A
OF
MOSSBAUER STUDY
INTERVALENCE
CHARGE
TEMPERATURE-DEPENDENT
TRANSFER IN
ILVAITE
by
ARTHUR NOLET
DANIEL
S.B.,
of Technology,
Massachusetts Institute
(1977)
SUBMITTED IN
THE
OF
PARTIAL FULFILLMENT
REQUIREMENTS
FOR
THE
DEGREE OF
MASTER OF
at
MASSACHUSETTS
SCIENCE
the
INSTITUTE OF
TECHNOLOGY
(May, 1977)
of Author--.-.-.-.-.Signature
Department of Earth and
Certified by
Planetary Sciences, May,
.
Thesis
Accepted by
1978
.
Supervisor
.
Chairman, Departmental
Lndgren
wfRM
MIT U-1,00RIES
Committee
A MOSSBAUER STUDY OF TEMPERATURE-DEPENDENT
INTERVALENCE CHARGE TRANSFER IN ILVAITE
by
Daniel Arthur Nolet
Submitted to the Department of Earth and Planetary
Sciences on May 12, 1978 in partial fulfillment of
the requirements for the Degree of Master of Science.
Abstract
The mixed valence compound ilvaite, CaFe 2 2 +Fe 3 +Si 2O 8
(OH), is reputed to show strongly temperature-dependent
Previous studies report the
Fe 2 +'-Fe 3 +- charge transfer.
The
resolution of discrete valence states below 320*K.
ilvaite structure is especially conducive to charge
Fe(A) octahedra share edges to form double
transfer.
Fe(B) octahedra share edges with
chains parallel to c.
The Mossbauer spectra of a suite
four Fe(A) octahedra.
of naturally occurring ilvaites at temperatures from 80
to 575*K were recorded.
A constrained fitting procedure
Fe 2 +(B),
Fe 3 +(A),
resolved doublets assigned to Fe 2 +(A),
Fe2+(A)-Fe 3 +(A)
Jc
and Fe2+(A)-Fe 3 +(A)
J
c.
The
doublets
temperature dependence of the charge transfer
work, a gradual
contrary to prior
that,
indicates
transition from discrete valence states to intervalence
charge transfer ta-kes place (with the appearance of
charge transfer peaks) over a broad temperature range
Models
and transfer is incompletely quenched at 80 0 K.
for charge transfer mechanisms have been explored and
necessary variables discussed.
Roger G. Burns
Thesis Supervisor:
Professor of Mineralogy and Geochemistry
Title:
ACKNOWLEDGEMENTS
Several
samples employed
in this
study were provided
by the Harvard Mineralogical Collection and I am very
grateful to Dr. Clifford Frondel for
would like to thank Dr.
his generosity.
Alan Parkes for his assistance
in microprobe analysis and Peter Cunningham
electronics consultation.
not least in typing
Useful discussions, devil's advocacy,
and much support and
and Kay P-arkin.
for
Also thanks are due Rcxanne
Regan for assistance in many things,
this manuscript.
I
love came from Margery Osborne
I owe the greatest debt to Dr.
Burns who introduced me to M'ossbauer
Roger
spectroscopy,
s.upported and encouraged me throughout my research, and
provides an example of a scientist:
one who measures,
probes, and analyzes the curious things of
the world,
in a harsh scientific environment and yet is above
a person --
all
considerate, caring, and gentle.-
This thesis is dedicated to my friend and wife,
Teresa.
Her support, personally and scientifically, has
been enormous.
in this world.
May she always remain a sign of Presence
TABLE OF CONTENTS
page
ABSTRACT
ACKNOWLEDGEMENTS
TABLE OF CONTENTS
LIST OF FIGURES AND TABLES
CHAPTER 1
1.1
1.2
1.3
1.4
1.5
Statement of the Problem
Occurrence and Paragenesis
Structure
Previous Mossbauer Work
Experimental Plan
30
CHAPTER 2
2.1
2.2
2.3
8
11
16
25
27
Microprobe Analyses
Unfit Mossbauer Spectra
M*ssbauer Fitting Procedures
CHAPTER 3
3.1
3.2
3.3
BIBLIOGRAPHY
APPENDICES
Final Mossbauer Parameters
Site Occupancy
Elect.ron
Hopping
-
A Model
52
64
66
80
5
LIST OF
FIGURES
AND TABLES
pag e
FIGURE
1.
PROJECTION
c (Beran
FIGURE
2.
PROJECTION OF
OF THE ILVAITE STRUCTURE
and Bittner,-1974)
THE
ILVAITE STRUCTURE
ib
FIGURE
3.
PROJECTION OF ONE
OCTAHEDRAL CHAINS
FIGURE
4.
UNFIT MOSSBAUER SPECTRUM OF IL
300 0 K
9
FIGURE
5.
UNFIT MOSSBAUER SPECTRUM OF
300 0 K
IL
8
FIGURE
6.
UNFIT MOSSBAUER SPECTRUM OF
300 0 K
IL
5
FIGURE
7.
FIGURE
UNIT OF Fe(A)
OF
IL
7
TEMPERATURE
PROFILE OF
IL
3
PROFILE OF
IL
4
MOSSBAUER TEMPERATURE PROFILE
80-390 0 K
MOSSBAUER
80-390 0 K
35
FIGURE
9.
MOSSBAUER TEMPERATURE
80-3900K
FIGURE
10.
FIT HIGH VELOCITY REGION OF THE
MOSSBAUER SPECTRUM OF IL 3 80 0 K
FIGURE
11.
FIT HI.GH VELOCITY REGION OF THE
MOSSBAUER SPECTRUM OF IL 3 200 0 K
FIGURE
12.
FIT HIGH VELOCITY REGION OF THE
MOSSBAUER SPECTRUM OF IL 3 320 0 K
45
FIT HIGH VELOCITY REGION OF THE
MOSSBAUER SPECTRUM OF IL 3 340 0 K
46
FIGURE
13.
page
FIGURE
14.
CORRELATION DIAGRAM FOR POSITIONS
OF THE FIVE HIGH VELOCITY PEAKS
0
OVER A SUITE OF SPECIMENS 300 K
FIGURE
15.
FULL FIT MOSSBAUER SPECTRUM OF
80 0 K
IL
3
FIGURE
16.
FULL FIT MOSSBAUER SPECTRUM OF
300 0 K
IL
3
FIGURE
17.
FULL FIT MOSSBAUER SPECTRUM OF
390 0 K
IL
3
18.
FIGURE
FIGURE 19.
50
POSITIONS OF PEAKS OF IL 3 TEMPERATURE
PROFILE VERSUS TEMPERATURE
54
WIDTHS OF PEAKS OF IL 3 TEMPERATURE
PROFILE VERSUS TEMPERATURE
55
OF
FIGURE
20.
NORMALIZED INTENSITIES OF PEAKS
IL 3 TEMPERATURE PROFILE VERSUS
TEMPERATURE
FIGURE
21.
SUMMARY OF ASSIGNMENTS
DOUBLETS IN ILVAITE
FIGURE
22.
POTENTIAL SURFACE PICTURE FOR INTERDay,
VALENCE CHARGE TRANSFER (after
1976)
FIGURE
23.
POTENTIAL SURFA.CE PICTURE FOR INTERVALENCE CHARGE TRANSFER INTERACTIONS
IN ILVAITE
OF MOSSBAUER
SAMPLES
TABLE
1
-
LOCATION INDEX FOR ILVAITE
STUDIED
TABLE
2
-
INTERATOMIC
TABLE
3
-
ELECTRON MICROPROBE ANALYSES
ILVAITE SAMPLES
DISTANCES IN
ILVAITE
OF
74
7
page
TABLE
TABLE
4
5
-
-
MOSSBAUER PARAMETERS
TEMPERATURE PROFILE
FROM
IL
3
REGRESSION FIT PARAMETERkS FOR
FUNCTIONS FIT TO FIGURES 18 AND
53
19
56
CHAPTER 1
Statement
1.1
of
the Problem
In recent years much interest has been
shown
Many of
these
concerning intervalence compounds.
compounds exhibit properties
such as anomalous color,
electrical conductivity, and magnetic ordering,
not
directly attributable to electronic transitions
of a
Various charge transfer mechanisms
single cation.
have been proposed to account for these properties.
+
Of greatest interest in geological materials are Fe
Fe
3+
,
Ti
3+
+Ti
transfers,
4+
,
Mn
2+
+Mn
3+
,
all homonuclear
charge
and various heteronuclear Fe-+-Ti and Fe+Mn
charge transfers.
Robin and Day
classification system for
(1967)
have proposed a
intervalence compounds.
Class
I compounds are those which show no charge transfer and
whose properties are the
components.
Class III
sum of the properties of their
compounds, on the other hand, show
charge transfer to the extent that it is impossible to
resolve discrete valence states and the compound
best characterized by an
is
"averaged" oxidation state
with anomalous properties.
Class II
compounds exhibit
an intermediate range of properties 'and various degrees
of "mixing" of oxidation states.
Ilvaite, CaFe
II
compound
Gerard,
2+
3+
Fe
Si 0 (OH) is regarded as a Class
2
2 8
(Gerard and Grandjean,
1975).
1971;
It is of particular
Grandjean and
interest that the
amount of "mixing" is believed to be strongly temperature
dependent.
The aim of the current investigation is
to
examine the effects of composition and temperature on
a suite
of naturally occurring ilvaites and to explore
for models of inter-
the implications of the results
valence charge transfer systems.
of the technique of Mossbauer
Principal use was made
spectroscopy.
The Mossbauer effect allows the resolution
of tiny energy differences
due to differences
number,
and
57
in
Fe atomic environments
in oxidation state, coordination
site symmetry.
Mossbauer
spectroscopy
measures transitions between nuclear quantum
analogous to electronic quantum levels.
appropriate energy to excit-e
57
Co radioactive decay.
