Mineralogical Magazine (2020), 84, 131–141
doi:10.1180/mgm.2019.57

ArticleUroxite and metauroxite, the first two uranyl oxalate minerals
Anthony R. Kampf1*, Jakub Plášil2, Barbara P. Nash3, Ivan Němec4 and Joe Marty5

1

Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA 90007, USA; 2Institute of Physics ASCR,
v.v.i., Na Slovance 1999/2, 18221 Prague 8, Czech Republic; 3Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112, USA;
4
Department of Inorganic Chemistry, Faculty of Science, Charles University, Hlavova 8, 128 43 Prague 2, Czech Republic; and 55199 East Silver Oak Road, Salt Lake
City, UT 84108, USA

Abstract
Uroxite (IMA2018-100), [(UO2)2(C2O4)(OH)2(H2O)2]⋅H2O, and metauroxite (IMA2019-030), (UO2)2(C2O4)(OH)2(H2O)2, are the firsttwo uranyl oxalate minerals. Uroxite was found in the Markey mine, Red Canyon, San Juan County, Utah, USA and in the Burro mine,
Slick Rock district, San Miguel County, Colorado, USA. Metauroxite was found only in the Burro mine. Both minerals are post-mining
secondary phases found in efflorescent crusts on mine walls. Uroxite occurs as light yellow striated blades exhibiting moderate neongreen fluorescence, ca 2 Mohs hardness with good {101} and {010} cleavages. Calculated density = 4.187 g/cm3. Optics are: biaxial(–), α = 1.602(2), β = 1.660(2), γ = 1.680(2) (white light), 2Vmeas. = 59(1)°, 2Vcalc = 59.1°, moderate r > v dispersion, orientation Y = b,
Z ∧ a = 35° in obtuse β and it is nonpleochroic. Metauroxite occurs as light yellow crude blades and tablets exhibiting weak green–
grey fluorescence, ca 2 Mohs hardness with good {001} cleavage. Calculated density = 4.403 g/cm3. Approximate optics are: α′ =1.615(5) and γ′ = 1.685(5). Electron probe microanalysis provided UO3 79.60, C2O3 10.02, H2O 10.03, total 99.65 wt.% for uroxite
and UO3 82.66, C2O3 10.40, H2O 7.81, total 100.87 wt.% for metauroxite; C2O3 and H2O are based on the structures. Uroxite is monoclinic, P21/c, a = 5.5698(2), b = 15.2877(6), c = 13.3724(9) Å, β = 94.015(7)°, V = 1135.86(10) Å3 and Z = 4. Metauroxite is triclinic, P1, a =5.5635(3), b = 6.1152(4), c = 7.8283(4) Å, α = 85.572(5), β = 89.340(4), γ = 82.468°, V = 263.25(3) Å3 and Z = 1. The strongest reflectionsof the powder XRD pattern [d, Å (I, %)(hkl)] are for uroxite: 10.05(38)(011), 5.00(100)(022, 111), 4.75(23)(031), 4.43(51)(120, 102),
3.567(33)(131), 3.341(29)(033, 132, 004), 2.623(28)(202, 015, 143, 220) and for metauroxite: 6.06(45)(010), 5.52(33)(100), 4.97(34)(011), 4.52(100)(011, 101), 3.888(80)(111, 002, 110), 3.180(51)(102, 012), 2.604(32)(201, 121). In the structure of uroxite (R1 = 0.0333
for 2081 I > 2σI reflections), UO7 pentagonal bipyramids share corners forming [U4O24] tetramers, which are linked by C2O4 groups
to form corrugated sheets. In the structure of metauroxite (R1 = 0.0648 for 1602 I > 2σI reflections) UO7 pentagonal bipyramids
share edges forming [U2O12] dimers, which are linked by C2O4 groups to form zigzag chains.
Keywords: uroxite, metauroxite, new mineral, uranyl oxalate, crystal structure, Burro mine, Colorado, USA, Markey mine, Utah, USA(Received 22 July 2019; accepted 26 August 2019; Accepted Manuscript published online: 02 September 2019; Associate Editor:
Peter Leverett)Introduction
The first synthetic uranium oxalates were reported in 1842 byEugène-Melchior Péligot (e.g. Péligot, 1842). From these, he
was able to show that the material that Martin Heinrich
Klaproth discovered and named ‘urane’ in 1789 was actually anoxide, rather than a pure element. Péligot obtained green and yellow uranium oxalates by adding oxalic acid to uranium chloride
in aqueous solution. He proposed the name ‘uranium’ for the
metallic element and ‘uranyl’ for the yellow salts of uranium.
As early as 1930, synthetic uranyl oxalate was being used in chemical actinometers. Today, actinide oxalates have important applications (Abraham et al., 2014), particularly in the separation
reprocessing of spent nuclear fuel, because of the chelating abilityof the oxalate group with actinides. In the last several decades,*Author for correspondence: Anthony R. Kampf, Email: akampf@nhm.org
Cite this article: Kampf A.R., Plášil J., Nash B.P., Němec I. and Marty J. (2020) Uroxite
and metauroxite, the first two uranyl oxalate minerals. Mineralogical Magazine 84,
131–141. https://doi.org/10.1180/mgm.2019.57

there have been numerous synthetic and structural studies ofuranyl oxalates because of the unusual array of linking geometries
that they demonstrate (Abraham et al., 2014; Giesting et al., 2006;
Loiseau et al., 2014); however, until now, uranyl oxalates have notbeen found in Nature.
Herein, we describe the first two uranyl oxalate minerals,
uroxite and metauroxite. The name ‘uroxite’ is based on the mineral being a uranyl (UR) oxalate (OX). The name ‘metauroxite’ isbased on the compositional similarity of the mineral to uroxite,
with one less H2O group per formula unit (pfu). These minerals
correspond to known synthetic phases and both were reportedwith their structures by Duvieubourg et al. (2005). The newminerals and their names were approved by the Commission on
New Minerals, Nomenclature and Classification of theInternational Mineralogical Association based upon proposals
IMA2018-100 (Kampf et al., 2018) for uroxite and 2019-030(Kampf et al., 2019) for metauroxite. The type specimens, allmicromounts, have been deposited in the collections of the
Natural History Museum of Los Angeles County, 900
Exposition Boulevard, Los Angeles, CA 90007, USA. Four uroxite© Mineralogical Society of Great Britain and Ireland 2019

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Anthony R. Kampf et al.

