
1.7979
1.7978
3
2 10
2
1.7592
1.7592
0
5 4
32
1.7139
1.7139
2
0 20
2
1.6239
1.6240
2
3 14
1
1.4547
1.4545
4
3 4
1
1.4287
1.4286
0
4 20
1
1.3108
1.3107
1
0 28
1
1.2930
1.2931
4
4 0
1
1.2505
1.2505
4
2 20
4
1.1721
1.1721
2
3 26
===================================================
Table 3. Summary of crystallographic data and refinement results for bobdownsite and whitlockite.
=======================================================================================
R050109 bobdownsite
R070654 bobdownsite
R080052 whitlockite
----------------------------------------------------------------------------------------------------------------------------------------------------Ca9Mg(PO4)6(PO3F)
Ca9Mg(PO4)6(PO3OH)
Ideal formula
Ca9Mg(PO4)6(PO3F)
Space group
R3c
R3c
R3c
a (Å)
10.3224(3)
10.3394(3)
10.3590(3)
c (Å)
37.070(2)
37.084(2)
37.086(2)
V (Å3)
3420.7(6)
3433.2(2)
3446.5(3)
Z
6
6
6
3
%calc (g/cm )
3.16(2)
3.055
3.135
& range for data collection (°)
2.4 - 34.41
1.7 - 35.5
1.7 - 32.4
No. of reflections collected
7868
11117
9233
No. of independent reflections
2282
3031
2518
No. of reflections with F>4'(F)
2055
2774
2326
Rint
0.0363
0.0301
0.0304
2
2
R1[F > 4'(F )]
0.0309
0.0274
0.0281
wR(F2)
0.0730
0.0711
0.0682
Goodness-of-fit
0.989
1.046
1.064
No. of parameters refined
149
146
150
=========================================================================================
135
x
y
z
U11
U22
U33
U23
U13
U12
Ueq
0.28036(7)
0.38694(8)
0
0
0
0.31741(11)
0.35016(11)
0.2746(3)
0.2480(3)
0.2734(2)
0.4895(3)
0.4034(3)
0.3984(3)
0.4155(3)
0.1779(3)
-0.0191(3)
0
0
Ca2
Ca3
M
P1A
P1B
P2
P3
O1
O2
O3
O4
O5
O6
O7
O8
O9
F1A
F1B
0
0
0.1299(3)
0.0789(3)
0.3033(3)
0.0473(3)
0.1966(3)
0.2418(2)
0.0020(2)
0.2337(3)
0.0922(3)
0.15740(9)
0.14328(10)
0
0
0
0.17961(7)
0.14639(7)
0.15618(8)
0.6893(7)
0.7971(2)
0.74208(7)
0.96373(7)
0.94638(6)
0.95383(6)
0.00672(6)
0.86809(7)
0.88653(6)
0.87774(6)
0.82484(6)
0.96736(2)
0.86398(2)
0.7323(3)
0.75327(6)
-0.00027(5)
0.76765(2)
0.56518(2)
0.67232(2)
0.035(2)
0.0178(12)
0.0085(11)
0.0164(11)
0.0131(10)
0.0239(13)
0.0078(11)
0.0110(10)
0.0174(11)
0.0234(13)
0.0086(4)
0.0092(4)
0.004(3)
0.0086(5)
0.0094(4)
0.0116(3)
0.0105(3)
0.0351(5)
0.035(2)
0.0134(10)
0.0109(10)
0.0124(11)
0.0158(11)
0.0165(11)
0.0084(10)
0.0082(10)
0.0195(12)
0.0203(12)
0.0086(4)
0.0089(3)
0.004(3)
0.0086(5)
0.0094(4)
0.0093(3)
0.0097(3)
0.0176(4)
0.045(4)
0.0274(14)
0.0142(10)
0.0122(10)
0.0118(9)
0.0076(10)
0.0179(12)
0.0100(9)
0.0137(10)
0.0091(10)
0.0072(3)
0.0080(3)
0.002(5)
0.0148(16)
0.0092(6)
0.0096(3)
0.0094(3)
0.0151(3)
0
0.0042(9)
0.0002(8)
0.0050(8)
-0.0027(8)
-0.0019(8)
0.0014(8)
0.0016(7)
0.0004(9)
-0.0001(9)
0.0004(3)
0.0005(3)
0
0
0
-0.0006(3)
-0.0014(2)
