Microlite: Mineralogy and Radiation Damage as a Function of Uranium-Content
in the Harding Pegmatite, Taos County, New Mexico
J. McLain Pray1,*, Zachary M. Weiss1,**
1
Department of Geological Sciences, University of Michigan, 1100 North University Avenue, Ann Arbor, MI 48109, U.S.A.
ABSTRACT
The Harding pegmatite mine is the world’s largest source of microlite, the Ta-rich endmember of the pyrochlore
group. The pegmatite is stratified into 8 mineralogically-unique zones, 3 of which contain microlite. Of these,
uranium-rich microlites have been found primarily in the latest-formed zone, the “spotted rock” zone, which
evidences extreme magmatic differentiation. Based on samples reportedly collected from the spotted rock zone,
U-rich microlites were found to be deficient in Na, suggesting that U is incorporated into the A-site of the
microlite structure. Lack of Na in the surrounding matrix suggests that U only substitutes into the microlite
structure when the surrounding melt is deficient in Na. Non-U-bearing microlites were found in Na-rich
matrices, suggesting crystallization in a less-differentiated magma. The occurrence of pucherite (BiVO4)
suggests non-U-bearing microlites may have been taken from a different zone within the pegmatite, such as the
quartz-lath spodumene zone. With increasing U-content, microlite samples showed an increase in O2-/F- ratios
in order to balance the high charge of U relative to Na. U-bearing samples were found to be amorphous,
indicating radiation damage over the course of the last 1.3 billion years since their emplacement. This suggests
a lack of annealing processes within the pegmatite, which is expected based on the temperature constraints
calculated for the post-crystallization pegmatite.
Keywords: microlite, uranium, magmatic differentiation, fractional crystallization, radiation damage, Harding
pegmatite, cation substitution
INTRODUCTION
A pegmatite represents the last stage of
crystallization of an igneous pluton, formed from
mature, highly differentiated magmas that remain
after common mineral-forming elements have
crystallized. Compared with less mature plutons,
pegmatites crystallize at lower depths (4-10 km)
and temperatures, and they contain large
concentrations of rare elements that are not favored
by the crystal structures of less mature melts. As
the melt volume decreases, the concentration of
these rare elements increases, eventually
crystallizing rare minerals. Crystallizing from the
outside inward, the mature magma that forms
pegmatites is rich in volatiles, such as water, which
facilitates the rapid transport of elements to
crystallization sites. These conditions lead to the
formation of large crystals, characteristic of
pegmatite bodies (Jahns 1953, Harding “Walking
Tour”).
*jmcpray@umich.edu
**zmweiss@umich.edu
The Harding Pegmatite, located near Taos, New
Mexico, in the Picuris Range 10 km east of Dixon
(Fig. 1), was discovered at the turn of the 20th
Century and was first mined in 1919. Mining at the
pegmatite occurred in three periods. The first period
of mining occurred from 1919-1930. Lepidolite, a
lilac-colored, lithium-bearing replacement mineral,
was mined and utilized in the manufacture of
opaque glass. In 1931, microlite was first located in
the main quarry. Wartime demand necessitated the
mining of microlite, and from 1942-1947 the mine
produced 10,000 kilograms of the mineral. The
final period of mining occurred from 1950-1958,
when beryl was excavated (Jahns and Ewing 1976).
The formation of the Harding Pegmatite occurred at
a depth of 6-7.5 km (Brookins et al. 1979),
intruding the country rock of Vadito Group
amphibolite (hanging wall) to the south and quartzmuscovite schists (footwall) to the north. The
pegmatite body is 2500 feet long, 150-500 feet
wide, and up to 25 m thick. The pegmatite
crystallized into eight mineralogically-distinct
subhorizontal zones, each between 5 and 20 feet
thick, which date in age from 1.529 Ga to 1.121 Ga,
The rose muscovite and lepidolite zones are
secondary, hydrothermal replacement zones, and
the perthite zone was formed post-crystallization
from exsolution processes (Figs. 2-3) (Jahns and
Ewing 1976, Lumpkin et al., 1986).
Microlite is an isometric oxide of space group
Fd3m, representing the Ta-rich end member of the
pyrochlore group. Minerals of the pyrochlore group
have the general formula A2-mB2X6Y1-n, where m
and n range from 0 to 1. The cubic A-site typically
contains Na and Ca, but may include Pb, K, Mg,
Mn, Bi, Ba, Ce, Cs, Sn, Sr, Th, U6+, U4+, Th, Y, Zr,
Fe2+, Sb, or REE; the octahedral B-site typically
contains Ta5+ and Nb5+, but may contain Ti, Fe, W,
Nb or Sn; the Y-site includes O, OH and F; and the
X-site normally contains only O (Lumpkin et al.
