Mineralogical Magazine, October 2017, Vol. 81(5), pp. 1039–1050

Leószilárdite, the first Na,Mg-containing uranyl carbonate
from the Markey Mine, San Juan County, Utah, USA
TRAVIS A. OLDS1,*, LUKE R. SADERGASKI1, JAKUB PLÁŠIL2, ANTHONY R. KAMPF3, PETER C. BURNS1,4,
IAN M. STEELE5, JOE MARTY6, SHAWN M. CARLSON7 AND OWEN P. MILLS8
1

Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, IN
46556, USA
2
Institute of Physics, Academy of Sciences of the Czech Republic, v.v.i., Na Slovance 1999/2, 18221 Praha 8,
Czech Republic
3
Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard,
Los Angeles, CA 90007, USA
4
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA
5
Notre Dame Integrated Imaging Facility, University of Notre Dame, Notre Dame, IN 46556, USA
65199 East Silver Oak Road, Salt Lake City, UT 84108, USA
7245 Jule Lake Road, Crystal Falls, MI 49920, USA
8Applied Chemical and Morphological Analysis Laboratory, Michigan Technological University, Houghton,
MI 49931, USA[Received 4 August 2016; Accepted 18 September 2016; Associate Editor: Stuart Mills]
ABSTR ACT

Leószilárdite (IMA2015-128), Na6Mg(UO2)2(CO3)6·6H2O, was found in the Markey Mine, Red Canyon,
White Canyon District, San Juan County, Utah, USA, in areas with abundant andersonite, natrozippeite,
gypsum, anhydrite, and probable hydromagnesite along with other secondary uranium minerals bayleyite,čejkaite and johannite. The new mineral occurs as aggregates of pale yellow bladed crystals flattened on{001} and elongated along [010], individually reaching up to 0.2 mm long. More commonly it occurs as paleyellow pearlescent masses to 2 mm consisting of very small plates. Leószilárdite fluoresces green under both
longwave and shortwave ultraviolet light, and is translucent with a white streak, hardness of 2 (Mohs), andbrittle tenacity with uneven fracture. The new mineral is readily soluble in room temperature H2O. Crystals
have perfect cleavage along {001}, and exhibit the forms {110}, {001}, {100}, {101} and {101}. Optically,leószilárdite is biaxial (–), α = 1.504(1), β = 1.597(1), γ = 1.628(1) (white light); 2V (meas.) = 57(1)°, 2V(calc.) = 57.1°; dispersion r > v, slight. Pleochroism: X = colourless, Y and Z = light yellow; X < Y ≈ Z. The
average of six wavelength dispersive spectroscopic analyses provided Na2O 14.54, MgO 3.05, UO3 47.95,
CO2 22.13, H2O 9.51, total 97.18 wt.%. The empirical formula is Na5.60Mg0.90U2O28C6H12.60, based on
28 O apfu. Leószilárdite is monoclinic, C2/m, a = 11.6093(21), b = 6.7843(13), c = 15.1058(28) Å, β =91.378(3)°, V = 1189.4(4) Å3 and Z = 2. The crystal structure (R1 = 0.0387 for 1394 reflections with Iobs >4σI), consists of uranyl tricarbonate anion clusters [(UO2)(CO3)3]4– held together in part by irregular chains ofNaO5(H2O) polyhedra sub parallel to [010]. Individual uranyl tricarbonate clusters are also linked together by
three-octahedron units consisting of two Na-centred octahedra that share the opposite faces of a Mg-centredoctahedron at the centre (Na–Mg–Na), and have the composition Na2MgO12(H2O)4. The name of the newmineral honours the Hungarian-American physicist, inventor and biologist Dr. Leó Szilárd (1898–1964).
K E Y WO R D S : leószilárdite, new mineral, uranium, uranyl carbonate, crystal structure, Markey mine.

