American Mineralogist, Volume 103, pages 143–150, 2018

Leesite, K(H2O)2[(UO2)4O2(OH)5]∙3H2O, a new K-bearing schoepite-family mineral fromthe Jomac mine, San Juan County, Utah, U.S.A.
Travis A. Olds1,*, Jakub Plášil2, Anthony R. Kampf3, Tyler Spano1, Patrick Haynes4,
Shawn M. Carlson5, Peter C. Burns1,6, Antonio Simonetti1, and Owen P. Mills7
Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, Indiana 46556, U.S.A.
2
Institute of Physics ASCR, v.v.i., Na Slovance 1999/2, 18221 Prague 8, Czech Republic
3
Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, California 90007, U.S.A.
4901 Sean Street, Socorro, New Mexico 87801, U.S.A.
5
245 Jule Lake Road, Crystal Falls, Michigan 49920, U.S.A.
6
Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, U.S.A.
7
Applied Chemical and Morphological Analysis Laboratory, Michigan Technological University, Houghton, Michigan 49931, U.S.A.
1

Abstract
Leesite (IMA2016-064), K(H2O)2[(UO2)4O2(OH)5]·3H2O, is a new uranyl-oxide hydroxyl-hydrate
found underground in the Jomac mine, Brown’s Rim, White Canyon mining district, San Juan County,
Utah. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analyses provided
the empirical formula K0.67Na0.004Ca0.012U4O20H15.31, based on 4 U and 20 O apfu. Sheets in the crystal
structure of leesite adopt the fourmarierite anion topology, and so belong to the schoepite family of
related structures that differ in the interlayer composition and arrangement, and charge of the sheet.
Leesite may form as one of the principal components of “gummite” mixtures formed during the alteration of uraninite, and the unit cell of leesite resembles the previously described, but poorly understoodmineral, paraschoepite. Uptake of dangerous radionuclides (90Sr, 135Cs, 137Cs, 237Np, 238Pu) into thestructure of leesite and other members of the family has important implications for the safe disposalof nuclear waste.
Keywords: Leesite, sheet anion topology, schoepite, uranium, uraninite, crystal structure

Introduction
Uranium dioxide nuclear fuel and uraninite, UO2+x, readilyalter in the presence of water and oxygen leading to the formation of uranyl-oxide hydroxyl-hydrate minerals (UOH) (Finch
and Ewing 1992; Wronkiewicz et al. 1992). UOH minerals areamong the first phases to form during the oxidation-hydration
weathering of UO2 (Finch et al. 1996a; Plášil 2014), and studies detailing their structure, solubility, and stability are numerous due to their importance for nuclear waste disposal and theenvironmental chemistry of uranium in general (Amonette etal. 1994; Finch and Murakami 1999; Klingensmith et al. 2007;
Kubatko et al. 2006). Schoepite, [(UO2)8O2(OH)12]·12H2O, themost hydrous UOH, was described by Walker (1923) nearly 95
years ago, yet the crystal-chemical details of phases produced
during its dehydration are still uncertain. Different minerals form
depending on the rate of dehydration, and the presence of cations
can impart variable (OH-) content in the sheets that build UOH
minerals (Table 1). The structures of several of these minerals are built from the same sheet topology found in schoepite(Finch et al. 1996b), and the so-named schoepite family includes
schoepite, metaschoepite, [(UO2)4O(OH)6]·5H2O (Weller et al.
2000), fourmarierite, Pb[(UO2)4O3(OH)4]·4H2O (Li and Burns
2000), paraschoepite, UO3·1.9H2O (Schoep and Stradiot 1947),* E-mail: tolds@nd.edu
0003-004X/18/0001–143$05.00/DOI: http://dx.doi.org/10.2138/am-2018-6083

paulscherrerite, UO2(OH)2 (previously “dehydrated-schoepite”)(Brugger et al. 2011), and heisenbergite, UO 2(OH) 2·H 2O(Walenta and Theye 2012). Describing the crystallography ofthese minerals has been challenging due to the lack of suitably
pure material, in sufficiently large crystals. Leesite is a newmember of the schoepite family containing monovalent cationsin the interlayer and marks the 22nd addition to the family of
uranyl-oxide hydroxyl-hydrate minerals. Plášil et al. (2016)
give an updated listing of the members of this family. Herein,
we provide a description of the crystal structure of leesite andobservations regarding substitutional variability between othermembers of the family, including Na-rich metaschoepite andK-rich fourmarierite from Jáchymov, Czech Republic.
The name leesite honors American mineral dealer and collector Bryan K. Lees (born 1957). Lees received his B.S. in
Geological Engineering from the Colorado School of Mines in
1985. In the same year, he founded Collector’s Edge Minerals,
through which he has developed innovative specimen extraction
techniques and created what is probably the world’s most advanced collector-specimen preparation laboratory. In the 1990s,
Lees spearheaded the mining of rhodochrosite at Colorado’s
Sweet Home mine. The rhodochrosite samples produced by thisventure are widely considered to be some of the most valuable
non-gem mineral specimens ever found, and many were personally collected with his advanced extraction techniques. Lees has
conducted 40 specimen-mining projects on five continents and
143