1986, Barthelmy 2009). In order to be considered
microlite, the mineral must contain more Ta than
Nb, and the sum of Nb and Ta must exceed double
the amount of Ti. The ideal end member formula of
microlite is NaCaTa2O6F. In the molecular
structure, B-site octahedra are arranged in
hexagonal channels, into which the A-, X-, and Ysite ions are found (Figs. 4, 7) (Lumpkin 1989).
In pegmatites, microlite is one of the last minerals
to crystallize, after the melt has become
differentiated and saturated in Ta. This late-stage
crystallization is evident in the Harding pegmatite,
where microlite is found protruding into cracks and
cavities and as replacement pseudomorphs after
spodumene (Brookins, Ewing et al. 1979). Primary
microlite is found in two central, late-forming
zones, the microcline-spodumene zone (“spotted
rock”, named for purple lepidolite replacements that
occur throughout the zone), and the quartz-lath
spodumene zone, as well as the perthite zone (see
Fig. 2). Microlite crystals mined at the pegmatite
ranged in size from microscopic crystals and grains
to large euhedral crystals between .2 and .8 inches
wide. Rarer microlite crystals were found to be up
to 3 inches long (Jahns and Ewing 1976).
The late crystallization of microlite within the
pegmatite led to the incorporation of many rare
elements other than Ta, especially in the final stages
of the cooling process. Of the three zones
containing microlite, U6+-bearing samples were
found primarily in the latest-forming spotted rock
zone (see Fig. 3), some containing up to 5 weight%
U (Brookins et al. 1979). The high concentration of
U, coupled with the old age of the pegmatite, makes
the spotted rock zone a likely type-locality to find
metamict crystals. Accordingly, fission and -recoil
paths have been discovered in many U-bearing
microlites from the Harding pegmatite (Lumpkin
1989).
The purpose of this paper is to present our analyses
of microlite samples reportedly collected from the
spotted rock zone. We examined the ionic content
of the microlites and surrounding matrix as a
function of U-content. Finally, we sought the
presence of radiation damage in the U-bearing
microlites.
EXPERIMENTAL METHODS
SEM: Microlite thin sections were carbon coated to
allow electrical conductivity. Samples were
analyzed at 20keV. Microlite’s possession of Ta
gives the mineral high density (S.G. 4.2-6.4), due to
the high atomic number of Ta and tight radical TaO bonds created by Ta’s high oxidation state. This
high density allows microlite grains to be
distinguished visually from the matrix in SEM
imaging. EDS analysis determined bulk chemical
composition of both microlite crystals and matrix.
Optical Microscopy: Microlite samples were
observed under optical microscopy utilizing PlanePolarized Light as well as Cross-Polarized Light.
Microlite’s isotropic properties allowed it to be
identified in XPL. Samples were analyzed for
presence of iron staining, which indicates secondary
precipitation in cracks and voids caused by
radiation damage.
Raman Spectroscopy: Raman spectra were taken
of microlite samples, utilizing an Argon Beam.
Raman spectra were compared to known microlite
spectra to confirm the presence of microlite. Ubearing samples were examined for broadening of
absorption peaks, indicative of amorphism that
suggests radiation damage. Raman spectroscopy
was used because the technique does not damage
the samples and requires little preparation.
2
RESULTS
SEM: Both U-bearing and non-U bearing
microlites were found. The microlites appeared
bright white, clearly delineated from the darker
matrix (Figs. 5-6). EDS analysis calculated ionic
ratios, which were consistent with the accepted
ratios for microlite (Figs. 7-9). No radiation
damage microfractures were observed in SEM
analysis. However, the U-bearing microlites
appeared generally less euhedral than non-Ubearing microlites, and the surrounding matrices
looked more disrupted in the U-bearing samples.
Matrices had distinct chemical compositions (Figs.
10-12). Non-U-bearing microlites were set in a Narich matrix (Albite), while U-bearing samples were
set in a Na-depleted matrix (K-spar and quartz).
In addition, the rare mineral pucherite, a Bismuth
Vanadate (BiVO4), was discovered in the non-Ubearing sample. Pucherite’s high density (6.25
S.G.) distinguished it clearly in the SEM, and EDS
analysis confirmed the correct ionic ratios (Figs. 1315).
Optical Microscopy: Microlites were identified
under XPL, where they appeared extinct. In PPL,
samples appeared brown, with no pleochroism and
high relief (refractive index 2-2.2). No iron staining
was observed in optical analysis (Figs. 16-17).
Raman Spectroscopy: The Raman spectra of
microlite were consistent with known microlite
spectra (Fig. 18-19), confirming the SEM
identification of microlite. The U-bearing microlite
spectra were flat curves with broad peaks at lower
intensities, indicating metamictization.