Introduction* E-mail: tolds@nd.edu
https://doi.org/10.1180/minmag.2016.080.149© 2017 The Mineralogical Society

TYPICALLY, uranyl carbonates are soluble ephemeral
solids and are the dominant phases that precipitate

TRAVIS A. OLDS ET AL.

from neutral to alkaline waters containing uranium(Finch and Ewing, 1992; Finch and Murakami,1999). Uranyl carbonate complexes are exceptionally stable and dictate the mobility of uranium incarbonated groundwater, where the uranyl ion(UO2)2+ is readily solubilized (Langmuir 1978;
Clark et al., 1995; Gorman-Lewis et al., 2008).
With the acceptance of leószilárdite, the family of
uranyl carbonate minerals now totals 34. The
majority of these contain the uranyl tricarbonate
complex (henceforth referred to as UTC) [(UO2)(CO3)3]4–, which is commonly present in solution
coordinated with other cations (K+, Na+, Ca2+, etc.)
and the phase that ultimately precipitates isinfluenced by the local groundwater chemistry,pH, pCO2 and evaporation rates (Garrels andChrist, 1959; Hostetler and Garrels, 1962).
Leószilárdite differs chemically from all others as
the first Na- and Mg-containing uranyl carbonate –a somewhat surprising occurrence considering thecations involved are commonly found together.
Leószilárdite is named in honour of the
Hungarian-American physicist, inventor and biologist Dr. Leó Szilárd (1898–1964). Szilárd held
many patents for his inventions, including those forthe linear accelerator, the cyclotron, the SzilárdEinstein refrigerator and the nuclear reactor with
Enrico Fermi in 1933 – however their reactor was
not based upon fission as the phenomenon had not
yet been discovered. He is perhaps most wellknown for envisioning the concept of, and patenting, the neutron-induced nuclear chain reaction.
Likewise, he played a compelling role in thedevelopment of the United States nuclear
program, by penning a letter to Franklin
D. Roosevelt warning that Germany may develop
nuclear weapons that harnessed the fission reactionof uranium. The famous letter, signed by Albert
Einstein, jumpstarted the research and developmentof the Manhattan project.
The new mineral and its name were approved by
the Commission on New Minerals, Nomenclature
andClassificationoftheInternational
Mineralogical Association (IMA2015-128). The
holotype specimen is deposited in the collections of
the Natural History Museum of Los Angeles
County, 900 Exposition Boulevard, Los Angeles,
CA 90007, USA, catalogue number 65645.

Occurrence
Leószilárdite was found underground at the Markey
mine, Red Canyon, White Canyon District, San
1040

Juan County, Utah, USA (UTM coordinates; 12 S
4155830 mN, 0561730 mE, NAD27). The workings lie 1 km southwest of the Blue Lizard mine,on the east-facing side of the canyon roughly
73 km west of Blanding, Utah and 35 km south ofHite on Lake Powell. Leószilárdite was later foundoccurring with abundant andersonite at the Pickett
Corral mine, Montrose County, Colorado, USA, but
this is not considered a type locality; the description
is based only on material from the Markey mine.
The geology of the Markey mine is quite similar tothat of the nearby Blue Lizard mine (Chenoweth,1993) although the mineralogy of the Markey mine
is notably richer in carbonate phases. As aninteresting aside, this difference in mineral chemistry may be reflected by the current mine air
chemistry. Preliminary underground gas measurements collected in 2016 using a hand-held
Crowcon Gasman CO2 monitor showed consistently elevated CO2 levels at the Markey mine,averaging ∼1000 ppm CO2 with a maximum
recorded value of 1600 ppm CO2, levels considerably higher than at the nearby Blue Lizard (andPosey) mines where carbonate mineral species areless abundant.
This geological and historical information following is taken from a report by Strand (1954)
contained within an application to the Defense
Minerals Exploration Administration (docket 4260)
for government aid to continue uranium explorationof the area surrounding the Markey mine. All large
uranium deposits in the area of Red Canyon arecharacterized as channel-type sediments fromstreams that deposited the Triassic Shinarump
conglomerate, and commonly cut into the underlying reddish-brown silty sandstones and mudstones of the Moenkopi Formation. Today, thecharacteristic red rocks of the Moenkopi Formation
form much of the lower floor and walls of thecanyon, assumed to be the origin of the name Red
Canyon. Location and exploration of the claims
encompassing the Markey mine area began in 1951
by Edward H. Eakland, Jr. and others, and the firstbatch of copper-uranium ore was mined andshipped to the Atomic Energy Commission millin Monticello, Utah by the Anaconda Copper
Mining Company in 1953. In total 1,214 tons ofuranium ore were removed averaging 20% U3O8.
The mine now lies abandoned and post-mining
oxidation of primary minerals in the humid
underground environment is responsible for
producing interesting secondary uranium minerals,
including abundant andersonite and natrozippeite.
Leószilárdite is a relatively rare mineral in theLEÓSZILÁRDITE, A NEW URANYL CARBONATE FROM THE MARKEY MINE