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Table 1.

OLDS ET AL.: LEESITE, A NEW MINERAL FROM UTAH, U.S.A.
Unit-cell parameters for analogous uranyl-oxide hydroxide hydrate phases

Phase
Schoepitea
Metaschoepiteb
Metaschoepitec
Paraschoepited
Heisenbergitee
Paulscherreritef
a-UO2(OH)2g

Occurrence
Shaba, DRC
Shaba, DRC
synthetic
Shinkolobwe, DRC
Menzenschwand, DE
Radium Ridge, AU
synthetic

a (Å)

b (Å)
Neutral sheets
14.337(3)16.813(5)14.680(2)14.029(2)14.6861(4)13.9799(3)
14.12
16.83
13.10(1)13.76(1)4.288(2)10.270(6)4.242(1)10.302(1)c (Å)

V (Å3)Space GroupΣM s.o.f.

14.73116.720(1)16.7063(5)15.22
14.50(1)6.885(5)6.868(1)3426(7)
3443
3439
3617
2613.7(2)303.2(2)300.1(1)

P21ca
Pbcn
Pbcn
Pbca
P212121; Pna21
P2; P21; P21/m
Cmca or C2cb

0
0
0
0
0
0
0

Charged sheets
LeesitehUtah, U.S.A.
14.866(7)14.126(7)16.772(8)3522(3)Pbca
0.71
Na-rich metaschoepitei
synthetic
14.7050(6)14.0565(5)16.7051(6)
3453
Pbcn
0.545
Na-rich metaschoepitej
Jáchymov, CZ
14.64(2)14.03(1)
16.69(2)3426(7)
Pbcn
0.91
K-rich fourmarieritej
Jáchymov, CZ
14.025(2)16.469(4)14.623(2)3378(2)
Bb21m
0.96
K-rich fourmarieritej
Jáchymov, CZ
13.442(5)16.611(6)14.447(2)3226(1)
Bb21m
1.03
Fourmarieritek
Shinkolobwe, DRC
14.010(1)16.401(1)14.317(1)
3290
Bb21m
1.022
Fourmarieritek
Shinkolobwe, DRC
14.018(1)16.468(1)14.368(1)
3317
Bb21m
0.863
Fourmarieritek
synthetic
13.938(2)16.638(3)14.672(2)
3402
Bb21m
0.497
Kroupaitel
Jáchymov, CZ
14.8201(8)14.0958(8)16.765(1)3502.3(3)
Pbca
0.69
a
Finch et al. (1996b), b Klingensmith et al. (2007), c Weller et al. (2000), d Schoep and Stradiot (1947), e Walenta and Theye (2012), f Brugger et al. (2011), g Taylor and
Hurst (1971), h This work, i Klingensmith et al. (2007), j Sejkora et al. (2013), k Li and Burns (2000), l Plášil et al. (2017).

his samples are displayed in the collections of museums andindividuals around the world. For his accomplishments, he hasreceived the Friends of Mineralogy Author of the Year Award(1998), Carnegie Mineralogical Award (1998), the Colorado
School of Mines Medal (2003), and the American Mineral
Heritage Award (2014).
The Commission on New Minerals, Nomenclature andClassification of the International Mineralogical Association
approved the new mineral and name (IMA2016-064). The
description is based upon two co-type specimens from the
Jomac mine, deposited in the Natural History Museum of Los
Angeles County, California, U.S.A., with catalog numbers
66285 and 66286.