57
These
Fe
levels
Gamma-rays of
nuclei are provided by
f-rays are absorbed and
re-emitted, setting up resonance in the sample.
Slight
energy differences between nuclei due to different
electronic
environments are compensated for by providing
the y-ray source with a constant acceleration Doppler
shift
energy and "sweeping" an energy spectrum.
typical
Common iron species in minerals exhibit two
Mossbauer parameters,
ting.
Isomer shift
isomer shift and quadrupole split-
is the
location of
the midpoint of
the spectrum of the absorbing species with respect
some
zero of energy.
to iron foil
to
Our spectra are reported relative
(re metal) calibration.
ting is the energy separation of the
Quadrupole splittwo peaks which
typically occur in 57Fe absorbers due to transitions
from a nuclear ground state with nuclear angular
momentum of +1/2,
to a split excited state with nuclear
angular momenta of +1/2
offset from zero energy,
and +3/2.
This doublet
is typical of all
57Fe absorbers,
except those which are magnetically ordered.
case,
the + degeneracy of the nuclear
and a six peak spectrum results.
spectrum,
In such a
levels is lifted
Since the lifetime of
sec,
the Mossbauer transition is on the order of 10
the excited nucleus "sees"
only electronic states which
are long-lived compared to 10
7
sec.
Short-lived
states
which occur with sufficient frequency may, however, be
observable as "averaged" between the ground and
states, though the excited
excited
state can correctly be
to be absent from the Mossbauer
spectrum.
said
For a more
complete discussion of Mossbauer theory and parameters,
including
tions,
1.2
some geochemical and
see Bancroft
Occurrence
inorganic chemical applica-
(1973).
.
and Paragenesis
The name ilvaite comes from the Latin for Elba, referring to the type locality of the mineral,
Calamita on the island of Elba, Italy.
called "lievrite" for M.
Ilvaite, also
Lelievre, its discoverer, occurs
mainly as a late-stage hydrothermal
skarn deposits.
a skarn at Capo
alteration product in
Mineral samples used in this
listed in Table 1,
study are
along with several other locations
where ilvaite has been reported.
Skarns are formed in contact metasomatic zones,
replacing limestones or dolomites
near granitic intrusives,
and have world-wide occurrence in space and
time.
Interest
in skarn deposits has been great for many years due to
their economic value as ore sources.
Burt
(1972) gave
a list of elements currently recovered from skarn
deposits:
ILVAITE
LOCATION INDEX FOR
SAMPLES
TABLE
1.
IL
1
-
ELBA,
IL
2
-
ELBA, ITALY
(UNIVERSITY OF
IL
3
-
ELBA, ITALY
(WILKE, PALO ALTO,
IL
4
-
SANTA EULALIA, MEXICO
(MIT COLLECTION)
IL
5
-
SANTA EULALIA, MEXICO
(MIT COLLECTION)
IL
6
-
SIORARSUIT, GREENLAND
(HARVARD 86189)
IL
7
-
DANNEMORA, SWEDEN
IL
8
-
SERIPHOS,
IL
9
-
LACEY MINE,
IL
10
-
HANOVER, NEW MEXICO
IL
11
-
TETJUKE,
ITALY
(MIT COLLECTION)
GREECE
#1413)
CA.)
(HARVARD 85091)
(HARVARD 93753)
IDAHO
USSR
CAMBRIDGE
(HARVARD 106085)
(HARVARD 91858)
(HARVARD 114518)
OTHER LOCALES WHERE ILVAITE OCCURS (Burt, 1972; Bartholme
and Dimanche, 1967; Hernon and Jones, 1969; Hewitt,
1968; Prescott, 1916; Schmitt, 1939; Shannon, 1918,
1926):
QUEBEC
NORANDA,
USSR
TYRNYAUZ,
DASHKESAN, USSR
BALKHASH (KAR-ATAS ORE
POIANA RUSCA, ROMANIA
BINNTAL, SWITZERLAND
SAN LEONE, SARDINIA
HORNSEELBACH, GERMANY
LIVORNO, ITALY
DEPOSIT), USSR
C
graphite
W
scheelite
CaWO 4
Mo
molybdenite
powellite
MoS 2
CaMoO
Sn
casiterite
SnO2
Be
helvite
chrysoberyl
(Mn,Fe,Zn)8(BeSiO 4)
BeAlO
Fe
magnetite
specular hematite
Fe 3 04
Fe 2 03
Cu
chalcopyrite
bornite
cubanite
CuFeS2
Cu 5 Fe 2 S4
CuFe 2 S 3
Zn
sphalerite
ZnS
4
S2
4
Of rarer
occurrence but still recoverable are
from the
following
elements
list:
PbS
Pb
galena
Au
with arsenides,
sulfides
Ag
with galena
As
arsenopyrite
FeAsS
Bi
bismuthinite
Sb
stibnite
Bi 2S3
B2S3
Sb 2S3
b2S3
Ba
barite
witherite
BaSO
BaCO
4
3
Impressive beyond the size and scope of the above lists,
is a list of those elements which are concentrated
skarn deposits but rarely recovered.
B,
F,
Ti, V,
These include Mn,
P, Cr, Ni, Co, Hg, Pt, U, Nb, Th, and
rare earth elements.
in
The size of skarn resources
the
is also
while
an economic consideration;
may be
small in
areal
extent,
enriched
an individual deposit
metasomatic
concentration
a given element.
Iron Mountain
may have
greatly
deposits
(southwestern Utah) have Be reserves of 30,000
tons with ore containing 0.4%
Be
(average crustal
abundance of Be is 2.8 ppm).
Many theories have been advanced to explain the
formation of skarn deposits.
these
The most wi.dely accepted of
is the infiltration-diffusion theory of Korzhinsky
(1948, 1950).
The
theory postulates the establishment
of an unstable chemical system at the intrusive-carbonate
contact.
This instability is due to high concentrations
Mg,
of Ca on one side of the contact and of Si, Al,
Na, and K on the other side.
The system is
Fe,
then opened
to hot aqueous solutions along planes of high porosity
and
low resistance.
Concentration gradients are set up
as the system proceeds toward the ultimate goal of uniform
composition throughout the areas
in contact with the
fluids.
Conditions of skarn formation have been estimated by
many geologists from field
evidence and mineral stability
experiments.
Most authors agree that skarns are
by hypabyssal
(shallow) intrusives and Smirnov
formed
(1976)
gives a range of
majority of
500-2.000 meters which likely covers
the
The thermal history of skarns
skarn deposits.
is complex, beginning with thermal metamorphism from
contact with a hot intrusive, through hydrothermal metato late-stage metamorphism
somatism and ore emplacement,
Burt
(normally retrograde hydration reactions).
estimates for these stages:
gives as
700-900 0 C
Solidification of the intrusive
500-700*C
Contact metamorphism
400-600 0 C
Hydrothermal metasomatism
300-500 0 C
Ore emplacement
0
(skarn formation)
Late-stage metamorphism
200-400 C
Smirnov
(1972)
(1976) gives a thermal range of 100-900*C, based
on experimental measurement of garnet and hedenbergite
stabilities and fluid inclusions in skarn minerals.
Burt
(1971, 1972) has done extensive work on the
stability of ilvaite and another hydrous phase, babingtonite, Ca2 Fe
2+
Fe
3+
Si 0 14(OH),
in Ca-Fe-Si skarns.
Ilvaite
in such skarns is formed primarily by late-stage hydrothermal alteration and retrograde metamorphism of
hedenbergite.
7 Hed
+
Relevant reactions are:
1/2 H 20
+
5/4 02
3 Hed + 2 CO2 + 1/2 H 20 +
=
Ilv +
6 Qtz
+
2 And
1/4 02 = Ilv + 2 Cal +
4 Qtz
Occasionally, however,
ilvaite can appear as a primary
constituent of the zoned skarn, i.e.
of metasomatism
Dimanche, 1967).
as a direct product
Sardinia, Bartholome and
(e.g. Elba and
Ilvaite occurs
in many different
assemblages, of which the most common are:
1.3
hedenbergite,
ilvaite,
'
andradite
-
Hanover, New Mexico
South Mountain, Idaho
Shasta, California
ilvaite, quartz
-
Santa
ilvaite, quartz,
andradite
-
South Mountain, Idaho
Hanover, New Mexico
Yaguki, Japan
ilvaite, quartz,
andradite, magnetite,
babingtonite
-
San Leone,
ilvaite, quartz,
calcite, magnetite,
hedenbergite,
ferroactinolite
-
Elba,
ilvaite, pyrrhotite,
sphalerite,
hedenbergite,
knebelite
-
Santa Eulalia,
Lima, Peru
Eulalia,
Mexico
Sardinia
Italy
Mexico
Structure
Early structural work on ilvaite was done by von
Gossner and Reichel
orthosilicates.
(1932) who placed
Takeuchi
ilvaite in the
(1948) agreed with this
determination, proposing a structure based on an
Sio 4
2+
framework of isolated tetrahedra linked by FeO
octahedra, with Fe
surrounded
by
3+
four 0
in an unusual 5-coordinate
2--
and one OH_.
The first work placing
sorosilicate
ilvaite in the
class was done by Belov and Mokeeva
A recent
(1954).
structural refinement by Beran and Bittner
confirmed the unambiguous existence of Si
the ilvaite structure,
site
(1974)
20 7
in
groups
and provides the primary structural
reference for this work.
Krause
(1960) performed UV
absorption analysis of ilvaite and Hanisch
the OH assignment by the OH
(1967) verified
stretching frequency in the
infrared.
The
b =
structure of
13.07A, c = 5.86A,
ilvaite
(Pbnm -
Z = 4 CaFe
characterized by isolated Si
2
2
Fe
16
D 2h'
a
0
=
8.82A,
Si 0 (OH))
2 8
is
0 7 groups linking together
double chains of Fe(A) octahedra parallel to c.
This
framework then provides the ligands for the octahedral
Fe(B) site and the seven-coordinate Ca site.