FIG. 1. Bladed pale yellow leószilárdite on andersonite.
Horizontal field of view is ∼1.2 mm. Travis Olds
specimen and photo.

secondary carbonate assemblage at the Markey
mine, and is found in areas with abundantandersonite and gypsum, along with the othersecondary minerals bayleyite, čejkaite, johannite,
natrozippeite and chalcanthite coating veins ofuraninite in sulfide-laden carbonaceous wood
fragments.
Physical and optical properties
Leószilárdite forms as aggregates of long thin paleyellow blades individually reaching 0.2 mm long,and pale yellow pearlescent masses to 2 mmcomposed of thin plates (Figs 1, 2 and 3).
Crystals are flattened on {001} and elongated on[010], and exhibit the forms {110}, {001}, {100},{101} and {101} (Fig. 4). No twinning was

FIG. 3. Secondary electron images of leószilárdite
microcrystals. Images by Owen Mills.

FIG. 2. Pearlescent masses of leószilárdite coat andersonite, with yellow natrozippeite and probable whitespherical hydromagnesite. Horizontal field of view is∼1.9 mm. Travis Olds specimen and photo.

1041

observed. Crystals are transparent with a vitreous
lustre, while crystalline masses exhibit a pearly
lustre. The mineral has a white streak, is weakly
fluorescent green under both longwave and shortwave ultraviolet light. The Mohs hardness is ∼2,
estimated by the behaviour of crystals when broken.
Crystals of leószilárdite are brittle with perfect{001} cleavage and even fracture. The density
could not be measured due to the limited
availability of material, and solubility in aqueous

TRAVIS A. OLDS ET AL.

yellow; X < Y ≈ Z. The optical orientation is X =b, Y ≈ a, Z ≈ c.
The Gladstone-Dale compatibility, 1 – (Kp/Kc),
is –0.011 (superior) for the ideal formula, and–0.053 (good) for the empirical formula(Mandarino, 2007). The value for k(UO3) = 0.118,
given by Mandarino (1976) was used for the
calculation as it yielded the best compatibilities.
Raman spectroscopy

FIG. 4. Crystal drawing of leószilárdite; clinographic
projection in non-standard orientation (b vertical).

heavy liquids. The calculated density is
3.256 g cm–3 based on the empirical formula.
Optically, leószilárdite is biaxial (–), with α =1.504(1), β = 1.597(1), γ = 1.628(1) (measured inwhite light). The measured 2V is 57(1)°, based on
extinction data collected on a spindle stage andanalysed using EXCALIBRW; the calculated 2V is
57.1°. Dispersion is slight, r > v. The mineral ispleochroic with X = colourless, Y and Z = light

The Raman spectrum of leószilárdite was recorded
using a Bruker Instruments Sentinel-785 laser head
mounted on a Nikon Optiphot-2 microscope with
Peltier-cooled integrated 785 nm diode laser,
operated at 200 mW, with a spot size of 100 μm
and resolution of ∼5 cm–1. The spectrometer wascalibrated with software-controlled procedures(Opus software) using neon emission lines (wavelength calibration) and Tylenol® Raman bands(frequency calibration). A background correction
was applied using the Opus software.
The Raman spectrum of leószilárdite is given in
Fig. 5, and the following assignments are based on
those given by various authors (Čejka, 1999;
Jolivet et al., 1980; Urbanec and Čejka, 1979).
The ν3 (CO3)2– antisymmetric stretching vibrations
are observed at 1535, 1396 and 1328 cm–1. Nobands related to the ν2 (δ)-bending vibration of

FIG. 5. The Raman spectrum of leószilárdite collected with a 785 nm laser.