Occurrence
Leesite is found underground in the Jomac mine, Brown’sRim, White Canyon mining district, San Juan County, Utah
(37°51′43″N 110°19′10″W), about 5.5 km SE of Hite Crossing.
Material containing the new mineral was collected in 1989 by
one of the authors (P.H.). The Jomac mine consisted of threeadits that are now closed and reclaimed. Collecting has beenprohibited since 1992 when the area was incorporated into the
Glen Canyon National Recreation Area. Leesite was found ina seam of gypsum, closely associated with compreignacite,
K2[(UO2)6O4(OH)6]·7H2O, and a later generation of blatonite
UO2CO3·H2O (Vochten and Deliens 1998) and owsaldpeetersite(UO2)2CO3(OH)2·4H2O (Vochten et al. 2001), two minerals forwhich the Jomac mine is the type locality. Other accessory minerals include alunite, chalcoalumite, probable mbobomkulite
or nickelalumite, sklodowskite, and boltwoodite. The depositof uranium exploited by the Jomac mine lies in the Shinarump
conglomerate member of the Triassic Chinle Formation. An
account of the geology and history of the mine is given by Trites
and Hadd (1958). Haynes (2000) summarizes this paper, andincludes the descriptions of minerals identified up to that time,
including two then unknown U minerals (designated “Unknown
number 1” and “Unknown number 2”). Unknown number 2
has since been described as oswaldpeetersite and Unknown
number 1 is described herein as the new mineral leesite.

Physical and optical properties
Leesite forms as aggregates of orange-yellow tablets up to
1 mm in diameter (Fig. 1). Tablets are flattened and stacked on{100}, the only well-developed crystal form (Fig. 2). Leesite
also occurs as powdery masses in the interstices of gypsum
crystals. Crystals are brittle with perfect cleavage on {100}and uneven fracture. No twinning was observed. Crystals aretranslucent with a vitreous luster, give a light yellow streak,and are non-fluorescent under LW and SW UV. The Mohs hardness is approximately 2, estimated by the behavior of crystals
when broken. The density was not measured due to the limited
availability of material. The calculated density is 3.256 g/cm3
based on the empirical formula. Leesite is readily soluble indilute HCl and HNO3, with no effervescence.
Leesite is optically biaxial (–), with a = 1.745(2), b =1.761(2), g = 1.765(2) (measured in white light). The 2V is50(2)°, measured directly by conoscopic observation on aspindle stage; the calculated 2V is 52.7°. Dispersion is strong,r > v. The mineral is pleochroic with X nearly colorless, Y andZ orange yellow; X < Y ≈ Z. The optical orientation is X = a, Y
= c, Z = b. The Gladstone-Dale compatibility, 1 – (Kp/Kc), is0.037 (excellent) for the ideal formula, and 0.028 (excellent) forthe empirical formula (Mandarino 2007). Raman and Fouriertransform infrared spectra of leesite can be accessed online
as Supplementary material (Appendix1 Figs. S1, S2, and S3).
Chemical composition
Electron microprobe analyses were deemed to be unreliable due to instability and decomposition of crystals under theelectron beam. Instead, six chemical analyses were performed
using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Six crystal aggregates were embedded
in epoxy and polished to provide a flat surface (~50 × 50 mm).
The ion signals for U, K, Pb, Na, and Ca were measured using
an Element2 sector field high-resolution inductively coupledplasma mass spectrometer (Thermo-Fisher Scientific) inmedium mass resolution mode coupled with a UP-213 (New
Wave Research) Nd:YAG deep UV (213 nm) laser ablation
system. Prior to the lasering of samples, the Element2 wasOLDS ET AL.: LEESITE, A NEW MINERAL FROM UTAH, U.S.A.
Table 2.

145

LA-ICP-MS data (wt%) for leesite, average of six analyses

Element

Mean ratio
Range
SD
Mean
Calculated(U/cation)			
apfu
wt% oxideNa
0.001
0.0006–0.0014 0.0004
0.004
–
Ca
0.003
0–0.006
0.482
0.012
–
K
0.168
0.1632–0.1748 0.0042
0.670
2.4
Ua
1.000
–
–
4
87.09 (as UO3)
H2Ob––
–		 10.51
Notes: The element ratios above are alternatively expressed in calculated wt%
oxide in the last column, based on the mean apfu derived from the count ratios
for each element.
aRatios normalized to 4 U apfu.
bCalculated according to the structure with charge-balance considerations on
the basis of 20 O apfu.

Figure 1. Orange-yellow aggregates of leesite rim the edges ofblocky orange compreignacite, with pale yellow sklodowskite. The wholeassemblage sits atop colorless gypsum, with probable white nickelalumite
or mbobomkulite. Horizontal field of view is 2 mm. (Color online.)