Fig. 1. Uroxite crystals from the Burro mine (cotype #73514). The field of view is0.68 mm across.

cotypes are assigned catalogue numbers 73514 (Burro), 73515(Burro), 73516 (Markey) and 73517 (Markey). Two metauroxite
cotypes are assigned catalogue numbers 67289 and 67290.
Occurrence
Uroxite and metauroxite were first collected on September 22,
2016 underground in the Burro mine, Slick Rock district, San
Miguel County, Colorado, USA (38°2′ 42′′ N, 108°53′ 23′′ W).
Uroxite was collected on the following day, underground in theMarkey mine, Red Canyon, White Canyon mining district, San
Juan County, Utah (37°32′ 57′′ N, 110°18′ 08′′ W).
The Burro mine is near the southern end of the Uravan
Mineral Belt, in which uranium and vanadium minerals occur
together in bedded or roll-front deposits in the sandstone of theSalt Wash member of the Jurassic Morrison Formation (Carter
and Gualtieri, 1965; Shawe, 2011). The uranium and vanadiumore mineralisation was deposited where solutions rich in U andV encountered pockets of strongly reducing solutions that had
developed around accumulations of carbonaceous plant material.
Mining operations have exposed both unoxidised and oxidised U
and V phases. Under ambient temperatures and generally oxidising near-surface conditions, water reacts with pyrite to form aqueous solutions with relatively low pH, which then react with theearlier-formed phases, resulting in diverse suites of secondary
minerals.
The Markey mine is located ∼1 km southwest of the Blue
Lizard mine, on the east-facing side of Red Canyon, ∼72 km
west of the town of Blanding, Utah, and ∼22 km southeast of
Good Hope Bay on Lake Powell. The geology of the Markey
mine is quite similar to that of the Blue Lizard mine
(Chenoweth, 1993; Kampf, et al., 2017), although the secondary
mineralogy of the Markey mine is notably richer in carbonate
phases. The information following is taken largely from
Chenoweth (1993). Mineralised channels are in the Shinarump
member of the Chinle Formation. The Shinarump member consists of medium- to coarse-grained sandstone, conglomeratic
sandstone beds and thick siltstone lenses. Ore minerals were
deposited as replacements of wood and other organic material
and as disseminations in the enclosing sandstone (Chenoweth,1993). Since the mine closed in 1978, oxidation of primary oresin the humid underground environment has produced a variety

Fig. 2. Crystal drawing of uroxite crystal; clinographic projection.

of secondary minerals, mainly sulfates, as efflorescent crusts on
the surfaces of mine walls (Kampf, et al., 2017).
Uroxite and metauroxite are both rare and occur onasphaltum–quartz matrix. At the Markey mine, uroxite is associated with feynmanite and gypsum. At the Burro mine, bothminerals are associated with abernathyite, gypsum, tyuyamunite,uranopilite and one other unidentified uranyl oxalate. At boththe Burro and Markey mines, the oxalate presumably derives
from the decomposed/fossilised organic matter noted above,
most likely directly from the asphaltum on which the new minerals have formed.
Physical and optical properties
UroxiteUroxite occurs in radiating sprays of striated prisms or blades, up
to ∼1 mm long, elongate and striated on [100], more or less flattened on {010} and with steeply sloping terminations (Figs 1 and 2).
The prism forms are {010} and {001} and the termination is {102}.
The colour is light yellow, the lustre is vitreous and the streak isvery pale yellow. Crystals are brittle, with Mohs hardness of ca
2, uneven fracture, one perfect cleavage on {102} and one faircleavage on {001}. Moderate neon-green fluorescence is observed
under 405 nm laser illumination. The density could not bemeasured with available density liquids; uroxite appears togradually dissolve in Clerici solution. The calculated density is
4.187 g cm−3 for both the empirical and ideal formulas. Uroxite
is optically biaxial (–) with α = 1.602(2), β = 1.660(2) and γ =1.680(2), determined in white light. The 2V measured directly
on a spindle stage is 59(1)°; the calculated 2V is 59.1°. There is

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superior range (Mandarino, 1981) for both the empirical andideal formulas. At room temperature, uroxite is insoluble in
H2O and slowly soluble in dilute HCl.
Metauroxite

Fig. 3. Yellow, bowtie-like intergrowths of metauroxite crystals (right-centre), yellow
radiating uroxite prisms (left) and colourless gypsum blades on asphaltum matrix(cotype #67289). The field of view is 0.50 mm across.

moderate r > v dispersion. The optical orientation is Y = b and Z ∧a = 35° in obtuse β. No pleochroism was observed. The
Gladstone–Dale compatibility, 1 – (Kp/Kc), is 0.005, in the