1042

LEÓSZILÁRDITE, A NEW URANYL CARBONATE FROM THE MARKEY MINETABLE 1. Chemical data for leószilárdite.
Constituent Mean wt.%
Na2O
MgO
UO3
CO2*
H2O**
Total

14.54
3.05
47.95
22.13
9.51
97.18

Range

Standard. deviation Probe standard

14.15–15.06
2.71–3.35
47.35–48.74

0.31
0.27
0.63

AlbiteOlivineUO2 (syn.)*Calculated based on the structure model for 6 C per formula unit.
**Calculated to provide charge balance.

water were observed. The split ν1 (CO3)2– symmetric stretching vibrations are observed at 1078,
1062 and 1052 cm–1. These three bands verify thepresence of three structurally non-equivalent carbonate units seen in the X-ray data. A very strong
Raman band at 824 cm–1 is assigned as the ν1

TABLE 2. Powder X-ray data for leószilárdite (d values in Å).
Iobs

dobs

36

7.59

13

5.82

dcalc Icalc
15.10167.55085.85705.80285.48325.46085.43855.37364.65544.54853.84863.84073.79493.77543.75803.39223.36063.26593.19103.13043.09433.04672.92852.88152.86212.83672.81312.76842.71922.70592.6712

f

100

5.46

f

28

4.64

f

30

3.82

33

3.383

6

3.209

25

3.105

31

20

2.864

2.704

f
f
f

f
f

4
45
7
7
25
15
60
4
24
9
6
11
14
4
3
21
11
4
7
9
18
2
3
9
9
9
2
3
7
9
10

hkl
001
002
110
200
111
201
111
201
112
202
203
113
113
004
203
020
310
311
114
204
022
312
220
221
401
401
023
313
222
205
115

Iobs

dobs

15

2.534

8

2.492

8

2.317

22

2.195

8

2.158

10

2.103

28 2.0283

5 1.9464

dcalc Icalc

f
f
f

f
f

f
12 1.8830 f
9 1.9174

2.54482.54032.53662.52332.51692.48802.48452.32772.32272.28912.25572.22342.19752.19622.19462.18042.17612.15742.13222.09602.03512.03332.02992.02762.01652.0062
1.9523
1.9244
1.9173
1.88471.8798The strongest lines are given in bold.

1043

2
4
7
2
2
7
3
2
6
3
2
4
4
2
5
3
4
6
2
6
4
8
4
3
3
5
3
2
5
2
7

hkl
223
403
314
024
006
403
314
404
116
206
025
315
131
510
131
511
421
007
132
512
316
423
513
133
117
423
330
601
134
602
514

Iobs

dobs

11 1.8178

dcalc Icalc

f1.8204
1.812814 1.7920

f

9 1.7444f

8 1.6953
9 1.6735

6 1.5771
11 1.5482

f
f

f
1.4587 f

5 1.4882

10 1.4370

1.80311.79851.79581.78471.76551.74711.74281.7385
1.69611.6803
1.67801.67371.66861.65481.6364
1.61971.5892
1.5783
1.55181.5495
1.5472
1.49131.47961.45911.45571.4375

6
2
2
2
4
3
2
2
3
2
6
2
2
3
2
2
2
2
2
4
3
2
2
2
2
2
3
4

hkl
027
333
118
317
515
135
425
227
334
604
040
620
009
426
136
042
516
241
242
532
336
533
624
244
534
441
441
535

TRAVIS A. OLDS ET AL.

the (UO2)2+ symmetric stretching vibration. Bartlett
and Cooney (1989) provide an empirical relation toderive the approximate U–Oyl bond length from theband position assigned to the ν1 (UO2)2+ stretchingvibration, which gives 1.79 Å. This value is inaccordance with the U–Oyl bond lengths given by
Burns et al. (1997) for the uranyl cation in hexagonal
bipyramidal coordination, and the U–Oyl bondlength of 1.79 Å derived from the X-ray data.
Raman bands at 742, 729, 705 and 695 cm–1 are
attributed to the split doubly degenerate ν4 (δ)(CO3)2– in-plane bending vibrations. A weak bandat 345 cm–1 is assigned as a ν (U–Oligand) stretch,and the doubly degenerate ν2 (δ) (UO2)2+ bending
mode is found at 290 and 254 cm−1, though bandassignment in this region is difficult due to overlap
with U–Oligand stretches. Remaining bands at 193,