-0.0081(3)
0
0.0084(10)
-0.0002(8)
0.0023(9)
-0.0019(8)
-0.0032(9)
0.0021(9)
0.0021(8)
0.0019(9)
0.0006(9)
-0.0001(3)
-0.0000(3)
0
0
0
-0.0021(2)
-0.0006(2)
-0.0111(3)
0.018(1)
0.0100(10)
0.0038(9)
0.0049(9)
0.0097(9)
0.0124(10)
0.0023(9)
0.0026(8)
0.0145(10)
0.0126(11)
0.0044(3)
0.0053(3)
0.002(1)
0.0043(3)
0.0047(2)
0.0051(2)
0.0057(2)
0.0197(4)
0.016(6)
0.038(2)
0.0186(5)
0.0117(5)
0.0147(5)
0.0125(4)
0.0150(5)
0.0122(5)
0.0107(4)
0.0146(5)
0.0169(5)
0.0081(2)
0.0084(2)
0.003(3)
0.0107(6)
0.0093(3)
0.0102(1)
0.0096(1)
0.0198(2)
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Note: (1) Ca1 = Ca0.92Na0.08; Ca2 = Ca; Ca3 = Ca.; M = (Mg0.72Fe3+0.13Al0.11Fe2+0.04)
P1A = 0.85 P + 0.15 vacancy; P1B = 0.15 P + 0.85 vacancy;
F1A = 0.85 F + 0.15 vacancy; F1B = 0.15 F + 0.85 vacancy
(2) The F1B atom was only refined with isotropic displacement parameter.
==================================================================================================================
0.29205(9)
Ca1
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
ATOM
==================================================================================================================
Table 4. Coordinates and displacement parameters of atoms in R050109 bobdownsite.
136
137
Table 5. Selected bond distances (Å) in R050109 bobdownsite
===============================================================
M-O4 x 3 2.049(3)
P1a-O9 x 3 1.508(2)
M-O8 x 3 2.079(3)
P1a-F
1.625(7)
<M-O>
2.064
<P-O/F>
1.537
Ca1-O1
Ca1-O2
Ca1-O3
Ca1-O5
Ca1-O5
Ca1-O6
Ca1-O7
Ca1-F1b
<Ca1-O/F>
2.535(2)
2.404(2)
2.385(2)
2.456(2)
2.535(3)
2.377(2)
2.654(3)
2.687(6)
2.504
P1b-O9 x 3 1.495(4)
P1b-F1b
1.59(3)
<P1b-O/F> 1.519
P2-O1
P2-O2
P2-O3
P2-O4
<P2-O>
1.532(3)
1.521(3)
1.540(2)
1.551(3)
1.536
Ca2-O2
Ca2-O3
Ca2-O4
Ca2-O4
Ca2-O5
Ca2-O6
Ca2-O7
Ca2-O9
<Ca2-O>
2.744(3)
2.495(2)
2.460(2)
2.478(3)
2.430(3)
2.460(2)
2.314(2)
2.458(3)
2.480
P3-O5
P3-O6
P3-O7
P3-O8
<P3-O>
1.540(3)
1.534(2)
1.520(3)
1.548(3)
1.530
Ca3-O1
2.367(3)
Ca3-O2
2.457(2)
Ca3-O3
2.365(2)
Ca3-O6
2.628(3)
Ca3-O7
2.628(3)
Ca3-O8
2.440(3)
Ca3-O8
2.463(2)
<Ca3-O>
2.464
===============================================================
Ca1
0.29139(8) 0.15560(7)
0.672376(16) 0.0370(4)
0.0196(3)
0.0172(2)
-0.0084(2)
-0.0118(2)
0.0212(3)
0.02147(13)
Ca2
0.28027(6) 0.14660(5)
0.565209(12) 0.0110(2)
0.0103(2)
0.01016(19) -0.00140(16) -0.00061(16) 0.00593(18) 0.01020(10)
Ca3
0.38790(6) 0.18026(5)
0.767658(12) 0.0121(3)
0.0091(2)
0.0107(2)
-0.00070(18) -0.00259(16) 0.00510(19) 0.01073(14)
M
0
0
-0.00029(5)
0.0125(4)
0.0125(4)
0.0124(5)
0
0
0.0063(2)
0.0125(3)
P1A 0
0
0.75328(4)
0.0084(4)
0.0084(4)
0.0162(11)
0
0
0.0042(2)
0.0110(5)
P1B 0
0
0.7314(2)
0.011(2)
0.011(2)
0.007(4)
0