Figure 2. Secondary electron image of tabular leesite stacked along(100). Photo by Shawn M. Carlson and Owen P. Mills.

tuned using a multi-element solution containing 1 ng/g of each
Li, In, and U to obtain maximum ion sensitivity. Laser ablation analyses involved acquiring background ion signals for
60 s with the laser on and shuttered, and this was followed by
60 s of data acquisition. Laser operating conditions involved
using an 8 mm spot size, repetition rate of 5 Hz, 100% powerout, which corresponded to a fluence of ~8.4 J/cm2. Six areas
on six crystals were examined using a raster scan or single
spot analyses depending on the size of the crystals. Leesite
contains appreciable U, K, Ca, some Na, and negligible Pb,
and the data are given in Table 2. No other elements were detected. The ion signals (cps = counts per second) obtained for
K, Ca, and Na are reported as a ratio relative to that recorded
for U, as absolute abundances could not be determined due
to a lack of an appropriate matrix matched external standard.
The H2O content was calculated according to the structure on
the basis of 20 O apfu with charge-balance considerations.
The empirical formula, K0.67Na0.004Ca0.012U4O20H15.31, is calcu-

lated on the basis of 4 U and 20 O apfu. The ideal formula isK(H2O)2[(UO2)4O2(OH)5]·3H2O, which requires: K2O 3.55,
UO3 86.27, H2O 10.18, total 100 wt%.
Single-crystal X-ray diffraction and refinement
A homogenous plate fragment with sharp optical extinction
in cross-polarized light was chosen for single-crystal X‑ray
diffraction study. Data were collected using MoKa X‑raysfrom a microfocus source and an Apex II CCD-based detector
mounted to a Bruker Apex II Quazar three-circle diffractometer.
Reflections were integrated and corrected for Lorentz, polarization, and background effects using the Bruker program SAINT.
A multi-scan semi-empirical absorption correction was applied
using equivalent reflections in SADABS-2012. An initial structure model was obtained by the intrinsic phasing method using
SHELXT (Sheldrick 2015) in space group Pbca with mostatoms located, except some O atoms of water molecules. The
SHELXL 2013 software package was used to refine the structure of leesite on the basis of F 2 for unique reflections, and theremaining O atoms of water were located in difference Fourier
maps. Hydrogen atom positions were not determined, due to theweak X‑ray scattering factor of hydrogen, and the dominanceof U in the difference Fourier density maps. Furthermore, thediffraction pattern suffered from a split crystal contribution,with the heaviest contribution to low angle data. An attempt todeconvolute this contribution was made but did not improve theresults. The split crystal, in combination with weak diffraction
led to some difficulties during refinement of anisotropic displacement parameters for several oxygen atoms, and rigid bondrestraints (RIGU) were applied to assists in their refinement.
Details regarding the data collection and refinement results are
given in the supplementary information1 and can also be foundwithin the CIF, and bond-valence analysis is given in Appendix1
Table S4. Powder X‑ray diffraction data (Appendix1 Table S1)and the profile fitting details (Appendix1 Fig. S4) are available
as supplementary information.

Crystal structure description
Cation coordination
The structure of leesite (Fig. 3) contains four symmetricallydistinct U sites. All adopt sevenfold pentagonal bipyramidal
coordination, where the apices of each polyhedron are comprisedof multiply bonded oxygen, forming the approximately linear
uranyl ion—UO2+2 (Burns et al. 1997a). Equatorially, each uranyl
cation is fivefold-coordinated by O or OH, and the polyhedra are

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OLDS ET AL.: LEESITE, A NEW MINERAL FROM UTAH, U.S.A.

Figure 3. A comparative view of the anion sheet topologies, (OH)– distributions, and cation positions for analogous uranyl-oxide hydroxyhydrates. Black circles highlight vertices containing (OH)–, and bare vertices represent O2–. The distribution of (OH)– in leesite is identical to thatof the synthetic Na-analog of metaschoepite. Potassium (blue), lead (orange), and sodium (yellow). (Color online.)