Figure 1
shows a projection of the unit cell perpendicular to c.
Figure 2 illustrates the FeO
chains and Fe(B)
sites in a projection perpendicular to b
and Ca
(note the Si
sites have been deleted for clarity).
The coordination polyhedra are all distorted from
highest symmetry.
Fe(A) can be modelled as a nearly
IC
Figure 1.
Projection of the ilvaite structure i c,
groups (Beran and Bittner,
showing Si O
1974) .
4
rt
U2
.J.
<
bi
(D
rt
P.C
- t
MH-
-
rt
o
Dh
D'
>rt
O
(D H
+
(DQ
<O >0
(D
r,(D
(0
< 0Mi
rtD
(
3- ' Mi
0
0
On O
regular octahedron with a range of M-0 distances from
2.006 to 2.172E, averaging
2.081 +
Fe(B) displays
.058A.
tetragonal distortion with four long planar and two short
2.039A,
apical M-0 distances approximately 2.266I and
0
respectively, with a mean of 2.190 +
0(2) -
Fe(B)
-
0(4)
noted that the Fe(B)
angles
of
site is
162.9*.
.129A and
It
planar
on the average
larger than
The seven-coordinate Ca -site has a mean
the Fe(A) site.
M-0 distance of 2.411 +
.053A and
is comparable to Ca
coordinatio-n polyhedra in the minerals titanite
1968).
(mean
2.459A,Mongiogi and Riva di Sanseverino,
Ca-0 distance of
1968),
also be
should
zoisite and clinozoisite
(2.461 to 2.67A,
Dollase,
Table 2 gives relevant interatomic distances for
the metal-oxygen coordination polyhedra and Figure 3
shows the intermetallic distances for one unit of the
octahedral chain.
Several theories have been proposed to describe the
distribution of Fe2+
Belov and Mokeeva
and Fe3+
over
the two Fe sites.
(1954) proposed a scheme with Fe
occupying Fe(A)
(eight sites per unit cell) and Fe
occupying Fe(B)
(four sites per unit cell).
Bittner
Beran and
(1974) proposed occupation of Fe(B) by Fe
a statistical distribution of Fe
2+
and Fe
3+
over
2+
and
FeCA).
21
TABLE
2.
INTERATOMIC DISTANCES IN ILVAITE
Fe(A)
Fe(A) -
0(1)
O(1)
0(2)
0(3)
0(4)
0(7)
2.172A
2.060
2.044
2.088
2.116
2.006
0
Average 2.081 +
.058A
Fe(B)
Fe(B)
-
O(1)
0(2)
0(4)
0(6)
2. 120A
2.246
(2x)
2.285
(2x)
1.957
Average
2.190 +
.129A
Ca
Ca -
0(2)
0(3)
0(4)
0(5)
0(7)
2. 399A (2x)
2.411
2.461
(2x)
2.440
2.306
Average 2.411 +
0
053A
Si(1)
Si(l)
-
0(2)
0(5)
O(6)
1.628A
1.640
1.598
(2x)
Average 1.624 +
.018A
Si (2)
Si(2)
-
0(3)
0(4)
0(5)
1.658A
1.619
(2x)
1.666
Average 1.641 +
. 025A
zt
Figure
3.
Projection of one unit of Fe(A) double chain
showing possible Fe-Fe edge-shared interactions
and metal-metal distances.
(Beran and Bittner
(1974) also advanced and rejected the
theory of complete statistical distribution of Fe2+ and
Fe + over both sites.)
The case for
the latter model is
-O and Fe
supported by a compilation of Fe
for octahedral iroa in silicates by Ghose
gave a mean Fe
of 2.025A.
(1969) who
3+
2+_
-o distance
-O distance of 2.135A and Fe
Beran and Bittner's
yielded a mean Fe
(1974) own compilation
2+_
o3
-o distance of 2.155A and Fe
distance of 2.014A, for
and sulfates.
-O distances
-O
six-coordinate iron in silicates
The average M-O distance for Fe(B) agrees
well with the values for octahedral Fe +,
especially
for values of silicates with similar components,
babingtonite, Ca 2Fe
distance of 2.169A
2+
Fe
3+
Si5 0
(OH),
e.g.
with a mean Fe
(Araki and Zoltai, 1972).
The
2+
-O
average
Fe(A)-O distance lying between the pure Fe 2+-O and Fe
distances supports an
of Fe(A).
-O
intermediate statistical population
Haga and Takeuchi
(1976) performed a neutron
diffraction study.on an ilvaite sample from the Kamioka
Mine, Japan and using Fourier mapping techniques derived
a site occupancy of:
Fe(A)
.478 Fe 2 +
.514 Fe 3 +
.006 Mn
Fe(B)
.815 Fe 2 +
.185 Mn
confirming Beran and Bittner's
Site
(1974) assignments.
occupancies derived from the Mossbauer spectra will be
discussed
in Section 3.2.
The structural possibilities
for Fe
2+
-Fe
3+
charge
transfer are many as can be seen in Figure 3.
Fe(A)
octahedra form a double chain by edge-sharing
both in
the xy-plane and along the z-axis.
there are
two M-M distances
In the
(2.83 and
z-direction
3.03A) as each pair
of Fe(A) octahedra does not lie flat in the xy-plane but
is
skewed with one Fe(A) down
the chain below it)
site above
it).
large Ca site
and one Fe(A) up
in
(closer to the
This distortion provides room for the
in the "hollow" created by the
elongated
Also available for electron transfer
Fe(A)-Fe(A) pair.
are Fe(A)-Fe(B) pairs sharing
shares
(closer to the Fe(A)
edges.
Each Fe(B)
site
edges with four Fe(A) sites, with M-M distances
varying slightly
(3.15 to 3.25A).
The intermetallic distances in ilvaite are well
within the range cited for minerals
known to show inter-
valence charge transfer transitions
(Loeffler et al.,
1976;
Burns et al.,
1977).
Loeffler et al.
(1976) argue
that, all other factors being equal,
intervalence charge
transfer occurs preferentially along
short metal-metal
distances.
Therefore,
be expected
to predominate
in
ilvaite,
Fe(A) 2+-Fe(A)
over Fe(B)
2+_
3+
-Fe(A)
theoretical grounds, that intervalence charge
occurs preferentially between sites of
1.4
transfer
similar symmetry.
leads to the expectation that in ilvaite
-Fe(A) + predominates over Fe(B)
Fe(A)
Hush
.
(1976) also advance the argument, on
(1967) and Day
Again, this
might
-Fe(A)
.
Early Mossbauer Work
Previous Mossbauer measurements of
ilvaites were
made by Herzenberg and Riley
(1969),
Borshagovskii et al.
(1971),
(1971),
and Grandjean and
Gerard
Gerard and Grandjean
(1975).
Herzenberg and Riley analyzed the spectrum
i-n terms of three quadrupole doublets assigned to ferrous
iron in two crystal sites and ferric iron in a distorted
site.
They reported Mossbauer parameters with isomer
shifts of
1.07,
splittings of
0.98,
2.36,
and 0.51 mm/sec and quadrupole
2.18,
and 1.24 mm/sec,
respectively.
They used their study to support the Belov and Mokeeva
(1954) structure determination and concluded
that the
ferric iron site should be distorted to produce
quadrupole splitting analogous to epidote
1967).
large
(Bancroft et al.,
Herzenberg and Riley also reported an asymmetry in
absorption intensities with total high velocity absorption
greater by about ten percent than total low velocity
absorption.
They suggest this asymmetry has only a minor
effect on fitting.
Borshagovskii et al.
(1971) studied Mn substitution
in ilvaite by M'ssbauer spectroscopy.
in the temperature variation of
Interested mainly
the Curie point with
composition, they resolved only two doublets
ilvaites studied, assigned to Fe +
(octahedral).
in the six
(octahedral) and Fe
In the range of Mn compositions 0 to 12%,
it was found that the Curie point was raised with increasing Mn, until magnetic order could be observed at 80K for
samples with 12% Mn.
Gerard and Grandjean
(1971) published data on
"electron-hopping process" in ilvaite.
Mossbauer spectrum of an
the
They recorded the
ilvaite sample from Livorno,
Italy over the temperature range of 4.2*K to 520*K, fitting the spectrum.to three doublets assigned to two
ferrous and one ferric iron sites.
Their attention was
drawn to the temperature range 300*K to 350*K, where an
additional absorption is readily apparent between the
high velocity ferrous and high velocity ferric peaks.
Gerard and Grandjean concluded that "one of the two
ions undergoes. a change which gives it a ferric
ferrous
In other words, one high velocity ferrous
character."
peak moves to lower velocity with increasing temperature
0
until at 370
K it overlaps with the ferric high velocity
Gerard and Grandjean attributed this movement to
peak.
to Fe
rapid transfer of an electron from Fe
compared
and
the process to electron transfer on the B sites
of magnetite at high temperature.
1.5
Experimental
Plan
In order to derive an experimental plan, the question
asked was,
"What was the status of knowledge of inter-
valence charge transfer
in ilvaite prior to this research?"
site preferences of Fe3+
Structural data gives
and Mn for Fe(B) which reduce
bilities to Fe
Fe
(A)
[ c.
and Grandjean
(A)-+Fe
The
(A)
the charge
transfer possi-
c and ic and Fe 2+(B)+
fitting of M'$ssbauer
spectra by Gerard
Grandjean and Gerard
(1971) and
for Fe(A)
(1975)
is
highly suspect due to the poor quality of the published
spectra and the lack of any published
such as line widths or X
of electron transfer
2
fitting parameters
These authors' interpretations
as "assuming a ferric character" are
also suspect on mechanistic grounds.
Borshagovskii et al.
(1971)
on
did not deal with the effects
of Mn substitution
the charge transfer process but rather concerned
themselves with variations in the Curie temperature and
magnetic ordering.