Metauroxite occurs as crude blades and lozenge-shaped tablets,
up to ∼100 μm in maximum dimension; commonly in irregularand bowtie-like intergrowths to ∼200 μm across (Fig. 3). No crystal forms could be measured because faces are generally poorly
formed; the tablets are apparently flattened on {011}. Twinning
is ubiquitous, by 180° rotation around the perpendicular to{011} (indicated by single-crystal diffraction data); observationof crystals under crossed polars suggests that the twinning is commonly polysynthetic. The colour is light yellow, the lustre is vitreous and the streak is very pale yellow. Crystals are brittle, withMohs hardness of ca 2, uneven fracture and two good cleavages
probably on {101} and {010} on the basis of the structure.
Weak green–grey fluorescence is observed under 405 nm laser
illumination. The density could not be measured with available
density liquids. The calculated density is 4.403 g cm−3 for boththe empirical and ideal formulas. Optical determinations were
problematic because of poor crystal quality and polysynthetic
twinning. It was only possible to obtain approximate minimum(α′ ) and maximum (γ′ ) indices of refraction: α′ = 1.615(5) and
γ′ = 1.685(5) measured in white light. The average of the estimated minimum and maximum indices of refraction is 1.65,

Fig. 4. Raman spectra of uroxite and metauroxite recorded using a 532 nm diode laser. The inset shows the expanded 1650–0 cm−1 region. Note that the uroxite
spectrum had to be recorded at much lower laser intensity and has been scaled by ×6 relative to that of metauroxite, which accounts for the lower signal-to-noise
ratio for the uroxite spectrum.

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Anthony R. Kampf et al.

Fig. 5. Infrared spectrum (micro-DRIFTS) of uroxite. Note changes in horizontal scale at dotted lines.

Table 1. Band assignments for the Raman and infrared spectra of uroxite and metauroxite.
UroxiteMetauroxite

Infrared

Raman

Raman

Assignment

3545, 3405, 3245

3400

3640
3298

ν(O–H) stretching of OH−
ν(O–H) stretching of H2O
overtones (2 × 1150 cm−1)
ν2(δ)(H–O–H) bendingν8(as,B2u)(C–O) asymmetric stretchingν1(Ag)(C–O) symmetric stretchingν5(B1u)(C–O) asymmetric stretchingν3(UO2)2+ asymmetric stretchingunassigned
ν1(UO2)2+ symmetric stretching
H2O libration
ν6(B1u)(δ)(O–C–O) asymmetric bending
ν(U–Oeq) and ν(U–OH) stretching; or H2O libration
ν3(Ag)(δ)(O–C–O) symmetric bending
four possibilities: γ(U–(OH)–U), ν(U=(OH)),H2O libration, ν7(ρw,s/ωs)(O–C–O) rocking
ν(U=(OH)) or H2O libration
ν2(δ)(O–U–O) and/or (δ)(U–(OH)–U) bending
ν4(τ)(C–C) torsion
phonons

2360–2100
1685
1620

1620
1499

1490

929

929

843

850
770

1170, 1150, 1100
1005

670
508, 504
400, 360

585
508
380

248
132
195, 98

330
275, 250, 226
135
170, 151,117

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Table 2. Chemical compositions (in wt.%) for uroxite and metauroxite.

Raman and infrared spectroscopy

Constituent

Raman spectra for both minerals were recorded from 4000 to
60 cm−1 using a 532 nm diode laser on a Horiba XploRA
PLUS. Uroxite suffered quick decomposition at laser power>1%; however, metauroxite was less susceptible to decomposition,
perhaps because its structure does not include an isolated H2Ogroup (see below). The uroxite spectrum was recorded at 1%
power and that of metauroxite was recorded at 10% power. The
intensity of the uroxite spectrum shown in Fig. 4 was factored
by ×6 relative to that of metauroxite.
Relatively large crystals of uroxite enabled us to perform
micro-diffuse reflectance infrared Fourier transform spectroscopy(micro-DRIFTS) using a Thermo Nicolet 6700 FTIR spectrometer

UO3
C2O3*
H2O*
Total

UroxiteMetauroxite

79.60(78.04–81.90)
10.02
10.03
99.65

82.66
(82.13–82.96)
10.40
7.81
100.87* Based on the structure.

which gives a Gladstone-Dale compatibility, 1 – (Kp/Kc), of
0.0195, in the superior range (Mandarino, 1981) for the empiricalformula. At room temperature, metauroxite is insoluble in H2O
and slowly soluble in dilute HCl.
Table 3. Powder X-ray diffraction data (d in Å) for uroxite*.
Iobs

Icalc

dobs

dcalc

hkl

38
7
9
8
100
23
51

38
7
9
9
51, 4922
14, 35
4
4, 7
12
7
12, 8
31
3
17, 4, 5
5
12, 8
10
3
6
9
3
4
4
9
6
5, 7, 7, 10
4, 5
6, 7
5
3
6
5
8, 3, 6
3
4
5, 4, 6
4, 9
8
8, 3
3, 4
3, 5
5, 4, 3, 4, 3
5, 6
3, 3, 4
4, 4
3
3
3
3

10.05
6.64
6.11
5.55
5.00
4.75
4.43

10.05126.6322
6.11335.5561
5.0256, 4.9750
4.76044.4943, 4.4240
4.33404.0493, 3.9863
3.84353.75553.6741, 3.6330
3.57123.50113.3504, 3.3407, 3.3349
3.31613.2806, 3.2583
3.07503.03782.98032.93852.89852.75382.73032.67872.64362.6307, 2.6282, 2.6193, 2.6110
2.5311, 2.5031
2.4370, 2.4259
2.40362.35342.33762.26102.2107, 2.2079, 2.1876
2.12482.05482.0378, 2.0300, 2.0160
1.9987, 1.9968
1.96141.9306, 1.9110
1.9042, 1.8910
1.8580, 1.8474
1.8270, 1.8075, 1.8045, 1.7981, 1.7922
1.7748, 1.7683
1.7557, 1.7543, 1.7408
1.6695, 1.6598
1.6576
1.61651.52091.4553