DISCUSSION
Microlite Structure Analysis
In microlite, research documents an inverse
correlation presence of U and the major A-site
cations (Na and Ca), suggesting that U occupies the
A-site in the crystal structure (Lumpkin et al. 1986).
However, the ionic radius of U6+ is much smaller
(0.073 nm) than that of Na (0.118 nm) and Ca
(0.112 nm). Using Pauling’s Coordination
Principal, assuming an ionic radius of O at 0.133
nm, RU/RO is 0.549, indicating preference for an
octahedral coordination. This suggests that
microlite would not preferentially incorporate U
into its cubic A-site, unless the melt were deficient
in the regular A-site cations. In such a melt,
however, U would be forced to occupy the A-site,
because its radius is larger than Ta (.064 nm), the
typical B-site cation. The Harding pegmatite is an
example of such a case. It has been noted that the
bulk chemical composition of the pegmatite is
“strikingly poor in Ca” (Jahns and Ewing 1976),
and that magmatic differentiation significantly
depleted Na in the late-formed spotted rock zone.
The inverse correlation between U-content and Asite cations was observed in our samples. However,
the correlation between U-content and Na-content
was much stronger than the correlation between Ucontent and Ca-content. The non-U-bearing
microlite sample contained 9.52 atomic% Na and
11.05 at% Ca. The U-bearing microlite sample
contained 2.59 at% Na, and 8.74 at% Ca (Figs. 8-9).
Given the deficiency of Ca in the melt, however, it
seems more likely that U should replace Ca and not
Na.
This contradiction may be explained through
examination of the chemical compositions of the
matrices surrounding the samples. The presence of
a Na-rich matrix (Fig. 10) surrounding the non-Ubearing sample and a Na-depleted matrix (Figs. 1112) surrounding the U-bearing sample indicate that
the U-bearing sample crystallized in a melt much
more depleted in Na. Given the depletion of Na
throughout the crystallization history of the
pegmatite, U-bearing microlites appear to have
crystallized later than non-U-bearing microlites.
Prior research also documents an increase in O2-/Fratio with increasing U-content, in order to balance
the high oxidation state of U relative to Na and Ca
(Lumpkin et al. 1986). This relationship is evident
in our samples. The U-bearing sample contains
60.84 wt% O and 3.97 wt% F, while the non-Ubearing sample contains 49.54 wt% O and 7.20
wt% F.
Radiation Damage
The near-level curve of the U-bearing microlite
indicates a completely amorphous structure,
signifying high levels of radiation damage. Such
heavy amorphization indicates a lack of annealing
3
processes in the pegmatite. Brookins et al. found
that metamict microlite begins annealing between
300-400C. However, assuming a 30C/km
geothermal gradient, the pegmatite’s emplacement
depth indicates an approximate post-cooling
ambient temperature of 180-225C. Furthermore,
lepidolite replacement in the pegmatite has been
constrained to temperatures of 265 + 25C
(Brookins et al. 1979). These temperature
constraints further confirm that annealing processes
have not likely played a large role in the postcrystallization pegmatite.
Sample Locations
Research documents an association of bismuth and
microlite, most prominently in the quartz-lath
spodumene zone (Spilde 1999). The Na-rich matrix
and lack of U in the microlites of the pucheritebearing sample are characteristic of formation in an
earlier stage within the pegmatite, such as the
quartz-lath spodumene zone. Thus, due to the lack
of knowledge regarding the exact locations of the
samples within the pegmatite, these observations
suggest that the pucherite-containing sample may
have been taken from the quartz-lath spodumene
zone.
would better constrain the fractional crystallization
history in the Harding pegmatite and also clarify the
methods by which U is incorporated into the
microlite structure.
Microlite analysis using Raman spectroscopy is still
in its infantile stage. Additional spectra should be
created from multiple microlite samples, both
crystalline and amorphous, collected from diverse
locations. By heating the metamict samples and
comparing the Raman spectra of the resultant
recrystallized microlite, the presence of radiation
damage can be further confirmed and the annealing
process better understood.
Acknowledgements
We are indebted to Professor Rod Ewing for
providing the slides and resources to complete this
project. We extend our thanks to Devon Renock for
his guidance in conducting our research and in
analyzing our samples in the SEM, and Lindsay
Shuller for conducting SEM analysis. Finally, we
would like to thank Maik Lang for his help in
detecting radiation damage and conducting the
Raman Spectroscopy analysis of our samples.
REFERENCES
CONCLUSIONS
The strong correlation between U/Na content and
the relatively weak correlation between U/Ca
content is intriguing. The ability of a deficient Ca
supply to linger until the late stages of
crystallization, while Na becomes thoroughly
depleted, contradicts Bowen’s Reaction Series.
Perhaps secondary processes rereleased Ca into the
melt shortly before crystallization of the spotted
rock zone began. These processes are addressed
elsewhere in the literature.