linked by sharing edges arranged into the so-called fourmarierite
anion sheet topology (Burns 2005; Li and Burns 2000; Lussier
et al. 2016). The sheets in schoepite and metaschoepite also
adopt the fourmarierite topology, which consists of sheets built
from topological pentagons and triangles (Fig. 3). Pentagons
are populated by U atoms, and triangles, arranged in alternating
bow-tie arrangements, are vacant.
The interlayer of leesite is populated with K+ cations andwater molecules. Dimeric clusters of K+ and H2O serve to connect
sheets of U polyhedra stacked along a by coordinating to theiroutstretched Oyl atoms (Fig. 4). Coordination about K+ is [9]-foldand each K+ binds six Oyl atoms and three water (Ow) molecules,
such that the clusters have the composition K2O10(H2O)4. There is
one symmetrically unique K site, and site-scattering refinement
reveals it is partially occupied (0.71), in agreement with theaverage empirical chemistry (0.67 apfu). This is not unexpected,
considering the analogous mineral fourmarierite also displays
variable Pb2+ content (0.86–1.02 apfu) in natural and synthetic
samples (Li and Burns 2000). Atoms of Pb2+ in fourmarierite
adopt a similar dimeric arrangement in the interlayer—withcomposition Pb2O10(H2O)4 (Fig. 3).
Relationship to other UOH minerals
The sheets of uranyl polyhedra in schoepite and metaschoepite are electroneutral, but can accommodate substitution ofOH– for O2– within the sheet (Finch et al. 1996b). This allows forvariably charged sheets and the presence of interlayer cations. A
synthetic Na-analog of metaschoepite is known, with an interlayer containing partially occupied Na+ sites (Klingensmith etal. 2007). The arrangement of OH– in leesite is identical to thatfound in the synthetic Na-analog of metaschoepite, and similar
to metaschoepite in most regards except that a single OH– groupin metaschoepite is deprotonated in leesite (atom O9). Naturally
occurring Na-rich metaschoepite is described by Sejkora et al.
(2013) from the Jan Evangelista vein, Svornost mine in Jáchymov, and was shown to contain appreciable amounts of otherelements (Na, 0.3 apfu; Cu, 0.13 apfu; Al, 0.13 apfu; Pb, 0.08apfu). The Na-rich material is poorly crystalline and powdery,
preventing its formal description as a mineral. Sejkora et al.
(2013) also describe K-rich fourmarierite (0.2–0.45 K apfu)
from the Evangelista vein, and recently, the schoepite family

mineral kroupaite, KPb0.5[(UO2)8O4(OH)10]·10H2O, has been
described (Plášil et al. 2017). Structurally, kroupaite is similar
to leesite, except K+ cations adopt slightly different positions.
Presumably, as Pb2+ content reaches ~50% in these phases,
structural transformation to the fourmarierite cell is promptedby interlayer rearrangement and increased O2– content within
the sheets, however more work is required to understand therelationship between leesite, kroupaite, K-rich fourmarierite,
and fourmarierite.
Foord et al. (1997) provide data for an unknown and incompletely characterized phase designated “mineral A” by Frondel(1956) that forms within “gummite” alteration rinds on uraninite from the Ruggles and Palermo granitic pegmatites in New
Hampshire, U.S.A. Powder diffraction analyses by Foord et al.
(1997) indicate it is a mixture of schoepite-family minerals andother UOH phases. Composite chemical analyses by Foord etal. (1997) indicate the material contains appreciable K, Pb, andCa; avg. (in wt%) UO3 83.5, PbO 4.85, BaO 0.68, CaO 0.167,
K2O 2.46, SrO 0.21, ThO2 0.85, H2O 6.9, S99.62. Given thesimilarity in chemical analyses to leesite, it appears that the
mixture of schoepite-family minerals found in some “gummite”
could contain leesite.

Bond valence analysis and role ofinterlayer H2O
The symmetry of minerals in the schoepite family is sensitive to the water content, and presence of cations. Some relations between the cation content and arrangement of interlayerwater are revealed by a bond-valence based approach, which
examines interactions between the structural unit and the role ofinterstitial species (Hawthorne 1992; Hawthorne and Schindler
2008; Schindler and Hawthorne 2004, 2008). The approachdeveloped by these authors is a measure termed the chargedeficiency per anion (CDA), and is defined as the averagebond-valence per O atom contributed by the interstitial species and adjacent structural units. Stable structures are formed
when the bonding availability of the structural unit matches
that of the interstitial complex (Hawthorne 2012, 2015). The
quantity is useful for crystal-chemical predictions, enablingcomparison between minerals with related topologies but differing interlayer constituents.

OLDS ET AL.: LEESITE, A NEW MINERAL FROM UTAH, U.S.A.

147

Figure 4. A polyhedral representation of the uranyl oxide hydroxide sheet (yellow) in leesite, with ball-and-stick interlayer containing water
oxygen (red) and potassium (blue). (Color online.)