011
021
012
100
11
022, 1031
02120, 1

121032, 112023
130041, 122131

113
32, 004
033, 1
042
113, 014
123
141
051

133

114

124
114
150

151

43, 220202,
015, 1
221, 202
134, 231

213

125

232
153
063, 162, 045

224
154
036, 163, 106
116, 145
243
172, 080

253, 017
235, 261
074, 225, 311, 056, 254
165, 322
174, 182, 331
304, 147
018

191
352

345

13
16
8
19
33
29
20
12
5
9
8
9
28
10
12
5
10
8
16
5
12
13
20
10
11
5
13
14
12
10
7
7
4
6
10

4.015
3.844
3.739
3.649
3.567
3.341
3.271
3.070
2.975
2.933
2.754
2.672
2.623
2.515
2.433
2.385
2.334
2.253
2.201
2.117
2.05162.0286
1.9985
1.9583
1.9216
1.89191.84681.80751.77641.7494
1.66691.65571.6241
1.52131.4532*The calculated intensities have been scaled such that the intensities of the adjacent 022 and 111 lines total 100. After scaling, only calculated lines with I ≥ 3 are included. Further data are in
the Supplementary material. The strongest lines are given in bold.

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Anthony R. Kampf et al.

Table 4. Powder X-ray diffraction data (d in Å) for metauroxite.
Iobs

Icalc

dobs

dcalc

hkl

9
45
33
34
100

10
61
38
55
24, 76
644, 25, 34
32
536, 20
4, 7
12
2
7, 12
19
5
7
24, 8
5
4
4
8, 16
3
6, 6, 93
3
5, 8
18, 3, 9
4, 10, 3
3
4
10, 5
10
4
7, 2
8, 7, 56, 5
5, 8, 85, 5, 6, 6, 4
4
2
3, 2, 5, 2, 4
4, 4, 5, 4, 2, 4
3, 2, 3, 4, 2
4
4, 3, 43
2
7, 2, 32
2, 2, 2, 3
3, 2, 3, 5, 2, 2

7.90
6.06
5.52
4.97
4.52

7.80166.04325.51484.96654.6087, 4.5031
4.37243.9081, 3.9008, 3.8287
3.72673.5056
3.1848, 3.1680
3.0216, 2.9948
2.89382.83352.8116, 2.8018
2.74732.66802.64452.5999, 2.5866
2.51332.48322.47932.4586, 2.4383
2.39172.3626, 2.3520, 2.3486
2.30662.30432.2673, 2.2518
2.2239, 2.2071, 2.2019
2.1862, 2.1823, 2.1555
2.13622.10392.0674, 2.0536
2.0144
1.9785
1.9534, 1.9504
1.9144, 1.9000, 1.8920
1.8681, 1.8634
1.8489, 1.8444, 1.8389
1.8274, 1.8238, 1.8193, 1.8157, 1.7967
1.7893
1.78851.7528, 1.7454, 1.7423, 1.7361, 1.7342
1.7230, 1.7213, 1.7130, 1.6994, 1.6921, 1.6863
1.6749, 1.6698, 1.6659, 1.6628, 1.6592
1.65101.6493, 1.6403, 1.6354
1.62081.59551.5923, 1.5840, 1.5790
1.5603
1.5513, 1.5362, 1.5347, 1.5209
1.5118, 1.5108, 1.5032, 1.5013, 1.4951, 1.4929

001
010
100
011
1, 10101110
10111, 002, 1
111
111
02, 012
1020, 112021
112120, 
112
021
112
210


21
201,
1
120
022
211
013, 
121
210
122, 103, 1
21
211
022
211, 202
212, 
122, 113
220, 
113, 2
12
221
113
212, 122030
130131, 004
220, 023, 203
123, 222032, 
123, 104
310, 
213, 114, 014, 
131
301
311
222, 114, 1
31, 032, 213
231, 223, 1
32, 024, 
132, 222
231, 312, 311, 302, 124
311321, 
312, 133
321
214
204, 024, 223005
232, 033, 214, 231
313, 040, 141, 105, 124, 233

80
22
51
7
7
15
14

32

22
20

13
27
11

16
8
9
18
22
21

17
13
16
11

12
8
16

3.888
3.748
3.180
2.999
2.888
2.796
2.746

2.604

2.455
2.352

2.26112.2136
2.16832.06642.0107
1.95101.8994
1.8454
1.8171

1.7382
1.7073
1.66521.64011.58851.53991.5012* The calculated intensities have been scaled such that the intensities of the adjacent 011 and 101 lines total 100. After scaling, only calculated lines with I ≥ 2 are included. The strongest
lines are given in bold.

(range 4000–600 cm−1, resolution 4 cm−1, 128 scans, Happ–
Genzel apodisation) equipped with a Spectra Tech InspectIR
Plus micro-spectroscopic accessory (liquid N2 cooled Hg–Cd–
Te detector). Uroxite crystals were pulverised, mixed with KBr
and analysed immediately without preparation of the pellet.
Pure KBr was taken as the blank reference. The infrared spectrum
for uroxite is shown in Fig. 5.
The band assignments for the infrared and Raman spectra
shown in Table 1 are based on papers by Bartlett and Cooney(1989), Čejka (1999), Colmenero (2019), Mancilla et al. (2009),
Peterson and Pullman (2016) and Shippey (1980). According to
the correlation given by Libowitzky (1999), the bands in the spectra attributed to ν (O–H) correspond to hydrogen bonds ranging

from ∼3.2 to 2.8 Å. Bartlett and Cooney (1989) provided anempirical relationship to derive the approximate U–OUr bondlengths from the band position assigned to the ν1 (UO2)2+ stretching vibration, which gives 1.77 Å for uroxite and 1.76 Å formetauroxite, in excellent agreement with U–OUr bond lengths
from the X-ray data: 1.763 and 1.767 Å for uroxite and 1.775 Å
for metauroxite.
Chemical composition
Chemical analyses were performed at the University of Utah on a
Cameca SX-50 electron microprobe with four wavelength dispersive spectrometers and using Probe for EPMA software. Analytical

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Table 5. Data collection and structure refinement details for uroxite and metauroxite.