Therefore, we know only that inter-
valence charge transfer has been ascribed to ilvaite,
that the
ilvaite structure is highly conducive to electron
delocalization, and that a phenomenon occurs in the
Mossbauer spectrum of
ilvaite with increasing temperature
(i.e. between 300 and
350 0 K).
The questions which this research attempts to answer
are:
1) How does one fit the Mossbauer spectrum of
ilvaite in
a self-consistent manner for a suite of specimens encompassing a range of temperatures, compositions, and formation
conditions?
2) What can be said about the
site occupancies and
ferrous/ferric ratios derived from Mossbauer measurement
as opposed
to chemical or x-ray determinations?
3) What are the implications for and restrictions placed
upon models of the charge
transfer mechanism in ilvaite
by the temperature variation of the observed Mossbauer
parameters?
To achieve answers to these questions, the
following
experimental plan was adopted.
First, a suite of
natural
ilvaites was assembled and chemically analyzed to determine
the range of cation
substitutions.
ilvaites was measured by Mossbauer
temperature and
Second,
the
suite of
spectroscopy at room
spectra fit to component peaks, attempting
to obtain consistency over the compositional range.
Finally, temperature profiles of the Mossbauer spectra
of several samples of differing compositions were obtained
at temperatures above and below room temperature and fit
to determine temperature variations of Mossbauer parameters.
Once these results were obtained, theoretical
for questions 2 and
drawn.
implications
3 above were explored and conclusions
30
CHAPTER 2
2.1
Microprobe Analyses
The
suits
a Materials
by
of
-samples of Table
Analysis Corp.
the MIT Department
1 were analyzed by
electron microprobe maintained
of Earth and Planetary Sciences.
Handpicked crystallites were mounted
and polished with
.5p corundum grit.
in an epoxy matrix
Thin carbon coating
The data
of the samples assured high conductivities.
were corrected using GEOLAB correction factors and
mineral standards maintained by the facility.
Table
gives the compositions in weight percent of oxides.
3
The
microprobe is unable to distinguish ferrous and ferric
iron and thus all iron is reported as FeO.
weight percentages are corrected
When total
for Fe 203 and FeO
(by estimates based on stoichiometry) and for the
(OH)
also not analyzed and present in the stoichiometric
formula, all totals came to equal
100 +
1 wt.%.
Mn contents of the suite vary from 6.44 wt.%
Il 7 to 0.30 wt.%
MnO in Il 8.
MnO in
On the basis of Mn content
and after examination of the room temperature Mossbauer
spectra, Il 3, Il 4,
and Il 7 were chosen as candidates
for temperature variant M'ossbauer study.
TABLE 3.
S iO2
TiO
ELECTRON MICROPROBE ANALYSES OF
SAMPLES
(in oxide weight percent)
IL 1
IL 2
IL 3
IL 4
IL 5
IL 6
IL 7
IL 8
IL 9
IL 10
IL l.
28.86
29.90
29.32
27.18
28.02
29.74
29.78
29.60
29.56
29.71
29.88
.00
.00
SnO2
*
*
ZrO
*
*
2
ILVAITE
.00
-*
*
.00
.00
.01
.01
.01
.00
.00
.00
*
*
.07
.07
.07
.05
.10
.07
*
*
.34
.21
.28
.27
.16
.21
2
Al20
Cr2 03
FeO
28
.00
51.88
.78
.63
.10
.10
.16
.28
.91
.35
.08
'.16
.00
.04
.07
.03
.04
.02
.02
.00
.04
.04
51.35
51.00
51.48
50.64
51.02
45.97
51.87
49.05
46.83
48.61
MgO
.10
.16
.11
.12
.08
.08
.22
.20
.02
.62
.02
MnO
1.23
.97
1.49
4.08
4.79
2.11
6.44
.30
4.08
5.49
4.30
CaO
12.93
12.70
12.84
11.29
11.09
12.66
12.19
12.19
12.30
12.04
12.50
Na20
.10
.00
.06
.09
.00
.00
.06
.00
.00
.00
.01
WO
.00
.20
.02
.00
.13
.00
.08
.08
.05
.00
.18
3
TOTAL
95.39
96.05
* Element not analyzed
95.50
94.41
94.84
96.23
95.32
95.84
95.61
94.96
95.33
2.2
Mossbauer SpectraMossbauer spectra were reccrded on an Austin
Associates
in
spectrometer
Each
a palladium matrix.
of a 1024 channel
channels
57Co source
with a 50 millicurie
recorded
spectrum was
multichannel
analyzer
on
512
and the
spectrum allowed to acquire greater than 106 counts per
Low temperature runs were made employing a "cold
channel.
finger"
arrangement of mounting samples
suspended
on a copper rod
in a reservoir of liquid nitrogen
ice/trichloroethylene
(200*K).
(80*K) or dry-
High temperature spectra
were recorded by means of a Ranger Electronics furnace
and temperature controller designed for M'dssbauer work
and calibrated with a chrome-alumel thermocouple and
external
ice point reference.
Unfit Mossbauer spectra at room temperature are
shown in Figures
4 -
6.
Figure
spectrum with resolution of
about
sec)
2 mm/sec)
4 shows a
typical ilvaite
two ferrous components
and one ferric component
(at
(about 1.2 mm/
at high velocity and strong overlap of the low
velocity components
(near 0 mm/sec) of all
three doublets.
Il 8 shown in Figure 5 displays some variation with the
innermost high velocity ferrous peak appearing as only a
IL9 300K
01
0
e(
/'
I
'S
*
S
/ \
*
*
S
I
S
S
S
S
S
*
z
*
10.C
0
S
S
*
I
St
CO
*
S.
0
'I'S
20.0I
MM/SEC
L
-2.0
Figure 4.
V
L-
-
--- -.--.-1l. -----.--
0
-
_L
_
- l.-..L
2.0
Unfit Mossbauer spectrum of IL 9 300 0 K.
Two
ferrous components resolved at about 2.2 mm/
sec.
4.0
IL 8
300K
0
-Ijyr* Af
I
I,
-"n
20
I
j
'
z
0
IMa0
4A
4.0
II
I
.
-2.0
Figure
5.
i
r
L
I
MM/SEC
I
I
l
I I I I
2.0
Unfit MO*ssbauer spectrum of IL 8 300*K.
Ferrous shoulder at 2.1 mm/sec and increased
absorption at 1.5 mm/sec.
4.0
IL5 300K
o
"
/M
v w,#
, wI
I
I #fp
z0
5.0
09
CA.
0
10.0
MM/SEC
..
-2.0
Figure 6.
0
0
20
20
--
. 4_._
0
0
Unfit Mossbauer spectrum of IL 5 300 K.
Large broad absorption centered near 1.5 mm/
sec.
4.0
shoulder and some
1.5 mm/
significant absorption in the
sec region under the minimum between ferrous and
in the high velocity region.
(Il 5)
Again low velocity components
Figure 6 shows yet another
are strongly overlapped.
ilvaite specimen
ferric
Here only
at room temperature.
one high velocity ferrous component can be resolved and
a large broad absorption is seen centered near 1.5 mm/sec,
at higher velocity than the previous posi.tion for ferric
iron absorption
(about 1.2 mm/sec).
The low velocity
region rema.ins overlapped.
Obviously, the variation in room temperature Mossbauer
spectra is extreme and fitting procedures must account
for the variations in changing positions, widths, or
intensities of absorbing species.
In order to obtain
another view of the variation in spectral profiles,
Mossbauer
spectra of three specimens were run over a
range of temperatures and are shown in Figures 7-9.
It is readily apparent in Figure 7
(Il 7)
that the
resolution of two ferrous components at 300*K is
with
lost
increasing temperature as the inner ferrous doublet
decreases
in intensity concurrent with increased absorp-
tion in the 1.4-1.5 mm/sec region.
Figure 8 shows the
temperature profile of Il 3 in which the 300 0 K
spectrum
IL7 TEMPRATURE PRON.E
0
zg
4-4
4-4
0 $4
4~4-rA
0 0
N 0
44~
-P ) M
4.)
4)
w
0
9
:0 .4
Zu -rl
340K
04-)
z
.
326K
44
04
.- I
4-4 0
0
4
>"4
-. r-i
k 0
0
3K
e
*4-)
44 -
'1-40
Z3
04
(1
4- 4
4-)
N~~0
0
0 ) En
:to
U) -
:0
0
20-K
0
1
2-1
Z r -,
80K
I
It
II'
I' I~
Ii,
I
I
'
f9
MM/SEC
~
2p
410
IL4
0
toL
OLO
0
4
0
2 ..
000
Q44
NO
400
0 U
0
4Q>
k
0
. 0
0
"Fi
zP
104343
4W
39-r
J
.
~fl(~0
:0
m .C.)
ma
0
-r4
Ne
4
O
2.0
I - - -MOMMUNNO
has
a shoulder in the high velocity ferrous region
Il 8, Figure 5).
This sample shows similar variation to
temperature and greater
(Figure 7) with increasing
Il 7
(cf.
resolution of a new peak at 390
0
K centered about 1.5 mm/
The room temperature spectrum of sample Il 4 in
sec.
Figure 9 shows significant absorption around
1.4-1.5 mm/
sec and only one ferrous component at high velocity.
Again, at 390
0
K,
a separate peak about
l..3-l.4 mm/sec is
clearly resolved.
Spectral Fitting Procedures
2.3
Given the variations of
4 -
9,
spectral profiles
it becomes necessary to describe these effects
in terms of component peak parameters and
their variations.
We have used a program written by A.J. Stone
1969)
in Figures
and modified by F.E. Huggins which fits
Lorentzian lines using
the Gauss
(Stone et al.,
a sum of
non-linear regression
procedure and allowing a variety of constraint packages.