0
0.0054(12)
0.009(2)
P2
0.31797(8) 0.14350(8)
0.864081(15) 0.0091(3)
0.0094(3)
0.0092(2)
0.0007(2)
0.0004(2)
0.0052(2)
0.00896(12)
P3
0.35050(8) 0.15790(7)
0.967265(14) 0.0087(3)
0.0098(3)
0.0083(2)
0.0007(2)
0.0002(2)
0.0044(2)
0.00905(12)
O1
0.2750(2)
0.0930(2)
0.82489(5)
0.0238(10) 0.0209(10) 0.0109(7)
-0.0002(7)
-0.0004(6)
0.0113(8)
0.0185(4)
O2
0.2478(2)
0.2336(2)
0.87747(5)
0.0175(9)
0.0191(9)
0.0152(7)
0.0007(6)
0.0025(6)
0.0140(8)
0.0151(3)
O3
0.2732(2)
0.0018(2)
0.88617(5)
0.0112(7)
0.0098(7)
0.0120(6)
0.0022(5)
0.0019(6)
0.0033(6)
0.0118(3)
O4
0.4892(2)
0.24185(19) 0.86830(5)
0.0082(8)
0.0088(8)
0.0181(8)
0.0018(6)
0.0026(6)
0.0031(7)
0.0122(3)
O5
0.4036(2)
0.1970(2)
0.00671(5)
0.0249(10) 0.0193(9)
0.0085(7)
-0.0010(6)
-0.0018(6)
0.0132(8)
0.0166(4)
O6
0.3991(2)
0.0484(2)
0.95381(5)
0.0146(8)
0.0181(9)
0.0132(7)
-0.0018(6)
-0.0010(6)
0.0111(7)
0.0140(3)
O7
0.4153(2)
0.3037(2)
0.94631(5)
0.0151(8)
0.0125(8)
0.0159(7)
0.0032(6)
0.0001(6)
0.0037(7)
0.0159(3)
O8
0.1790(2)
0.0788(2)
0.96363(5)
0.0083(8)
0.0106(8)
0.0157(7)
0.0002(6)
0.0008(6)
0.0041(7)
0.0118(3)
O9
-0.0201(2) 0.1298(2)
0.74149(5)
0.0177(9)
0.0113(8)
0.0297(10)
0.0050(6)
0.0070(7)
0.0083(7)
0.0191(4)
F
0
0
0.79715(15)
0.043(2)
0.043(2)
0.049(3)
0
0
0.0217(12)
0.0454(19)
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Note:
M = 0.96Mg + 0.04Al
P1A = 0.81 P + 0.19 vacancy; P1B = 0.16 P + 0.84 vacancy;
F = 0.85 F + 0.15 vacancy
=================================================================================================================
Atom x
y
z
U11
U22
U33
U23
U13
U12
Ueq
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Table 6. Coordinates and displacement parameters of atoms in R070654 bobdownsite.
==============================================================================================
138
0.29278 (8)
0.28035 (7)
0.38769 (7)
0
0
0
0.31690 (9)
0.35083 (10)
0.2758 (3)
0.2462 (2)
0.2724 (2)
0.4877 (2)
0.4029 (3)
0.3999 (2)
0.4167 (2)
0.1793 (2)
0
0
(0.0193 (2)
0.15662 (7)
0.14623 (6)
0.18008 (6)
0
0
0
0.14343 (9)
0.15788 (9)
0.0936 (3)
0.2328 (2)
0.0016 (2)
0.2415 (2)
0.1961 (2)
0.0489 (2)
0.3045 (2)
0.0794 (2)
0
0
0.1295 (2)
0.672195 (18)
0.565199 (14)
0.767655 (13)
(0.00028 (3)
0.75326 (5)
0.7318 (4)
0.864280 (17)
0.967182 (16)
0.82513 (5)
0.87771 (5)
0.88627 (5)
0.86884 (6)
0.00664 (5)
0.95358 (5)
0.94644 (5)
0.96326 (6)
0.79758 (15)
0.6893 (7)
0.74202 (5)
0.0364 (5)
0.0104 (3)
0.0107 (3)
0.0082 (3)
0.0074 (5)
0.000 (4)
0.0085 (4)
0.0086 (4)
0.0251 (12)
0.0158 (10)
0.0110 (10)
0.0096 (11)
0.0211 (12)
0.0148 (10)
0.0147 (10)
0.0075 (10)
0.030 (2)
0.015 (9)
0.0183 (12)
0.0188 (3)
0.0095 (3)
0.0089 (3)
0.0082 (3)
0.0074 (5)
0.000 (4)
0.0089 (3)
0.0090 (3)