172, 161, 144 and 125 cm−1 are assigned to eitherν2 (δ) (UO2)2+ bending modes, U–Oligand stretches,or external lattice vibration modes.
Chemical composition
Several crystals of leószilárdite were analysed usinga Cameca SX50 electron microprobe (University ofNotre Dame), operating with an accelerating
voltage of 15 kV, a beam current of 10 nA andbeam diameter of 25 μm. The following standards
were used: NaKα (albite), MgKα (olivine), UMα(synthetic UO2). Other elements were sought but
not detected. The counting time for each peak was
8 s, as was the counting time for each background.
Matrix effects were accounted for using the PAP
correction routine (Pouchou and Pichoir, 1985).
The mineral contains a high proportion of volatile
sodium, and the fragility of crystals under the beam
led to slightly low totals (∼97%) and required theanalysis of multiple crystals using a wide beam.
The CO2 and H2O contents could not be measured
due to the limited amount of material available;
instead they are calculated based on the structural
formula with 6 C, and 28 O atoms per formula unit,
and with charge balance considerations satisfied by
H. Analytical data averaged for six analyses frommultiple crystals are given in Table 1. The empiricalformula is Na5.60Mg0.90U2O28C6H12.60. The ideal
formula is Na6Mg(UO2)2(CO3)6·6H2O, which
requires Na2O 15.88, MgO 3.45, UO3 48.88, CO2
22.55 and H2O 9.24, total 100 wt.%.

TABLE 3. Data collection and structure-refinement
details for leószilárdite.

Diffractometer
X-ray radiation/power
Temperature
Structural formulaSpace group
Unit-cell dimensions

VZ
Density (for above
formula)Absorption coefficientF(000)
Crystal size (μm)
θ range
Index ranges
Reflections collected/
uniqueReflections with
Iobs > 4σ(I)Completeness to θ =25.24°
Refinement method
Parameters refined
Goof (obs/all)
R (obs), wR (obs)R (all), wR (all)Largest diff. peak/hole

Bruker Quazar II with Apex II
detector
MoKα (λ = 0.71075 Å)/50 kV,
60 mA
298(2) K
Na6Mg(UO2)2(CO3)6·6H2OC2/m
a = 11.6093(21) Åb = 6.7843(13) Å
c = 15.1058(28) Å
β = 91.378(3)°1189.4(4) Å3
2
3.269 g cm−3

Powder X-ray diffraction
Room temperature powder diffraction data
(Table 2) were recorded using a Rigaku R-Axis
Rapid II curved imaging plate microdiffractometerwith monochromated MoKα radiation. A Gandolfilike motion on the j and ω axes was used torandomize diffraction from the sample. Observed dvalues and intensities were derived by profile fitting
using JADE 2010 software (Materials Data, Inc.).
Unit-cell parameters refined from the powder data
are as follows: a = 11.588(6) Å, b = 6.767(6) Å,c = 15.081(6) Å, β = 91.453(9)°, V = 1182.2(13) Å3.

13.862 mm−11044.045 × 30 × 6
1.35 to 29.26°–15 ≤ h ≤ 15, –8 ≤ k ≤ 8, –20 ≤l ≤ 197517/1634; Rint = 0.0719
1394
100%
Full-matrix least-squares on F 2
1221.006/0.966
0.0387, 0.0847
0.0504, 0.0892+2.34/–1.64 eSingle-crystal X-ray diffraction

1044

Single crystal data were collected from an opticallyhomogeneous bladed crystal using MoKα X-rays
from a microfocus source and an Apex II CCDbased detector mounted to a Bruker Apex II Quazar