The first attempts to fit the spectra were based on
the following assumptions:
1)
Since there is no evidence
for magnetic ordering, all Fe species will
quadrupole doublets;
absorb as
2) Components of a doublet can be
expected to be equal in width and
inttensity to a reasonable
mlwftk
approximation;
to be equal
3) All ferrous species can be expected
in width to a reasonable approximation;
4) Criteria for goodness of fit are convergence, reduction
of X2 ,
consistency between samples and temperatures, and
Assumptions
crystallochemical reasoning.
1 and 2 are
supported by theoretical considerations of Mossbauer
absorbers
(Bancroft, 1973)
doublets of equal width and
which predict quadrupole
intensity for non-magnetically
Assumptions
ordered species.
Mossbauer practice and
range of minerals.
2 and 3 are based on
common values found over a wide
Assumption 4 is
supported by statisti-
cal arguments for convergence and chi-squared reduction
and by geochemical evidence
for consistency.
Work is in
progress to verify fits as well by Ruby's MISFIT parameter
(Ruby,
1973)
Due to the extreme overlap in the low velocity region,
initial fitting attempts were made on the high velocity
Three peaks corresponding to one ferric
envelope only.
and two ferrous species were postulated and additional
peaks added to take up excess area.
Resolution of more
than three peaks was further justified by significant
decreases in X
2
.
It was found
peaks were required to fit all
that two such additional
spectra.
More than two
42
such peaks made convergence difficult and did not reduce
X 2.
Figures
10 -
at various temperatures.
for IL 3
fits
13 show high velocity
These fitted
spectra point out
the gross variation in peak parameters with temperature.
The correlation diagram in Figure 14 shows
the consistency
of the positions of the five high velocity peaks
suite of
specimens at room temperature.
for the
Constraints
employed during this stage of fitting wer.e ferrous widths
held equal and widths of the two additional peaks held
equal.
Having now located the positions, widths, and
intensities of
the high velocity peaks,
components were added to each doublet.
and intensities
in the
to estimated values
their low velocity
Initial positions
low velocity region were constrained
and allowed to converge before
releasing these constraints.
holding all ferrous widths
Final
fits were obtained
equal, both ferric widths
equal, and all additional peak widths equal.
originally held to estimates and later
pairs, were freed,
Intensities,
constrained in
except for the tightly overlapped
low velocity areas which required constraints to
avoid divergence.
Figures 15 -
17 show full ten peak,
IL3
80K
0
z5
0
0
MM/SEC
-2.0
Figure 10.
0
2.0
High velocity region of IL 3 80 0 K Mossbauer
spectrum fit to five peaks.
44
IL3
200K
z0
I-rn
0.
D4.
Do
6.0
MM/SEC
Figure 11.
High velocity region of IL 3 200*K Mossbauer
spectrum fit to five peaks.
IL3
320K
z0
CA
0
4.01
MM/SEC
4.0
Figure 12.
High velocity region of IL 3 320*K Mossbauer
spectrum fit to five peaks.
IL 3
340-
z
0
0
4f.
MM/SEC
Figure 13.
High velocity region of IL 3 340 0 K
spectrum fit to five peaks.
Mossbauer
Figure 14.
Correlation diagram for the positions of the
five high velocity M'ossbauer peaks over the
suite of samples, all at 3000K.
0
* FE
)
0 FE
)
2.2
2.0
LU
U,
1.8
0
FE-FE CT(xY)
0
1A
.
0
O~1.4
0
0
a
o FE-FECT(z)
a.
1.2
0
1
0
o
FE3 +
2 3 7 8 9 10 11
SPECIMEN NUMBER
IL3 80K
z
0.,
0
C|
CO
EC
2.0
Figurd 15.
Full ten peak five doublet fit for the
Mossbauer spectrum of IL 3 80*K.
4.0
IL3
300K
z 2.0
0Z2.0
0
CO
4.01MM/SEC
-2.0
Figure, 16.
0
2.0
Full ten peak five doublet fit for the
Mdssbauer spectrum of IL 3 300 0 K.
4.0
IL 3
390K
0
1.0
z0
Q.
0
0A
2.0
MM/SEC
-2.0
Figure.1
7
.
0
2.0
for the
Full eight peak four doublet fit
0
K.
390
3
'Mossbauer spectrum of IL
4.0
51
five doublet fits
for Il 3 at several temperatures.
Figure 15 at 80*K shows a low intensity for both additional
peaks.
These peaks can be seen to gradually increase in
intensity relative to the ferric and innermost ferrous
doublets as temperature increases.
Finally at 390*K,
the innermost ferrous doublet lies below the resolution
of the Mossbauer effect and only one ferrous doublet can
be fit.
OIL,
CHAPTER 3
3.1
Final Mossbauer Parameters
All
spectra taken were fit acc6rding to the above
procedure, resulting in ten peak, five doublet fits.
Table 4 gives fitting parameters for the
Il 3 temperature
profile, including positions, widths, isomer shifts,
quadrupole splittings,
2
and X2.
Appendix 1 shows
fit
spectra from a range of samples and temperatures.
It becomes obvious,upon inspection of
the fitted
spectra, just how the electron transfer process varies
with temperature and composition.
The variation is best
illustrated by plotting Mossbauer parameters versus
temperature.
Figure 18
shows
such a plot for the posi-
tions of the Il 3 temperature profile peaks.
Peak
positions vary in a near linear fashion and lines were
calculated by a least squares program.
coefficients for the
Figure 19 shows
temperature.
Correlation
lines are given in Table 5.
the variation of line widths with
It has been reported that widths of peaks
associated with intervalence charge transfer
decrease with
increasing temperature, i.e.
in magnetite
show inverse
temperature variation to "pure" ferrous and ferric
species
TABLE 4.
MOSSBAUER PARAMETERS FROM IL
PROFILE
POSITIONS (mm/sec)
3 TEMPERATURE
80 0 K
200*K
300 0 K
320 0 K
340 0 K
390 0 K
-.
297-
189
-.
149
-.
131
-. 135
-. 180
-. 119
-.
161
-.113
-. 230
-. 133
-. 085
-.
-.
-.
039
-. 037
-
.030
-.
019
-.
-.
-.
009
-.
043
021
170
017
-.
010
-. 005
.110
1.160
1.341
1.611
.075
1.225
1.617
2.155
2.336
2.435
.075
1.218
1.563
1.904
2.142
2.295
.110
1.204
1.530
1.760
2.010
2.250
.088
1.192
1.488
1.695
1.908
2.206
.119
1.156
1.396
1.624
1.847
2.180
.228
.261
.482
.254
.378
.247
.284
.361
.240
.301
.448
.249
.303
.431
.244
.290
.465
.241
1.069
1.088
1.021
.787
.649
1.032
1.004
.910
.771
.647
1.030
.940
.860
.760
.657
1.018
.887
.829
.735
.640'
1.000
.864
.797
.989
2.376
2.043
1.732
1.506
1.104
2.361
1.966
1-653
1.406
1.036
2.126
WIDTHS (mm/sec)
Fe 2+
Fe+Fe 3 +
Fe2++*Fe3+
ISOMER SHIFT (mm/sec)
1-10
2-9
3-8
4-7
5-6
.693
.637
.796
.668
.635
QUADRUPOLE SPLITTING (mm/sec)
1-10
2-9
3-8
4-7
5-6
2.732
2.497
2.268
1.661
1.153
2.525
2.274
1.989
1.584
1.143
2.439
2.141
1.799
1.539
1.094
X2
1.445
1.285
2.001
2.275
1.630
1.346
1.050
1.044
2.5
~A
~*
2
~
A
_
-
A
'-~*0
-~~0
0m0
-0-
1.5
0
P 0O
C.
0
Oi
In
++cn.1 d H.CDJ0 (D~U
z
0
MOFl
+
00O
H 0
O
1
0)
±OD
pi H rt s
oU)J H
0
(D (n
tfl
CDf
w'a
a.
+
0.5
-
o0
e
O
0
(Dl
) (D H
..
,........
..
.-
3--
v
(D~
R-7 76 ---
t-i
:jA)
- P)
rt td
(D
0i
-
-
200
TEMPERATURE (K)
40u0
Figure 19.
Widths of the peaks of the IL 3 temperature
Fe 2 +, Fe 3 + fi
profile versus temperature.
to regression calculation lines; Fe 2 ++Fe 3 +
charge transfer
fit
to I/P c e-E/kT
0
(Sawatzky et al., 1969) (Table 5).
X
i
44
1
\
xx
1
t
I
X
O
\
41
411
TABLE
Figure
5.
REGRESSION FITS FOR FUNCTIONS IN FIGURES 18
18
-
lines
All
fit
to y
a1 x +
=
80*K Position
(mm/sec)
Peak
and 19
a0
a0
Correlation
Coefficient
r
a1
1 Fe
A
-. 297
-.
330
.0005
.991
2 Fe
A
-. 161
-.
167
.0001
.892
2+
3+
3 Fe +Fe
0
-. 113
-. 142
.0003
.991
4 Fe
+-Fe
0
3+
5
5 Fe
3+
6 6Fe +
7F2+ +Fe 3+0 0
7 Fe
-. 043
-. 049
.0001
.952
.075
.059
- .0001
.865
1.225
1.252
-.
0002
.877
1.617
1.713
-.
0008
.896
8 Fe
+Fe
2.155
2.287
-.
0018
.990
9 Fe
A
2.336
2.489
-. 0018
.985
10 Fe
A
2.45
2.507
-. 0010
.988
Figure
19
0
-
Fe
2+
and Fe
3+
fit
y =
to
charge transfer fit to y
a 1x + a0,
1bx
= ae
Fe
where
2+
Fe
y =
3+
1/r
and x = 1000/K
a
Fe + A
3+
Fe
X
1
.2115
.0002
.959
.2214
.0006
.961
b
3+
2+
Fe, +Fe
Note:
Correlation
Coefficient
r
5.198
-. 072
Correlation
Coefficient
r
.958
Plot of regression fit for l'/P a e-E/kT model of
Fe 2 ++Fe 3 + charge transfer widths (after Sawatzky
et al., 1969).