0.0207 (12)
0.0174 (11)
0.0081 (9)
0.0075 (9)
0.0164 (11)
0.0156 (11)
0.0114 (11)
0.0098 (10)
0.030 (2)
0.015 (9)
0.0095 (9)
0.0160 (3)
0.0088 (2)
0.0095 (2)
0.0088 (4)
0.0184 (14)
0.000 (8)
0.0074 (3)
0.0070 (3)
0.0077 (8)
0.0149 (8)
0.0115 (8)
0.0170 (9)
0.0075 (8)
0.0126 (8)
0.0133 (8)
0.0140 (8)
0.047 (3)
0.032 (14)
0.0269 (11)
(0.0126 (3)
(0.0004 (2)
(0.0024 (2)
0
0
0
0.0012 (3)
0.0000 (3)
0.0002 (7)
0.0027 (7)
0.0021 (7)
0.0031 (8)
(0.0022 (7)
(0.0008 (7)
0.0021 (7)
0.0004 (8)
0
0.0086 (9)
(0.0086 (3)
(0.0011 (2)
(0.0009 (2)
0
0
0
0.0012 (3)
0.0004 (3)
(0.0005 (8)
0.0017 (8)
0.0017 (7)
0.0012 (7)
(0.0022 (7)
(0.0030 (7)
0.0046 (7)
(0.0004 (7)
0
0.0040 (7)
0.0208 (3)
0.0056 (2)
0.0048 (2)
0.0041 (2)
0.0037 (3)
0.000 (2)
0.0049 (3)
0.0045 (3)
0.0130 (10)
0.0133 (9)
0.0028 (8)
0.0031 (9)
0.0114 (9)
0.0110 (9)
0.0033 (9)
0.0026 (8)
0.0151 (11)
0.007 (4)
0.0089 (9)
0.0206 (2)
0.0093 (1)
0.0098 (1)
0.0084 (3)
0.0111 (6)
0.000 (4)
0.0080 (2)
0.0082 (2)
0.0171 (4)
0.0138 (4)
0.0111 (4)
0.0119 (4)
0.0141 (5)
0.0128 (4)
0.0146 (4)
0.0112 (4)
0.0359 (18)
0.020 (7)
0.0174 (5)
M = (0.62Mg + 0.35Fe + 0.02Mn + 0.02 Al)
P1A = 0.85 P + 0.15 vacancy; P1B = 0.09 P + 0.91 vacancy;
O9 B = 0.17 O + 0.83 vacancy.
============================================================================================
Note:
-----------------------------------------------------------------------------------------------------------------------------------------------------------
Ca1
Ca2
Ca3
M
P1A
P1B
P2
P3
O1
O2
O3
O4
O5
O6
O7
O8
O9
O9B
O10
Atom x
y
z
U11
U22
U33
U23
U13
U12
Ueq
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Table 7. Coordinates and displacement parameters of atoms in R080052 whitlockite.
===============================================================================================
139
3.5912
3.5867
3.5801
3.5715
3.531
3.5647
3.5832
3.5918
3.5591
3.5922
3.6
3.75
4
4.25
4.5
4.75
bobdownsite
bobdownsite F0.84
whitlockite
aluminocerite-(Ce)
cerite-(Ce)
cerite-(La)
merrillite
whitlockite
whitlockite
whitlockite
model
model
model
model
model
model
c/a
0.28936
2.07529
5.49086
10.53606
17.21089
21.99801
23.74322
25.49985
23.77217
23.9021
26.81851
28.34486
26.03169
24.33555
24.00177
23.75728
Ucp
Ca
Ca
Ca
Ca
La,Ce,Ca
Ce
Nd,Ce,Pr
Ca
Ca
Ca
REE1
Ca
Ca
Ca
Ca
La,Ce,Ca
Ce
Ce,La,Ca
Ca
Ca
Ca
REE2,3
V
Mn
Mg,Fe
Mg,Fe
Fe,Ca,Mg
Mg
Al,Fe
Mg
Mg
Mg
M
P
P
P
P
Si
Si
Si
P
P
P
T
Tsirlin et al. 2006
Kostiner and Rea 1976
Calvo and Gopal 1975
Hughes et al. 2006
Pakhomovsky et al. 2002
Moore and Shen 1989
Nestola et al. 2009
this work
this work
this work
reference
Table 8. A comparison of some crystallographic parameters for bobdownsite with isostructural
compounds and some model structures.