1045

Atoms

x

y

z

Ueq/iso

U 11

U 22

U 33

U 23

U 13

U 12

U1
Na1
Na2
Mg1
C1
C2
C3
O1
O2
O3
O4
O5
O6
O7
O8
O9
O10
Ow1
Ow2

0.312623(3)0.3146(4)0.4274(2)0.5
0.6045(10)0.3150(9)0.0336(10)0.6033(6)0.0303(6)–0.0659(6)0.4069(6)0.2193(6)0.3153(7)0.3151(5)0.5152(7)0.7062(7)0.1271(7)0.5647(7)0.6259(6)0.5
00.2611(5)
0
0
0.5
0
0
0
0
0.5
0.5
0.5
0.2359(9)00000.2133(12)0.72748(2)0.83965(29)0.3783(2)10.8216(7)0.5369(7)0.8234(7)0.7349(4)0.7359(5)0.8611(5)0.5869(5)0.5825(5)0.4549(5)0.7269(3)0.8644(5)0.8605(5)0.8665(5)0.4168(6)1.0210(4)0.01418(13)0.0220(6)0.0274(7)0.0163(9)0.0199(20)0.0202(14)0.0201(20)0.0142(14)0.0149(14)0.0201(15)0.0225(12)0.0205(12)0.0243(12)0.0237(10)0.0221(14)0.0345(22)0.0401(22)0.0293(16)0.0493(21)0.0113(2)0.0188(14)0.0298(18)0.0128(19)0.0189(19)0.016(3)0.0177(18)0.017(4)0.019(4)0.016(2)0.014(3)0.014(2)0.019(4)0.030(3)0.0183(16)0.0179(20)0.0173(16)0.020(4)0.041(3)0.0176(2)0.0249(17)0.0259(17)0.0216(25)0.032(6)0.029(4)0.028(6)0.017(4)0.013(3)0.027(4)0.038(4)0.031(3)0.038(4)0.0208(16)0.036(4)0.075(7)0.089(7)0.037(4)0.078(5)0.0137(2)0.0223(18)0.0267(17)0.0146(17)0.0093(23)0.0165(14)0.014(3)0.008(2)0.01290.017(3)0.0158(19)0.017(2)0.0155(14)0.0196(21)0.0124(20)0.010(3)0.014(3)0.030(5)0.029(3)00–0.0019(12)
0
0
0
0
0
0
0
0
0
0
–0.001(1)0000–0.025(3)0.00213(13)0.0011(13)0.0042(13)0.0050(14)0.0015(19)–0.0008(18)0.0003(20)0.002(2)–0.002(2)0.001(2)0.0022(16)–0.001(2)0.0002(20)–0.0036(17)0.0029(16)0.001(2)–0.0004(17)0.002(3)0.0176(23)00–0.0032(13)
0
0
0
0
0
0
0
0.0225(12)00–0.001(2)0000–0.042(3)LEÓSZILÁRDITE, A NEW URANYL CARBONATE FROM THE MARKEY MINETABLE 4. Atomic coordinates and displacement parameters (Å2) for leószilárdite.

TRAVIS A. OLDS ET AL.

TABLE 5. Selected bond distances in leószilárdite.
U1−O7 (×2)
U1−O1
U1−O2
U1−O3
U1−O4
U1−O5
U1−O9
<U1−OUr><U1−OEq>
Mg1−O8 (×2)
Mg1−Ow2 (×4)
<Mg1−O>1.792(6)2.477(7)2.486(7)2.410(7)2.392(7)2.438(7)2.407(8)
1.792
2.429
2.061(7)2.075(6)2.070

Na1−O10
Na1−O7 (×2)
Na1−O8
Na1−Ow2 (×2)
<Na1−O>

2.223(9)2.337(6)2.350(9)2.633(9)2.32

three-circle diffractometer. Reflections were integrated and corrected for Lorentz, polarization andbackground effects using the Bruker program
SAINT. A multi-scan semi-empirical absorption
correction was applied using equivalent reflectionsin SADABS-2012. An initial structure model was
obtained by the charge-flipping method using
SHELXT (Sheldrick, 2015) in the space group
C2/m. Refinement proceeded by full-matrix leastsquares on F 2 using SHELX-2013 (Sheldrick,2008), and refined to an R1 of 3.87% for 1394
reflections with Iobs > 4σ(I ). Rigid bond restraints
were applied to the anharmonic displacement
parameters of all atoms using the DELU and
RIGU instructions in SHELXL. Hydrogen atom
positions were sought but were not located indifference-Fourier maps, as a result the nature of
H-bonding interactions is not discussed. Further
details regarding the data collection and refinement
are given in Table 3. Final atom coordinates anddisplacement parameters are listed in Table 4,
selected interatomic distances in Table 5, andthe bond-valence sums are given in Table 6. The
original crystallographic information file (cif )has been deposited with the Principal Editorof Mineralogical Magazine and is available fromhttp://www.minersoc.org/pages/e_journals/dep_mat_
mm.html