Model yields an activation energy
of .0062 eV for ilvaite thermal transfer.
(cf. page 57)
10,
8
6-4.
.1
,
-
'.
. -
"
2-
' 4'6
I
I
10
I
ONO.
(Sawatzky et al.,
1969).
Figure 19 with ferrous
Such variation can be seen in
and ferric widths of Il 3 spectra
increasing approximately linearly and widths of
mediate species decreasing with
Sawatzky et al.
inter-
increasing temperature.
(1969) model the change in peak width,
Ar, as an activated process with activation energy E and
e - /kT
1/AF
Figure 20 illustrates the cause of
temperature variation of the
the enormous
spectra and observed
The intensity of the
differences from sample to sample.
outermost ferrous doublet showed little variation with
temperature within the counting statistics of the
Hence, the axis labelled normalized
represents the ratio of
intensity in Figure 20
intensity of the peak, x,
intensity of the highest velocity ferrous peak.
tion is required
times,
spectra.
to the
Normaliza-
to eliminate variations due to counting
source activity, and sample thickness.
For the
sake of clarity, and due to the constraints necessarily
imposed on the tightly overlapped low velocity region,
only high velocity intensities are
included in Figure
20.
It is immediately apparent that, compared to the
"normal" Fe 2+
species, the intensities of the other
ferrous peak at high velocity and
the ferric peak decrease
Figure
20.
Normalized intensities of the peaks of the
IL 3 temperature profile versus temperature.
Peaks normalized to Fe 2 +(B).
Only high
velocity intensities are shown due to
constraints on low velocity intensities.
0-I
~
-
-~
it
-
7
'S
%%
~~/' ~
\\
a%3/
/
4~3
/
IC~
'1
I,
'I
-I
r
,1
/
/
/
0'
I
I
1'
I
I
I
I
I
1/
I I
I
I
ii
'I
I
I
I
I
I
I
I
I
.4
I
~
~
/
/
/
/
I
I
I
I
~
j
~
I
10 CI
I
I
I
III
I
I
I
I
i
;
I
I'
I,
If
''I
C4
I
I
1'
.1
10
I
I
I
I
OAW
I..
I.U
U
I"'.
U
U
ALISN3JNI G3ZI1VWUON
with increasing temperature.
Contemporaneous with the
intensity decrease of the Fe3+ and Fe
peaks,
the
additional peaks increase with temperature, one with a
gentle slope and the second with a sharp rise between
320 and 370*K.
Grandjean
The interpretations of Gerard and
(1971),
who cited movement of one ferrous
peak to lower velocity "undergoing a change which gives
it a ferric character,"
are
thus rejected by the above
The "electron-hopping process" in ilvaite is now
data.
interpreted
as population of intervalence molecular
orbitals centered no longer on a single Fe
cation but spread
instead over an Fe
Such an interpretation explains
of intensity of Fe
2+
-Fe
2+
3+
or Fe
3+
couple.
the coordinated decrease
and Fe3+ peaks with increase of
the
additional peaks now assigned to Fe 2+Fe 3+intervalence
charge transfer.
These
"trapped" and "pure" valence
levels decrease as electrons acquire the thermal energy
necessary to occupy
the intervalence charge transfer
levels.
The task remains to assign the M'dssbauer peaks
crystallographic positions
in the
ilvaite structure.
The one ferric doublet is easily assigned to Fe
because almost all Fe
is
located at the A site
(A)
to
61
according
Bittner
to Haga and Takeuchi
(1976) and Beran and
Such an assignment is
(1974).
chemically because the A
site is the smaller
No evidence was found for
and Mn).
(compared to Fe
a second doublet with parameters
iron on the B site.
in Section 1.3.
symmetries
to be
expected
for ferric
The outermost ferrous doublet was
(B) on the basis of arguments advanced
Since Fe
predominate over Fe
metal distances
of the two
shows a preference for smaller sites
iron sites and Fe
assigned to Fe
supported crystal-
2+
3+
(A)
(A)+Fe
(B)-Fe
to
(A) due to shorter metal-
(Loeffler et al.,
(Day, 1976; Hush,
.
is expected
1976) and similar
1967),
the Fe
(B) doublet
should be invariant with temperature and increased
charge transfer.
Similarly, the innermost ferrous
doublet, showing dramatic decrease
in intensity with
increasing temperature, was assigned to Fe
(A).
This
doublet varies in intensity from approximately equal
to Fe
2+
(B) at 80K
(as expected from stoichiometry and
from Haga and Takeuchi's site occupancy study) to below
the resolution of our spectrometer at 390K.
The two
intervalence charge transfer peaks were
assigned as follows.
transfer is
As noted above, Fe(B)*Fe(A) charge
expected to contribute weakly, if at all,
Furthermore, the
to observed electron delocalization.
Fe(A)-+-Fe(A)
(i.e. along
interaction in the c-direction
the Fe(A) octahedral chains) occurs.with the shortest
metal-metal distance and has
the possibility of extended
delocalization along the chain.
such metal chain compounds
Day
(1976) discussed
and showed that extended chain
delocalization may be important in increasing intervalence
charge transfer probability
compound showing
(e.g. PtenBr 3., a Class II
some intervalence charge transfer,
with a very. large Pt-Pt distance of 5.60A).
this long-range delocalization effect and
yet
Considering
the fact that
a chain delocalization would be more dependent on a
thermal activation mechanism than a pair delocalization,
the
the innermost charge transfer doublet which shows
largest temperature increase was assigned to Fe
Fe 3+(A) parallel
charge
to c
(in the z-direction).
(A)-+-
The second
transfer doublet was, therefore, assigned to
Fe 2+(A)-+-Fe
(A)
I c
(in the xy-plane),
also correlating
with a short Fe-Fe distance, but not showing dramatic
temperature increase due to the restriction
delocalization.
Figure 21.
These assignments are
to pair
summarized in
Intensity plots in Figure 20 have been
labelled appropriately.
Fe(Ar
80K
IL3
z
5.0
a.
O
04.
MM/SEC
-2.0
Figure
21.
0
Summary of assignments
in ilvaite.
4.0
2.0
of M*ossbauer
doublets
No correlation has been found with Mn content and
extent of charge transfer
in room temperature spectra.
This is to be expected as Mn prefers the Fe(B)
(Haga and Takeuchi, 1976) and would be expected
have no affect on Fe
(A)-+Fe
(A)
to
intervalence charge
The observed variations of room temperature
transfer.
spectra can, however, be directly correlated
amount of
sites
to the
intervalence charge transfer occurring and
relative intensity of charge transfer peaks compared to
trapped valence absorption.
This effect should be
explainable in terms of other sample-to-sample differences
such as formation conditions affecting the
ordering of Fe + and Fe
on Fe(A)
extent of
sites and the ferrous/
ferric ratios.
3.2
Site Occupancies
The site occupancy data of Beran and Bittner
and Haga and Takeuchi
(1976) is further supported by the
Mossbauer results of this research.
Evidence shows that
at 80*K, approximately equal amounts of
as Fe
(A),
Fe
(A),
and Fe
iron are present
(B) with small contributions
from intervalence charge transfer species to the A
occupancies.
(1974)
site
These iron species account for 31.30, 40.03,
and 28.67
percent, respectively, of the total iron when
corrected for Fe
2+
(A) and Fe
3+
(A) present as intervalence
charge transfer peaks.
Little can be said about the site preferences of
Fe
2+
and Fe
preferences
3+
for A or B from
the Mossbauer data as site
from neutron diffraction and x-ray experiments
concern-
Two points
were employed to argue assignments.
ing site occupancies can be made, however.
is a
There
negative correlation between the relative intensity of
the Fe 2+(B)
doublet and Mn content by
electron microprobe.
This indicates that Mn does substitute primarily for Fe
in the B site as expected
from crystal-chemical arguments.
It is, however, difficult to quantify this correlation as
normalization of
impossible due
transfer.
intensities to A site doublets is
to varying amounts of intervalence
Herzenberg and Riley
distorted environment
(i.e. B
(1969) argued for a
site) for ferric iron in
ilvaite in order to produce the large quadrupole
analogous to epidote
(Bancroft et al.,
study, Fe3+
regular
octahedron, on the basis of
site,
the more
site preference data
preference for a small
preference for a distorted site).
splitting
However,
1967).
is assigned to the A
in this
and normal Fe
charge
site
(with Fe2+
It remains
necessary
the large quadrupole
to explain
of Fe
splitting
3+
(A).
The probable answer lies in the effects on "trapped"
ferric valence sites of chain delocalized electrons which
exist even at 80 0 K.
of the electronic
Such electrons may cause distortion
environments throughout the chain and
account for the large Fe
the small Fe
Fe
3.3
(A) quadrupole
(A) quadrupole splitting
the more distorted
site).
Electron Hopping -
A Model
(B) ,
Hush
(1967)
and Day
splitting and
(relative to
some
(1976) have discussed
qualitative theoretical aspects of intervalence
transfer as have Loeffler et al.
(1976) and others.
general, intervalence charge transfer
frequently in the geochemical and
is
In
invoked fairly
spectrochemical litera-
ture with little regard for the theory behind
mechanisms.
charge
such
Similarly, in solid state physics and
materials literature much has been said about theory and
this or that model with little reference to physical
systems which support the model.
In this work, I have
attempted to demonstrate the existence of
intervalence
charge transfer in ilvaite and turn attention now to
constraints upon and
hopping."
implications for models of
"electron-
The foundations for any such model must lie
-Fe
3+
To
couple.
of an Fe-
picture
orbital
a molecular
review briefly,
2+
the Fe
of
treatment
molecular orbital
in a
coordination sphere describes a "mixing" of Fe atomic
(s, p,
orbitals
d)
ligand orbitals
and
These ligand orbitals
most minerals, oxygen orbitals).
are combined in symmetry-adapted
(in ilvaite and
linear combinations
(SALC's) which transform similarly under
operations.
symmetry
The Fe atomic orbitals and oxygen SALC's
are then combined under
symmetry constraints to form
molecular wave functions and corresponding molecular
energy levels.