No matter what processes were responsible for Ca’s
concentration in the spotted rock zone, the
assumption that all samples came from the spotted
rock zone suggests that the same magmatic
differentiation processes characteristic of the bulk
pegmatite were still active at the late stages of
crystallization. Further studies should look for
subzones within the spotted rock showing Na-rich
matrix around non-U-bearing microlites and Nadepleted matrix around U-bearing microlites, which
may illustrate this differentiation. Such studies
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5
Fig. 1: Location of the Harding pegmatite (Fig.
from Brookins et al. 1979).
Fig. 2: The main zones of the
Harding pegmatite, as seen on the
main quarry wall. Notice central
location of U-bearing microcline
spodumene zone (spotted rock)
and quartz-lath spodumene zone.
(Figure from Lumpkin 1998).
6
Fig. 3: Ages of Harding pegmatite zones. Notice
spotted rock zone is designated “whole rock,”
indicating the age of the original crystals and not
the replacement minerals. The rose muscovites and
lepidolites are replacement minerals, and the
perthites formed post-crystallization from
exsolution processes. The ages marked with
asterisks were extrapolated, and are viewed as
anomalous. Thus the spotted rock zone is the
youngest primary crystallization zone within the
pegmatite (Figure from Brookins et al. 1979).
Fig. 4: Unit cell of microlite. B-site octahedra
are in blue, creating hexagonal channels, into
which large octhedrally coordinated A-site
cations, as well as X- and Y-site anions are
found.
7
Fig. 5: Microlite in non-U bearing sample. Notice euhedral nature of the grains and generally undisturbed
matrix.
Fig. 6a: SEM image of U-bearing microlite sample.
Notice subhedral nature of the grain and fracturing
in the matrix.
Fig. 6b: SEM image of U-bearing microlite sample.
Notice subhedral nature of the grain and fracturing
in the matrix. Color contrast has been reduced in
this image in order to see the surface texture.
8
Microlite
Molecular Weight = 529.02 gm
Sodium
Calcium
Tantalum
Hydrogen
Oxygen
Fluorine
6.52
3.79
68.41
0.06
20.87
0.36
%
%
%
%
%
%
Na
Ca
Ta
H
O
F
8.79
5.30
83.53
0.51
%
%
%
%
Na2O
CaO
Ta2O5
H 2O
0.36 % F
Fig. 7: Accepted compositional weight percentages of microlite (Figure from Barthelmy 2009).
Fig. 8: Graph shows SEM analysis of non-U-bearing microlite. Chart shows EDS
calculations of ionic weight percentages of same sample. Compare with Fig. 7.
9
10
Fig. 9: Graph shows SEM analysis of U-bearing microlite. Chart shows EDS calculations of
ionic weight percentages of same sample. Notice significant reduction in Na-content and
increase in O2-/F- relative to Figs. 7 and 8.
11
Fig. 10: Albite matrix surrounding non-U-bearing microlite sample. Note presence of Na.
12
Fig. 11: K-spar matrix surrounding U-bearing microlite sample. Notice absence of Na.
13
Fig. 12: Quartz matrix surrounding U-bearing microlite sample. Note absence of Na.
14
Fig. 13: Pucherite (white) occurring in non-U-bearing microlite sample.
Pucherite
Molecular Weight = 323.92 gm
Vanadium
Bismuth
Oxygen
15.73
64.52
19.76
______
100.00
%
%
%
%
V
Bi
O
28.07 % V2O5
71.93 % Bi2O3
______
100.00 % = TOTAL OXIDE
Fig. 14: Accepted compositional weight percentages of microlite (Figure from Barthelmy 2009).
15
Fig. 15: SEM/EDS data for pucherite, taken from non-U-bearing sample. The sample size was smaller
than the resolution of the SEM, so the data contains elements from the background matrix (Al, Fe, O)
that are not included in the Pucherite. Note Bi/O/V ratios compare favorably with the accepted values in
Fig. 14. High Th contents were found, but the reasons for its presence are beyond the scope of this paper.
16
Fig. 16:
Photomicrograph
image of Ubearing microlite
from Fig. 5 (on
left) in PPL.
Notice brown
color and high
relief.
Fig. 17:
Photomicrograph
image of Ubearing microlite
from Fig. 5 (on
left) in XPL.
Notice extinction.
17
Fig. 18: Raman spectra of non-U-bearing crystals (blue) and U-bearing crystals (green) from the spotted rock
zone. Notice the correlation between the blue curve and the known microlite Raman spectra in Fig. 19,
confirming the SEM detection of microlite. Notice green curve lacks significant peaks, indicating amorphous
structure due to severed chemical bonds, indicating radiation damage.
Fig. 19: Known Raman spectra for microlite (from Downs 2006). Notice similarity to blue curve in Fig. 18.
18