Na2−O1Na2−O2Na2−O4Na2−O5
Na2−O6
Na2−Ow1<Na2−O>2.482(6)2.426(6)2.560(7)2.537(6)2.394(7)2.444(7)
2.47

C1−O1
C1−O8
C1−O9
<C1−O>
C2−O4
C2−O5
C2−O6
<C2−O>

1.309(12)1.235(13)1.307(14)1.28
1.293(13)1.322(13)1.239(13)
1.28

C3−O2
C3−O3
C3−O10<C3−O>

1.321(12)1.300(13)1.253(13)
1.29

Features of the crystal structure1046

In the structure of leószilárdite there is one
independent U, three C, one Mg, two Na and 12
O sites (Z = 2). It contains hexagonal uranyl
bipyramids chelated by three CO3 groups, and has
a cluster topology based upon uranyl tricarbonate
anions (UTC) of the formula [(UO2)(CO3)3]4–(Burns, 2005). Leószilárdite is a member of theuranyl tricarbonates, Strunz class 05.ED, with UO2:
CO3 = 1 : 3. It bears most resemblance to čejkaite(Plášil et al., 2013), synthetic Na4(UO2)(CO3)3(Li et al., 2001; Císařová et al., 2001) and ježekite(Plášil et al., 2015) in that the structures of all ofthese phases contain chains of face-sharing sodium
polyhedra linked to the uranyl tricarbonate clusters.
However, the structure of leószilárdite is uniqueamongst the uranyl carbonates with respect to thearrangement of Na+ and Mg2+ cations, andrepresents the first natural uranyl carbonate containing both Na and Mg as essential constituents.
The uranyl hexagonal bipyramid shares twoedges with face-sharing chains of Na(H2O)O5
trigonal prisms (Na2) that extend along [010](Fig. 6). The structure also contains a distinctive
three-octahedron
unit
of
the
formulaNa2MgO12(H2O)4 consisting of two Na-centredoctahedra that share the opposite faces of a Mg-

0.46
0.35
1.45
1.77
1.75

1.25

2.12

0.37
× 2→
1.52

0.20

1.91
2.03
2.00

1.25

C1
C2
C3
Σan

Mg1

Na2

Na1

0.14
× 2↓

1.20
2.01

0.16
× 2↓

1.28
1.77

2.08

1.30

0.11
× 2↓

1.20

0.12
× 2↓

1.50

0.18
× 2↓

1.88

1.65
× 2→
0.21
× 2→
0.47
0.52
0.50
0.43
0.44
U1

*Multiplicities indicated by × 2→, × 2↓ or × 4→; Na+–O from Wood and Palenik (1999); Mg2+–O and C4+–O bond strengths taken from Brown and Altermatt (1985);
U6+–O bond strength from Burns et al. (1997).