To a good approximation,
these molecular
energy levels, when filled, correspond to our traditional
picture of a metal
cation with its valence electrons
The molecular wave functions take
surrounded by anions.
the form:
$(a
$*(a
lg
lg
) = c (4s) + c
1
)
= c
1
(4s)
The constants, c 1
-
-(a
+a +a
1
c -(a
2
1
2
-
2
+a
3
4
+a
5
+a
+a +a +a +a +a
2
3
4
5
6
6
)
(bonding)
)
(antibonding)
and c 2 , describe the amount of metal or
ligand "character" to the molecular wave function;
c2 =
0 would be equivalent to a non-bonded Fe(4s)
c1
= 1,
atomic
orbital and cl = 0,
c2 =
c
on the oxygens.
1 would similarly be
and c2 are further
localized-
constrained by
requiring the wave functions to be normalized or
c1
where G
fM
2
+ c2
2
+ c1c2G
= 1
is the group overlap integral given explicity as
L dT and measuring
the amount of interaction between
metal atomic wave functions and ligand SA.LC wave functions.
The amount of "mixing" is,
therefore, dependent on the
degree of overlap between atomic orbitals and SALC's and
on the ratio c 1/c2.
The energies of
the molecular orbitals
may be calculated from the secular equation
H.
- WG.. = 0
(Sample calculations for MnO
Ballhausen
and Gray,
4
3and CrF
6
are given in
1964, Chapter 8).
In order to apply such a molecular orbital treatment
to an intervalence charge transfer couple,
it is necessary
to form Fe SALC's and then to mix these with ligand SALC's
of appropriate symmetry.
Even for the highest symmetry
(each Fe in pure octahedral 0 h),
however, the
reduction on going to an Fe-Fe couple
symmetry
is great.
An edge-
shared octahedral pair can have, at most, D 2 h symmetry
69
and complex computer calculations
are required to generate
energy levels which are, at best, oversimplified models
for real, less symmetric mineral
systems.
This method provides an analytic
solution for
the
molecular wave functions and energy levels but is extremely
An alternate way
difficult to solve.
of picturing
the
interaction is as a small distortion of a normal octahedral
FeO 6 system employing perturbation calculations.
and Day
(1974) describe the wave
Mayoh
function for the Fe
d6
electron as
3+
2+
.O
$0A
B
and for the electron transfer as
3+
$' =
A
2+
IPB
These wave functions transform with the
the ground and excited states are,
same symmetry, and
therefore, represented
by the normalized.linear combinations,
$=
(1a-
ex=
(1
The constant a
-
1/2
) /
+
a$
)$
+
4$
is the delocalization
coefficient of the
tion of the system is
sites.
'$0
If a is very small, $
system.
If
a
is
and Fe
as discrete Fe
large
(
.7071) ,
$
the best descrip-
and
f
electron is shared equally by both sites,
over the entire system.
x0+
1 and x >> y.
If a
i.e. delocalized
=
is small but finite, $
The valence states are essentially
trapped but there exists a small,
exchange.
and the
+$
a0
octahedral
In any system with 10
23
finite probability of
or mor.e atoms, even
assuming a certain statistical disorder to the necessary
Fe 2+-Fe3+ pairs as in ilvaite, such a
finite probability
should affect mineral properties.
In the case of a
small but finite,
the probability of
transfer can be increased by addition of energy to increase
the stability of $P
can be seen
relative to $0.
Such energy effects
in the M*ssbauer spectrum of
ilvaite.
Thermal
vibrational energy of the electrons increasing with temperature increases the number of 57Fe nuclei which see, over
the lifetime of
the Mossbauer transition
statistically averaged
(10
-7
sec),
a
electronic environment due to rapid
intervalence charge transfer.
Similarly, other materials
may require larger energy inputs to show charge transfer,
an example of which would be
semiconductor materials
requiring electric activation over a "band gap."
A qualitative method of picturing
ments is supplied by Day
(1976).
the energy require-
Day pictures the energy
levels of the Fe + d6 electron as a potential well with
vertical energy scale
and horizontal vibrational scale
centered at r 0 , the equilibrium radius of the atomic
orbital.
Such a well would be similar to a Hartree-Fock
potential modified to consider only distance greater than
r0 from the Fe nucleus and, therefore,
symmetric about r 0 *
Within such a well exist discrete vibrational
energy levels
the
with a ground state dependent on the configuration of
system.
Day proposed "mixing" two such potential surfaces
analogous to "mixing" two FeO
sets.
Figure
molecular orbital function
identical potential wells representing
Fe
Two
22 illustrates such an interaction.
2+
3+
3+
2+
or
and Fe Fe
Fe
B
A
B
A
$
0
and $
1
the configurations
have been drawn.
to optical transitions
Energies corresponding
from the
2+*
3+
3+
2+
ground state Fe A FeB
to the excited state FeA FeB
2+
-
(E OP
3+
and thermal transitions from the ground state FeA FeB
the delocalized
state
2+
3+
3+
2+
)
Fe
+Fe
(FeA Fe
B
A
B
A
labelled appropriately.
(E
TH
)
are
At the crossing point of
the two
configurations, there occurs a resonance interaction
similar to the symmetry-constrained interaction which
bombines $0
and $
to generate $i
and $e
.
to
Two new
surfaces are then formed, corresponding to $p
g
= fI
2+
$P
3+
H$J
B
A
H
+
0
ex
H
A
2+
-
=
2+
A
-
B
3+
IP
B
3+
dT4+1)
H$P A
$B
2+
2+
H$i
res
2+
dT
dT
in an energy
field
in the
the extent of delocalization
state in a wave function field.
For H
the electron is completely delocalized and
H
$B
determines the extent of electron
res
just as a determines
is a Class
3+
A
res
delocalization in the ground state
ground
B
A
3+
$~
Hres
1
The magnitude of H
3+
$
,
and given explicity by,
separated by the energy, H res,
E
Pe
and
III compound
(dashed lines
res
large,
the material
in Figure 22).
For
small, the electron is trapped in a localized ground
state and has
some probability with appropriate thermal
excitation of undergoing transfer.
is illustrated by the dotted
This Class II
lines in Figure 22.
This potential energy surface picture
the ilvaite system in Figure 23.
trates the Fe
(A)+Fe
(A) J
system
is applied to
The top diagram illus-
c tran'ition with the
inter-
E
A
ET
Figure 22.
Potential surfaces for the configurations
A3 +PB 2 + (after Day, 1976).
*A 2 +V B 3 + and
Interactions shown for Hres large giving
delocalized ground state electrons and a
Class III system (dashed lines) and Hres
small giving trapped valence ground states
and a Class II system.
XY
Figure 23.
Potential surface pictures for ilvaite
intervalence charge transfer interactions.
Top diagram shows the pair interaction
Fe 2 +(A)+Fe 3 +(A) i c and the lower diagram
2
illustrates the chain interaction Fe +(A)+
Fe 3 +(A)
11
c.
75
action of two potential surfaces of small H
res
.
Valences
are trapped and thermal activation provides a small number
of electrons with the necessary energy
to overcome the
Increasing temperature
transfer barrier.
should increase
level
this electron population of the transfer
concurrent
with depopulation of the trapped valence ground states.
supported by experimental results showing
This picture is
absorption with
(A) and Fe 3+(A) Mossbauer
decreased Fe
increasing temperature simultaneous with increased Fe
Fe
3+
intervalence charge transfer
(A)
absorption.
lower diagram shows the potential surface
for Fe
(A)--Fe
(A)
c.
3+
2+
B
A
3+
A
2+
B
3+
C
2+
.*D
$$
A
2+
D
$
.
,
*
2+
possible over more
D
C
3+
B
3+
3+
B
3+
A
interactions
surface representing
Potential
3+
2+
C
$
2+
.C
2+
D
.
.,have been
allowed to interact with the requirement that H
.res
H
res
.
3k
.
The
Because the octahedral chains
run parallel to c, delocalization is
than two sites.
(A)+
Valences' remain trapped
..
i3
=
in the ground state
but thermal activation now puts the delocalized electron
into a chain-wide molecular energy level.
The opportunity
for this long range delocalization should increase the
stability of this energy level relative
ization and hence Hres
11
to a pair delocal-
c has been drawn as greater than
76
i C.
Hres
The long range delocalization effects on the
stability of Fe
(A)-Fe
c should be reflected
(A)
in experiment, as has been reported above.
Fe
(A)
range
Fe 2+(A)+
c increases dramatically over the temperature
320-370*K as thermal activation facilitates
population of the intervalence charge transfer molecular
energy levels.
In light of the molecular orbital model of intervalence
charge transfer just described,
the variables
it is necessary to examine
important in physical systems.
The inter-
valence charge transfer probability depends on the
magnitude of Hres
compared to
2+
3+
3+
Hl0 and
2+
$4) dT,
B
Hyi
explicitly as
f
integral H01
which Ballhausen and Gray
A
$B
B
H4 A
A
H res,
.
given
is just the exchange
(1964) approximate
as
H
The H .. terms
11.
and G..
res
2G. .(H..
x
H
)1/2
JJ
1].
are just the energies of each configuration
is the group overlap integral
coordination
H
= 131)
spheres.
for the two metal
The presence of G..
suggests
that
will be a function of polyhedral connection with
face-shared > edge-shared >>
of the above connections, G.
corner-shared.
.
(and hence H
iJ
res
Given any
) will
increase with decreasing metal-metal distance, as
tion sphere orbitals increase overlap.
Hres
coordina-
should also
depend on geometries of polyhedral orientations with
special regard to the relative orientations of orbitals
in the metal
coordination sphere.
Hres will also be maximized when energy differences
between configurations are minimized, as occurs for all
interaction integrals of
the form H...