140
2"6=4.9
0
a (Å)
10.3224
10.3394
10.3590
10.347
10.330
10.330
10.361
%F
100
84
0
0
0
0
0
37.096
37.103
37.103
37.099
37.086
37.084
37.070
c (Å)
3.5803
3.5918
3.5918
3.5855
3.5801
3.5867
3.5912
c/a
Mg0.53Fe0.38Mn0.09
Mg0.585Fe0.415
Mg0.8Fe0.2
Mg
Mg0.62Fe0.35Mn0.02Al0.02
Mg0.96Al0.04
Mg0.72Fe0.17Al0.11
M-site
OH0.69O0.31
OH0.81O0.19
OH0.84O0.16
OH
OH
F0.84OH0.16
F
F-site
Whitlockite (Hughes et al. 2008)
Whitlockite (Gopal & Calvo 1975)
Whitlockite (Gopal & Calvo 1972)
Whitlockite (Gopal et al. 1974)
Whitlockite (this work)
Bobdownsite (this work)
Bobdownsite (this work)
reference
Table 9. The relationship between composition and cell parameters for the bobdownsite-whitlockite solid solution.
ideal CCP
141
Table 10. Tentative Raman band assignments of bobdownsite.
=============================================================
Assignment
Bands (cm-1) Intensity
-----------------------------------------------------------------------------------------------------------1080-1100
Weak
)3 (PO4) asymmetric stretching
950-975
Very Strong, sharp
)1 (PO4) symmetric stretching
923
Relatively strong, sharp
)1 (PO3F) symmetric stretching
500-600
Weak
)4 (PO4) / (PO3F) asymmetric bending
400-500
Weak
)2 (PO4) / (PO3F) symmetric bending
<400
Weak
Lattice, Mg-O and Ca-O vibrational modes
=============================================================
142
Figure 1. Tabular, colourless, euhedral crystals of bobdownsite from Big Fish River, Yukon, Canada.
143
Figure 2. Comparison of thermal gravimetric analyses (TGA) of R050109 bobdownsite and R080052 whitlockite. The weights of the
starting materials are 5.8912 mg and 2.3223 mg for R050109 and R080052, respectively.
144
Figure 3. A portion of a layer perpendicular to c showing [Mg(PO4)6]16- “pinwheels” (Hughes et al. 2006). The anionic units form a
perfectly closest-packed planar arrangement. The spheres represent intralayer Ca1 cations.
145
Figure 4. A portion of two layers from a cubic closest-packed arrangement of Mg octahedra looking down c showing an isolated PO3F
tetrahedron in the large octahedral void formed by six of the octahedra. The “arms” of the Mg-pinwheels, the six PO4 groups for every
Mg octahedron, have been left off for clarity (see Figure 3), as have the intralayer calcium cations. The gray octahedra are in the lower
layer (zMg = 0.3331), the white in the upper (zMg = 0.4997). The spheres are interlayer Ca2+ cations with gray ones (Ca2) at z = 0.3985
and white (Ca3) at z = 0.4343. The base of the tetrahedron is in the midplane between the two octahedral layers. In this illustration, P5+
has adopted the upper possibility of its split position, just above the midplane (instead of just below), and so the tetrahedron points
along +c, instead of –c.
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Figure 5. A plot of packing efficiency, Ucp, vs. axial ratio for the observed whitlockite-type structures of Table 8 (solid circles) and
several model whitlockite-type structures with a fixed 10 Å and variable c / a (hollow circles). A value of zero for Ucp represents
perfect eutaxy, while values for the observed structures represent extreme distortion.
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Figure 6. Crystal structure of NaK3(PO3F)2 (Durand et al. 1975), viewed down c.
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Figure 7. Crystal structure of LiNH4PO3F (Durand et al. 1978), viewed down b.
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The best-fit line is drawn to guide the eye.
Figure 8. The relationship between fluorine content and cell parameters for the bobdownsite-whitlockite solid solution.
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Figure 9. Raman spectra of (a) R050109 bobdownsite, (b) R070654 bobdownsite, and (c) R080052 whitlockite.
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APPENDIX G:
STRUCTURE OF WALSTROMITE, BaCa2Si3O9, AND ITS RELATIONSHIP TO
CaSiO3-WALSTROMITE AND WOLLASTONITE-II
Madison C. Barkley, Robert T. Downs, and Hexiong Yang
Department of Geosciences, University of Arizona, Tucson, Arizona 85721, U.S.A
Published in American Mineralogist 96, 797-801
Paper is given as the scanned reprint
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