4.02
4.01
3.93

2.16
0.36
× 4→

0.16
× 2↓
0.28
0.50

O10
O9
O8
O7
O6
O5
O4
O3
O2
O1
Atom

TABLE 6. Bond-valence sums for leószilárdite.*

0.87

1.08
0.09
× 2→

6.16

Ow1

Ow2

Σcat

LEÓSZILÁRDITE, A NEW URANYL CARBONATE FROM THE MARKEY MINE

1047

centred octahedron at the centre (Na1–Mg1–Na1).
These three-octahedron units, aligned along [101],
further connect the UTC units (Fig 7).
Chains of Na-centred polyhedra found in čejkaite
coordinate to UTC units by sharing only apices withcarbonate triangles from six individual uranyl
polyhedra. In contrast, each Na-centred polyhedronin the chains of ježekite shares one edge with auranyl polyhedron and one apex with the carbonate
triangle of a different UTC unit. Individual
Na-centred octahedra in the chains of leószilárdite
coordinate to three UTC units, sharing two edges ofuranyl polyhedra and one apex of a CO3 triangle,
each (Fig. 8). The structural formula obtained fromthe X-ray data is Na6Mg(UO2)2(CO3)6·6H2O,
Z = 2, with Dcalc = 3.269 g cm3.
Hazen et al. (2016) predicted 16 yet undiscovered UTC minerals on the basis of isomorphous
substitutions of some elements common to natural
groundwater into known structures. They also
predict 11 other UTC phases, bearing similar
compositions or structures to known synthetic
phases. Leószilárdite was not amongst the predicted phases, but highlights an unfortunate
downside to estimations like these; in that ourinability to predict novel arrangements of cation
polyhedra in uranyl carbonate minerals prevents us
from fully understanding these systems. Many of
our recent descriptions of uranyl carbonates fromRed Canyon, Utah, USA and Jáchymov, Czech
Republic bear little to no resemblance to thesensible chemical substitutions expected innatural minerals, nor to any synthetic phases not
yet found to occur naturally. Rather, nearly all
recent new uranyl carbonate minerals constitutenovel topological arrangements; in addition to aunique class of hybrid uranyl carbonate cluster
minerals that contain bridging uranyl pentagonal
bipyramid clusters. We are in the process ofdescribing two such minerals; the first includes a
combination of uranyl tricarbonate (UTC) anddicarbonate units, which share apices with trimeric[(UO2)3O(OH)3]+ cluster units in ewingite(IMA2016-012). The second is an unnamed Ca,
UTC mineral found by one of the authors (J.M.) in
the Markey Mine, and includes UTC-bridged
monomeric UO7 units. Several uranyl carbonate
minerals containing uranyl pentagonal bipyramids
arranged into sheets are known; including fontanite(Hughes and Burns, 2003), wyartite (Burns andFinch, 1999), bijvoetite-Y (Li and Burns, 2001)
and roubaultite (Ginderow and Cesbron, 1985).
However, the structures of ewingite and theunnamed Markey mine phase are unique as theTRAVIS A. OLDS ET AL.

FIG. 6. A polyhedral representation of the structure of leószilárdite down b. The unit cell is shown as dashed lines.

FIG. 7. Irregular chains of Na-polyhedra display varying degrees of coordination to UTC units in the related mineralsčejkaite, leószilárdite and ježekite. Uranium (yellow), sodium (blue), carbon (black). Various cations have been omitted
to highlight the chains of Na-polyhedra in each structure.

FIG. 8. The coordination environment about three-octahedron Na1–Mg1–Na1 units in the structure of leószilárdite, andtheir connectivity to uranyl and carbonate oxygen atoms.

1048LEÓSZILÁRDITE, A NEW URANYL CARBONATE FROM THE MARKEY MINE

first cluster, and open sheet topologies to containthis combination, respectively.
The rarity of leószilárdite is due mostly to theephemeral nature of the mineral, and it may be amore common constituent in uranyl carbonate
assemblages where the rate of dissolution is slow,
and Na+ and Mg2+ are available. Newly describeduranyl carbonate mineral structures motivate ourwork while supplementing our observations of thechemistry of uranium in carbonated groundwater.
The search for rare uranyl carbonate structurescontinues, as it aids in delineating the complicatedseries of dissolution and precipitation reactions thatthese minerals are involved in.
Acknowledgements
Support for this work is provided by the Chemical
Sciences, Geosciences and Biosciences Division,
Office of Basic Energy Sciences, Office of Science,
U.S. Department of Energy, Grant No. DE-FG0207ER15880. We thank the ND Energy Materials
Characterization Facility for the use of the singlecrystal X-ray diffraction and Raman spectroscopy
instruments. A portion of this study was funded by theJohn Jago Trelawney Endowment to the Mineral
Sciences Department of the Natural History Museumof Los Angeles County. Jakub Plášil is thankful forthe support from the project No. LO1603 under the
Ministry of Education, Youth and Sports Nationalsustainability programme I of Czech Republic.

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