Intervalence
charge transfer will, therefore, be favored for systems
where
E$0 -
1
is small.
Since the
ionization potential
is equal and opposite to the electron affinity
of Fe
of Fe
E$
,
energy differences in such a homogeneous couple
can only arise from differences
in ligand fields.
This is
not the case for heterogeneous couples where ionization
potential and electron affinity contribute to the
energies
E40
and E$
.
Energy differences
in ligand
fields are those due to coordination number,
and mean M-0 dista.nce
requirement
(or bond strength).
is that the energies of the
the perturbation calculation, $
similar.
2+
A
p
B
3+
over mixed systems.
The primary
levels involved in
and $p
A
Therefore, octahedral-octahedral,
tetrahedral, or cubic-cubic
site symmetry,
3+
$2
B
2+
,
be
tetrahedral-
systems will be preferred
Likewise, transitions between
octahedral sites of' similar symmetry are preferred over
transitions between regular and distorted octahedra.
ilvaite
In
(and in magnetite) these requirements are met by
intervalence charge transfer between crystallographically
equivalent sites.
For mixed cubic-octahedral, cubic-
tetrahedral, and octahedral-tetrahedral systems, the
preference would be cubic-tetrahedral >>
tetrahedral >
the t
2g
and
octahedral-
e
cubic-octahedral due to the inversion of
g
levels.from octahedral to cubic or
tetrahedral symmetries.
Such mixed systems would require
crystal field splittings of appropriate magnitude to bring
t2 and e energy levels into correspondence.
Experimental results from ilvaite M'$ssbauer studies
require intervalence charge transfer models to show increasing transfer with increasing temperature.
satisfies
this requirement.
The
above model
(This model also explains
intensity of
the inverse temperature dependence of
optical transitions for other Fe
2+
3+
+-Fe
In such transitions, the ground state
mineral
systems.
is depopulated with
increasing temperature and optical transition from the
ground state to an excited transfer
intensity.)
state decrease in
Results also require models to explain
decreased peak width with
Sawatzky et al.
increasing temperature.
(1969) employed a mathematical model with
an activation energy 6 and
energy is
1/AP
= e-
simply ETH in Figure 22,
/kT.
the
This activation
thermal energy
required to transfer the electron from a ground state to
a delocalized
E.nergy level.
Other requirements of
polyhedral connection, coordination number,
symmetry,
and mean metal-metal distance have been discussed
and are supported by this research.
above
80
BIBLIOGRAPHY
Araki, T.
Zoltai, 1972, Crystal structure of
and T.
Z. Krist.,
babingtonite.
Ballhausen,
C.J. and H.B.
Gray,
135,.355-373.
1973,
Introduction for
Inorganic Chemists and Geochemists,
A.G. Maddock, and R.G. Burns, 1967,
Bancroft, G.M.,
Applications of
.mineralogy:
the Mossbauer effect to silicate
I. Iron silicates of known crystal
Geochim. Cosmochim. Acta, 31,
structure.
of
Mossbauer Spectroscopy: An
N.Y.
McGraw-Hill,
Bartholome, P.
and F.
Dimanche, 1967,
ilvaite in Italian skarns.
Belgique,
Baur, W.H.,
Orbital
N.Y.
Theory, W.A. Benjamin, Inc.,
Bancroft, G.M.,
Molecular
1964,
90,
1970,
(1966-1967),
2219-2246.
On the paragenesis
Ann. Soc. Geo.
Bull.
5,
17-48
(B533-564).
Bond length variation and distorted
coordination polyhedra in inorganic crystals.
Trans. Amer. Cryst. Assoc.,
Belov, N.V. and V.I.
of ilvaite.
89
6,
129-155.
Mokeeva, 1954, The crystal
structure
Trudy Inst. Krist. Akad. Nauk. SSSR, 7,
402 (Russ.).
Beran, A.
and H. Bittner, 1974, Untersuchungen zur
kristallchemie des ilvaites.
Petr. Mitt.,
21,
11-29.
Tschermaks Mineral.
-- wmw
Borshagovskii, B.V.,
A.S.
Marfunin, A.R. Mkytchyan,
1971, Mossbauer
G.N. Nagyaryan, and R.A. Stukan,
study of isomorphous
IL
substitution in ilvaite.
Phys. Stat. Sol., (B)43, 479-482.
Burns, R.G.,
K.M. Parkin, B.M. Loeffler, I.S.
R.M. Abu-Eid, 1977,
Leung, and
Further characterization of
spectral features attributable to titanium on the
7th Lunar Sci. Conf.
Proc.
moon.
(1976),
2561-2578.
1971, Multisystems analysis of the relative
Burt, D.M.,
stabilities of babingtonite and
Inst. Washington Yearbook 70,
ilvaite.
Carnegie
189-197.
1972, Mineralogy and Geochemistry of Ca-Fe-Si
Burt, D.M.,
Skarn Deposits.
Ph.D. Dissertation, Harvard Univ.,
256 pp.
Day, P.,
1976,
Mixed valence chemistry and metal chain
compounds.
In: Keller, H.J.
(ed.),
Low Dimensional
Cooperative Phenomena, Plenum Press, N.Y.
Dollase, W.A.,
1968, Refinement and comparison of
structures of
53,
zoisite and clinozoisite.
the
Amer. Mineral.,
1882-1898.
Gerard, A.
and F.
Grandjean, 1971, Observation by Mossbauer
effect of an electron hopping process in ilvaite.
SSC,
9,
1845-1849.
82
Ghose, S.,
1969, Crystal chemistry of
iron.
(K.H. Wedepohl,
ed.),
of Geochemistry,
In:
Handbook
Vol. II/l,
26A, Springer, Berlin-Heidelberg.
Gossner, B.
Reichel, 1932,
and C.
Centralbl.
einiger sog. orthosilite.
Palaont. Abt. A Jg.
Grandjean, F.
and A.
spectroscopy of the
Haga, N.
of
and Y.
electronic hopping process
in
Takeuchi, 1976, Neutron diffraction study
ilvaite.
Hanisch, K.,
1975, Analysis by Mossbauer
16, 553-556.
SSC,
ilvaite.
Min. Geol.
255-229.
1932,
Gerard,
Uber das kristallogitter
Z. Krist.,
144,
1967, Untersuchungen
von OH-gruppen in mineralen.
S-161-174.
Uber del orientierung
Dissertation Univ.
Gottingen.
Hernon, R.M. and W.R. Jones,
1969, Ore deposits of
central mining district Grant Co.,
In:
Ridge, J.D.
1933-1967, v.
the
New Mexico.
Ore Deposits of the U.S.,
(ed.),
2, AIME, N.Y.,
p.
1211-1237.
Herzenberg, C.L. and D.L. Riley, 1969, Oxidation states
and
site symmetries of iron
Mossbauer spectroscopy.
Hewitt, W.P.,
1968,
in ilvaite using
Acta Cryst.,
A25, 389-391.
Geology and mineralization of the
San Antonio mine, Santa Eulalia District, Chihuahua,
Mexico.
AIME Trans.,
241,
228-260.
1967,
Hush, N.S.,
Intervalence-transfer
absorption:
Part
2, Theoretical considerations and spectroscopic
Prog. Inorg. Chem.,
data.
391-444.
1960, uber lievrit aus dem huttal bei clausthal.
Krause, H.,
N.
8,
Jb. Min. Abh.,
94,
1277-1283.
R.G. Burns, and J.A.
Loeffler, B.M.,
Tossell, 1976, Metal-
metal charge transfer transitions:
Interpretation
of visible-region spectra of the moon and
Proc. 6th Lunar Sci. Conf.,
materials.
lunar
2663-2676
3,
(1975).
and P.
Mayoh, B.
Day, 1972, Trapping of valence states in
a ruthenium(II,III)-pyrazine
Soc.,
Mayoh, B.
and P. Day, 1974,
Charge transfer in mixed
Part VII,
of valence delocalization
silicates.
Mongiori, R.
Perturbation calculations
in iron(II,III)
1968, A
846-852.
reconsid-
structure of titanite CaTiOSiO4.
Petr. Acta (Bologna),
14,
Min.
123-141.
1916, The main mineral zone of the Santa
Eulalia District, Chihuahua, Mexico.
51,
cyanides
J. Chem. Soc. Dalton Trans.,
and L. Riva di Sanseverino,
eration of the
Prescott, B.,
J. Amer. Chem.
2885.
94,
valence solids:
and
complex.
57-99.
AIME Trans.,
84
Robin, M.B.
and P.
Day, 1967, Mixed valence chemistry.
Adv. Inorg. Chem. Radiochem.,
247.
10,
1973, Why MISFIT when you already have X 2
Ruby, S.L.,
In: Mossbauer Effect Methodology 8,
(J. Gruverman,
Plenum Press, N.Y.
ed.),
Sawatzky, G.A.,
J.M.D. Coey,
Mossbauer study of
sites of Fe 3 0 .
1939,
Schmitt, H.,
J.
electron hopping in the octahedral
Appl. Phys.,
Shannon, E.V.,
187-208.
115,
1918, On the occurrence of
ilvaite in the
South Mt. mining district, Owyhee Co.,
Am. J.
Sci. 4th Ser.,
Shannon, E.V.,
1402-1403.
40(3),
The central mining district, New
AIME Trans.,
Mexico.
and A.N. Morrish, 1969,
45,
Idaho.
118-125.
U.S. Nat.
1926, The minerals of Idaho.
Museum Bull. 131.
H.J. Augard, and J.
Stone., A.J.,
Fenger, 1969, General
constrained non-linear regression for Mssbauer
spectra.
Publ. Danish Atomic Energy Comm.,
R150-M-
1348.
Takeuchi, Y.,
HCaFe
3+
1948, The structure of lievrite
2+
Fe 2 Si 0
2 9
2
x-rays.
Osaka Univ.,
5,
8-14.