Diffractometer
X-ray radiation/power
X-ray power
Temperature (K)Structural formula
Refined formula
Crystal system, space group
Unit-cell dimensions

UroxiteMetauroxite

Rigaku R-Axis Rapid II
MoKα (λ = 0.71075 Å)
50 kV, 40 mA
293(2)[(UO2)2(C2O4)(OH)2(H2O)2]⋅H2O
U2 C2 H8 O13
Monoclinic, P21/c
a = 5.5698(2) Å
b = 15.2877(6) Åc = 13.3724(9) Å

Rigaku R-Axis Rapid II
MoKα (λ = 0.71075 Å)
50 kV, 40 mA
293(2) K(UO2)2(C2O4)(OH)2(H2O)2U2 C2 O12
Triclinic, P
1
a = 5.5635(3) Å
b = 6.1152(4) Åc = 7.8283(4) Å
α = 85.572(5)°β = 89.340(4)°γ = 82.468(5)°.
263.25(3)1
4.366
30.775292
30 × 30 × 20
2.61 to 25.05−6 ≤ h ≤ 6−6 ≤ k ≤ 7−9 ≤ l ≤ 96484/1700; Rint = 0.0704
1602
99.1%
Full-matrix least-squares on F 2
74/0
1.146
R1 = 0.0648, wR2 = 0.1830
R1 = 0.0683, wR2 = 0.1849+3.30/−2.65
0.272
0.196 −0.729
0.001 −1.000 −0.001−1.270 −0.196 −0.272
0.453/0.547

β = 94.015(7)°V (Å3)
Z
Density (refined formula, g cm−3)
Absorption coefficient (mm−1)
F(000)
Crystal size (μm)
θ range (°)
Index ranges

Refl. collected/uniqueRefl. with I > 2σI
Completeness to θ max
Refinement method
Parameter/restraints
GoF
Final R indices [I > 2σI ]R indices (all data)
Largest diff. peak/hole (e − A−3)
Twin matrix

1135.87(10)44.11828.5421232
80 × 30 × 30
3.05 to 25.02−6 ≤ h ≤ 7−8 ≤ k ≤ 18
−15 ≤ l ≤ 15
12103/1995; Rint = 0.054
1777
99.8%
Full-matrix least-squares on F 2
178/11
1.071
R1 = 0.0253, wR2 = 0.0511
R1 = 0.0308, wR2 = 0.0531+1.14/−0.90–

Twin fractions–

Rint = Σ|Fo2–Fo2(mean)|/Σ[Fo2]. GoF = S = {Σ[w(Fo2–F2c )2]/(n–p)}1/2. R1 = Σ||Fo|–|Fc||/Σ|Fo|. wR2 = {Σ[w(Fo2–F2c )2]/Σ[w(Fo2)2]}1/2; w = 1/[σ2(Fo2)+(aP)2+bP] and P is [2F2c +Max(Fo2,0)]/3; for uroxite a is 0.0169 andb is 6.577; for metauroxite a is 0.0746 and b is 28.4245.

Table 6. Atom coordinates and displacement parameters (Å2) for uroxite.

U1
U2
C1
C2
O1
O2
O3
O4
O5
O6
O7
O8
OH1
H1
OH2
H2
OW1
H1a
H1b
OW2
H2a
H2b
OW3
H3a
H3b

x/a

y/b

z/c

Ueq

U 11

U 22

U 33

U 23

U 13

U 12

0.21995(5)0.53127(5)0.8707(14)0.8959(13)0.7406(10)0.9836(10)0.7848(10)0.0279(10)0.9670(10)0.4749(10)0.2879(10)0.7771(10)0.4801(9)0.638(7)0.2640(9)0.135(11)0.3998(12)0.365(13)0.270(11)0.2141(13)0.208(16)0.300(15)0.6771(15)0.719(16)0.816(13)0.46057(2)0.18517(2)0.3512(5)0.2888(5)0.3287(3)0.4233(3)0.2178(3)0.3128(3)0.5150(4)0.4080(4)0.2312(4)0.1393(4)0.5628(3)0.549(5)0.1037(3)0.091(5)0.2775(4)0.336(3)0.258(5)0.5825(4)0.635(3)0.597(5)0.5578(5)0.498(3)0.577(6)0.28584(2)0.04232(2)0.1192(7)0.2060(6)0.0432(5)0.1295(5)0.1970(4)0.2817(4)0.3275(4)0.2422(5)0.1011(5)0.9830(5)0.3569(4)0.338(7)0.9428(4)0.984(5)0.8952(5)0.914(6)0.850(6)0.1631(5)0.203(5)0.106(4)0.0483(7)0.071(8)0.014(8)0.01947(10)0.01866(10)0.0244(19)0.0194(18)0.0297(14)0.0290(14)0.0262(14)0.0252(13)0.0273(14)0.0317(14)0.0323(15)0.0306(14)0.0232(13)0.035
0.0225(13)0.034
0.0385(17)
0.046
0.046
0.0399(17)
0.048
0.048
0.064(2)
0.077
0.077

0.01833(18)0.01823(17)0.022(4)0.010(4)0.039(4)0.033(3)0.030(3)0.029(3)0.025(3)0.033(3)0.034(3)0.022(3)0.015(3)0.01818(17)0.01727(17)0.025(5)0.030(5)0.022(3)0.021(3)0.016(3)0.022(3)0.027(3)0.031(3)0.030(3)0.031(3)0.023(3)0.0216(2)0.02025(19)0.027(5)0.018(5)0.027(4)0.032(4)0.032(4)0.025(4)0.029(4)0.032(4)0.034(4)0.039(4)0.031(4)–0.00236(12)–0.00089(12)–0.001(4)–0.001(3)0.000(3)0.000(2)–0.001(2)0.002(2)0.001(3)–0.010(3)–0.010(3)–0.004(3)–0.010(2)–0.00050(13)–0.00033(13)0.006(4)–0.002(3)–0.009(3)–0.008(3)–0.003(3)–0.003(3)–0.002(3)0.007(3)0.005(3)0.005(3)0.002(3)–0.00055(11)–0.00030(11)0.003(4)0.007(3)–0.011(3)–0.011(3)–0.005(2)–0.003(2)0.002(2)–0.002(3)–0.003(3)0.002(2)–0.002(2)0.018(3)0.023(3)0.026(4)0.001(2)0.000(2)–0.004(2)0.053(4)0.020(3)0.040(5)0.003(3)–0.018(3)–0.008(3)0.063(5)0.033(4)0.025(4)0.000(3)0.006(3)–0.016(3)0.080(6)0.031(4)0.078(7)0.006(4)–0.012(5)0.017(4)Downloaded from https://pubs.geoscienceworld.org/minmag/article-pdf/84/1/131/5020593/s0026461x19000574a.pdf
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Anthony R. Kampf et al.

Table 7. Atom coordinates and displacement parameters (Å2) for metauroxite.

U1
C
O1
O2
O3
O4
OH5
OW6

x/a

y/b

z/c

Ueq

U 11

U 22

U 33

U 23

U 13

U 12

0.3654(2)0.023(6)0.606(4)0.131(5)0.176(4)0.079(4)0.370(4)0.522(5)0.3519(2)0.379(6)0.416(4)0.268(4)0.244(4)0.650(4)0.688(4)–0.029(4)0.20317(15)0.545(4)0.325(3)0.077(4)0.474(3)0.314(3)0.040(3)0.254(3)0.0207(4)0.027(7)0.028(5)0.038(6)0.028(5)0.032(5)0.028(5)0.034(6)0.0182(6)0.025(17)0.022(11)0.032(13)0.030(12)0.026(12)0.019(11)0.045(15)0.0240(7)0.033(19)0.051(15)0.029(13)0.016(11)0.030(13)0.036(14)0.028(13)0.0200(6)0.026(17)0.011(10)0.055(16)0.037(13)0.040(14)0.028(12)0.026(12)–0.0014(4)0.005(14)0.004(10)0.009(11)–0.011(9)–0.003(11)–0.004(10)–0.001(10)0.0004(4)–0.017(14)–0.008(8)–0.012(12)0.013(10)0.015(11)0.011(9)–0.008(11)–0.0027(4)–0.012(14)–0.001(10)–0.017(11)0.003(9)–0.010(10)–0.002(10)0.006(11)conditions were 15 kV accelerating voltage, 10 nA beam current
and beam diameters of 15 μm (for uroxite from the Burromine) and 5 μm (for metauroxite). Counting times were 30 s
on peak and 15 s on background. Raw X-ray intensities were corrected for matrix effects with a wρ(z) algorithm (Pouchou andPichoir, 1991); ideal formula concentrations of O and C wereused for the matrix calculations. Synthetic UO2 was used as theprobe standard for UO3. A time-dependent intensity correction
was applied for ingrowth of U. Because insufficient material isavailable for a direct determination of C2O3 or H2O, they are calculated based upon the structure determination. For uroxite fromBurro, eight points on four crystals were analysed and, formetauroxite, eight points on five crystals were analysed. The
results are given in Table 2.
For uroxite, the empirical formula (calculated on the basis of
13 O atoms pfu) is [(U1.00O2)2(C2O4)(OH)2(H2O)2]⋅H2O andthe ideal formula is [(UO2)2(C2O4)(OH)2(H2O)2]⋅H2O, which
requires UO3 79.88, C2O3 10.06, H2O 10.06, total 100.00 wt.%.
For metauroxite, the empirical formula (calculated on the basisof 12 O atoms pfu) is (U1.00O2)2(C2O4)(OH)2(H2O)2. The ideal
formula is (UO2)2(C2O4)(OH)2(H2O)2, which requires UO3
81.94, C2O3 10.32, H2O 7.74, total 100.00 wt.%.
X-ray crystallography and structure refinement
Powder X-ray studies were done using a Rigaku R-Axis Rapid II
curved imaging plate microdiffractometer, with monochromatised
MoKα radiation (λ = 0.71075 Å). A Gandolfi-like motion on thew and ω axes was used to randomise the samples and observed dTable 8. Selected bond distances (Å) for uroxite.
U1–O5
U1–O6
U1–OH1U1–OH2U1–OW2
U1–O4
U1–O7<U1–OUr><U1–Oeq>1.760(6)1.766(6)2.292(5)2.315(6)2.458(6)2.483(5)2.498(6)
1.763
2.409

U2–O7
U2–O8
U2–OH2U2–OH1
U2–O3
U2–O1
U2–OW1<U2–OUr><U2–Oeq>1.760(6)1.774(6)2.294(5)2.309(5)2.473(6)2.485(5)2.490(6)
1.767
2.410

C1–C2
C1–O1
C1–O2
C2–O3
C2–O4
<C–O>

1.501(12)1.254(10)1.271(9)1.252(9)1.263(9)1.260

Hydrogen bonds
D–H⋅⋅⋅A

D–H

H⋅⋅⋅A

D⋅⋅⋅A

<D–H⋅⋅⋅A>

OH1–H1⋅⋅⋅O5
OH2–H2⋅⋅⋅O8
OW1–H1a⋅⋅⋅OW3
OW1–H1b⋅⋅⋅O4
OW2–H2a⋅⋅⋅O3
OW2–H2b⋅⋅⋅O1
OW3–H3a⋅⋅⋅O2
OW3–H3b⋅⋅⋅O2

0.96(2)0.96(2)0.96(2)0.96(2)0.96(2)0.96(2)0.98(2)0.98(2)

1.92(3)2.12(6)
1.71(2)
1.91(3)
1.84(3)2.29(6)
1.99(6)2.28(7)2.863(8)2.854(8)2.671(9)2.840(8)2.788(9)3.100(9)2.840(9)3.153(11)168(8)
132(7)
176(7)
164(7)
168(8)
142(8)
144(8)
148(10)values and intensities were derived by profile fitting using JADE
2010 software (Materials Data, Inc.). The powder data for uroxite(Table 3) and those for metauroxite (Table 4) show good agreementwith the patterns calculated from the structure determinations.
For both minerals, the single-crystal structure data were collected at room temperature using the same diffractometer andradiation noted above. The Rigaku CrystalClear software package
was used for processing the structure data, including the application of an empirical absorption correction using the multi-scanmethod with ABSCOR (Higashi, 2001). For uroxite, a single crystal from the Burro mine was used for the data collection. For
metauroxite, the crystal used was twinned by 180° rotation aroundthe perpendicular to {011}, as indicated by the TwinSolve program
within the Rigaku CrystalClear software package. Initial structure
models were obtained using SIR2011 (Burla et al., 2012).
SHELXL-2016 (Sheldrick, 2015) was used for the refinement of
the structures.
For uroxite, difference-Fourier syntheses located all nonhydrogen atoms not located in the original structure solution,and subsequent cycles located all H sites. The structure wasfound to be identical to that of synthetic compound (#3 ofDuvieubourg et al., 2005), which lacks the positions of H atoms.
The H atoms were refined with soft constraints of 0.97(2) Å on
the O–H distances and with the Ueq of each H set to 1.5 times
that of the donor OH and 1.2 times that of the donor OW
atom. For metauroxite, difference-Fourier syntheses failed tolocate H atoms. The high residuals are attributed to the relatively
poor crystal quality, to the difficulty in integrating the twinned
reflections and to the inability of the ABSCOR program to completely correct for the absorption by the twinned crystal. The
metauroxite structure was found to be identical to that of synthetic compound #1 of Duvieubourg et al. (2005). For both structures, data collection and refinement details are given in Table 5.
Atom coordinates and displacement parameters for uroxite are in
Table 6 and those for metauroxite in Table 7. Selected bond distances for uroxite are in Table 8 and those for metauroxite are in
Table 9. A bond-valence analysis for uroxite is in Table 10 andthat for metauroxite is in Table 11. The crystallographic information files have been deposited with the Principal Editor of
Table 9. Selected bond distances (Å) for metauroxite.
U–O1
U–O2
U–OH5
U–OW6
U–OH5
U–O3
U–O4
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1.75(2)1.80(2)2.34(2)2.38(2)2.40(2)2.44(2)2.46(2)
1.775
2.404

C–C
C–O3
C–O4

1.58(6)
1.26(4)1.25(4)

Hydrogen bonds (D···A)OH5⋅⋅⋅O2
2.91(3)OW6⋅⋅⋅OH5
2.71(3)OW6⋅⋅⋅O3
2.85(3)

Mineralogical Magazine

139

Table 10. Bond-valence analysis for uroxite. Values are expressed in valence units.*
U1
O1
O2
O3
O4
O5
O6
O7
O8
OH1
OH2
OW1
OW2
OW3
C1
C2
Σ

U2

C1

0.40

1.43
1.37

0.42
0.41

H1

H2

H1a

H1b

H2a

H2b

H3a

H3b

0.19

0.14

0.81

0.86

1.00

1.00

0.14
1.44
1.40

0.38
1.83
1.81

0.59
0.57

C2

0.21
0.20
0.20

1.83
1.78
0.57
0.59
0.39

0.18
0.80
0.82
0.77

0.80

0.40

0.79

0.86

0.23

Σ
1.97
2.12
2.06
1.98
2.03
1.81
1.83
1.96
1.96
1.98
1.96
2.05
1.90

1.11
6.00

5.97

1.11
3.91

3.95

1.00

1.00

1.00

1.00

1.00

1.00

*Bond-valence parameters are from Gagné and Hawthorne (2015). H –O bond valence is based on the graphical relation from Brown (2002).
+

Mineralogical Magazine and are available as Supplementarymaterial (see below).
Descriptions of the structures
In the uroxite structure, there are two U, two C, 13 O sites(including two OH and three H2O sites) and eight H sites.
Both U atoms in the structure of uroxite are surrounded byseven O atoms forming a UO7 pentagonal bipyramid. This is
the most typical coordination for U6+ in the solid state, where
the two short apical bonds of the bipyramid constitute the UO2
uranyl group (Burns et al., 1997; Burns, 2005). Two carbon
atoms and four equatorial O atoms (Oeq: O1, O2, O3 and O4)of the UO7 bipyramids form oxalate groups. The basic connectivity of cation polyhedra is via sharing of two OH vertices (OH1and OH2) of each bipyramid with other two UO7 bipyramids;
this results in a tetrameric fundamental building block (FBB),[U4O24]. Oxalate groups connect one tetrameric FBB to fourothers to form corrugated layers parallel to {102} (Fig. 6). There
are two distinct types of ring linkages in the structure of uroxite,
four-membered rings (U–OH)4 creating an unoccupied diamondshaped void and a much larger eight-membered ring (U–OH–U–
Ox)4, in which uranium atoms are alternately connected by OH
and oxalate groups (Fig. 7). These large voids are occupied by
the OW3 H2O groups that are linked strongly by hydrogenbonds to the uranyl oxalate network (Table 6). Adjacent sheetsof U polyhedra are linked only via hydrogen bonds (Table 6;
Fig. 7) and there are no interlayer atoms, resulting in a rather

short spacing of adjacent sheets, as seen by the (102) powder
X-ray diffraction line of 4.424 Å (Table 3).
The metauroxite structure has one U, one C and six O sites(including one OH and one H2O site). As in the structure ofuroxite, the U atom is surrounded by seven O atoms forming asquat UO7 pentagonal bipyramid, with the two short apicalbonds of the bipyramid (Oap O1 and O2) constituting the UO2
uranyl group. The carbon atom and two equatorial O atoms(Oeq: O3 and O4) of the UO7 bipyramids form C2O4 oxalategroups. Two equivalent UO7 bipyramids share an equatorial
OH–OH edge to form a dimer. Oxalate groups connect adjacenturanyl dimers to form chains along [101]. Uranyl oxalate chainsare linked to one another only via hydrogen bonds (Fig. 8).

Table 11. Bond-valence analysis for metauroxite. Values are expressed invalence units.*
U1
O1
O2
O3
O4
OH5
OW6
C2
Σ

1.87
1.68
0.43
0.42
0.54,0.47
0.49
5.90

C

H5

H6a

H6b

0.15
1.41
1.45

0.17
–0.15

0.22
–0.22

–0.17

Σ
1.87
1.83
2.01
1.87
1.08
0.10

0.90
3.76

*Bond-valence parameters are from Gagné and Hawthorne (2015). Hydrogen-bond strengths
are based on O–O bond lengths from Ferraris and Ivaldi (1988).

Fig. 6. The layer in the structure of uroxite viewed perpendicular to the {102} sheets.
Hydrogen bonds are shown as slender black lines. The unit cell outline is shown bydashed lines. Two coloured areas highlight voids at the centres of two different typesof ring linkages: the small vacant area (lime green) built by tetramers of UO7 polyhedra and the large area (sky blue) formed by four pairs of UO7 polyhedra alternatingwith four oxalate groups. Each large area accommodates two OW3 groups.

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140

Anthony R. Kampf et al.

one to the other. In fact, their close association on some specimens indicates that they form under similar conditions, moreor less contemporaneously.
Discussion

Fig. 7. Stacking of [(UO2)2(C2O4)(OH)2(H2O)2] sheets (parallel to {102}) in the structureof uroxite. The OW3 groups are omitted. The sheets are seen to be linked to one
another by hydrogen bonds (single black lines). The unit cell outline is shown bydashed lines.

The C2O4 groups link the UO7 bipyramids in the same way inthe structures of both uroxite and metauroxite; however, the UO7bipyramids link to one another in quite different ways in the two
structures. In the structure of uroxite, each UO7 bipyramid shares
two equatorial OH corners with two other UO7 bipyramids,
resulting in a corner-linked uranyl tetramer, instead of the edgelinked dimer in metauroxite. The quite different structural topologies suggest that it is unlikely that the structures convert fromThe extensive body of research on the synthesis, structural characterisation and properties of actinide oxalates has been driven
by the selective affinity of the oxalate group for the actinides intheir various oxidation states and, as a consequence, by the wideranging assortment of important applications including (but notlimited to) radioactive waste management, selective actinide separation and recovery, nuclear fuel recycling and actinide isotope
production (cf. Abraham et al., 2014). Given the pronounced
affinity of oxalate for uranyl cations and the intimate associationof uranium mineralisation and carbonaceous plant material in theuranium deposits of the Colorado Plateau, it seems surprising thatnaturally occurring uranyl oxalates have not been discovered until
now. There are two likely reasons: (1) the oxalate group is quite
ephemeral in natural systems; its presence in the fossilised carbonaceous plant material would require it to have been quite
recently generated and probably only locally in very small
amounts; (2) most syntheses of actinide oxalates reported in theliterature have utilised conditions quite different from those currently prevailing in the Colorado–Plateau uranium deposits.
As an example of the latter, Duvieubourg et al. (2005) synthesised
the equivalent of uroxite using (UO2)(NO3)2⋅6H2O, KNO3, oxalic
acid and water, maintained at 120°C for four days. They synthesised
the equivalent of metauroxite using U3O8, oxalic acid and water,
maintained at 190°C for two weeks under autogeneous pressure.
At both the Burro and Markey mines, it is likely that the oxalate, C2O2–
4 , was derived from the asphaltum matrix (probably fossilised wood), while the uranyl, UO2+
2 , was derived fromearlier-formed primary (esp. uraninite) and/or secondary
uranium-bearing minerals. Based upon secondary minerals associated intimately with uroxite and metauroxite and presumably
contemporaneously formed, additional cations and anions werealso present in the solutions: Na+, Ca2+ and SO2–
4 at the Markey
3–3–mine and K+, Ca2+, SO2–
4 , AsO4 and VO4 at the Burro mine.
It is also clear from the modes of occurrence in post-mining secondary assemblages that these minerals formed under ambientconditions: atmospheric pressure, moderate relative humidity
and temperatures probably not exceeding 30°C.
Acknowledgements. Structures Editor Peter Leverett and an anonymous
reviewer are thanked for their constructive comments on the manuscript. A
portion of this study was funded by the John Jago Trelawney Endowment to
the Mineral Sciences Department of the Natural History Museum of Los
Angeles County. JP acknowledges support through project GACR 17–09161Sof the Czech Science Foundation.
Supplementary material. To view supplementary material for this article,
please visit https://doi.org/10.1180/mgm.2019.57.

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Fig. 8. The structure of metauroxite viewed along [100] (top) and [010] (bottom).
Hydrogen bonds are shown with single solid black lines. Note that the hydrogenbond that appears to be between O1 and O3 actually is from an OW6 in a chain
behind the one shown. The unit-cell outline is shown by dashed lines.

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