Bond valence analysis of the X‑ray structure permits distinction of O atom types within the structure of leesite, andthe ideal structural formula assuming full K+ occupancy isK(H2O)2[(UO2)4O2(OH)5]·3H2O. The CDA of the sheet in leesite
is 0.133, identical to the value for Na-metaschoepite. The value isintermediate to those of schoepite and metaschoepite (0.08) and
fourmarierite (0.19). The range in Lewis basicity of the structuralunit in leesite is 0.13–0.24 using the method of Schindler and
Hawthorne (2008), and is comparable to ranges calculated forother uranyl-oxide hydroxyl-hydrate minerals (e.g., schoepite:
0.11–0.20).
When present in the interstitial complex (H2O) molecules
act as transformers of bond-valence from cations within theinterlayer to atoms in the structural unit (Schindler and Hawthorne 2008). The interlayer of leesite contains 5 unique (H2O)
molecules. The atoms Ow1, Ow2, Ow3, and Ow4 are [5]-coordinated and transfer weak bond valence from K+ to uranyl ion
oxygen atoms within the sheet through various interactions, andaccording to the designation given by Schindler and Hawthorne(2008), are considered inverse-transformer (H2O) groups. Atom
Ow1 forms two bonds to K+, two weak H-bonds with uranyl
ions of the sheet (~0.2 v.u.), and one H-bond with Ow2. Atoms
Ow2, Ow3, and Ow4 each link to two H atoms (2 × 0.8 v.u.)of other (H2O) molecules, and all form at least one H-bond to
uranyl ions of the sheet. They also each accept one H-bond (0.2v.u.) from (OH) groups of the sheet. The remaining (H2O) group,
Ow5, does not bond to a cation and has coordination number[4]. Thus, Ow5 acts as a non-transformer group by propagating
weak bond valence from cations to anions that are too distantto bond directly to the cation. Details regarding the number of
H-bonds donated and accepted, and their interatomic (O-H…O)

distances are given in Appendix1 Tables S4 and S5 (Supplementary1 Information), respectively.
It is difficult to establish if hydronium (H3O+) is present inschoepite family phases with non-stoichiometric cation contents(Wilkins and Mateen 1974), but this would be in accord withthe observation that leesite and other members of the family areformed from acidic solutions. Site-scattering refinement indicates slightly higher K+ occupancy (0.71) than obtained from
LA-ICP-MS analyses (0.67 apfu), which suggests the presenceof disordered O (H3O+) or N (NH4+) atoms at this site, instead of
vacancies. However, the discrepancy in refined occupancy may
be related to an inadequate absorption correction. It is currently
unclear if partial K+ occupancy is accounted for by OH– ↔ O2–
substitution in the sheet, or through interlayer NH+4 or H3O+
substitution, but due to their similar sizes, H3O+ and NH+4 could
readily substitute for K+ in leesite.

Speciation of interlayer cations
With details of the range of bonding availability of thestructural unit and interlayer (H2O), we now have the necessary
information to describe the species of cations that can be expected
to occur in the interlayer in schoepite family minerals as demonstrated by Schindler and Hawthorne (2004, 2008). Figure 5
depicts the variation in Lewis acidity for various cation coordination numbers and charges with differing numbers of coordinatingtransformer (H2O) groups. A stable structure is formed wherelines of variable Lewis acidity overlap the Lewis basicity rangeof the structural unit (shown in blue). As revealed in Figure 5,
cations with coordination number >[8] must bond to at least one
inverse-transformer (H2O) group to produce a stable structure(Schindler and Hawthorne 2008). In the structure of leesite,

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OLDS ET AL.: LEESITE, A NEW MINERAL FROM UTAH, U.S.A.

Figure 5. The variation in Lewis acidity for particular cation coordination numbers and charges of a general interstitial complex with differing
numbers of transformer (H2O) groups. The range in Lewis basicity of the structural unit of leesite is shown in blue. A stable structure is formed
where the lines of variable Lewis acidity overlap the Lewis basicity of the structural unit. Figure adapted in part from Schindler and Hawthorne(2004). (Color online.)

potassium cations are coordinated by three inverse-transformer(H2O) groups (2 × Ow1, and Ow3), and this agrees with thepredicted coordination number of 0 transformer (H2O) groups.
Monovalent cations with a wide range of coordination
numbers can be incorporated into the interlayer of leesite,and may include H3O+, NH4+, Na+, or Cs+. Incorporation of[8]-coordinated Ca2+ is possible; however, others have documented conversion of metaschoepite to becquerelite in the presenceof Ca2+ at elevated temperatures (Sandino and Grambow 1994;
Sowder et al. 1996, 1999). Although leesite occurs intimately
with gypsum, it contains relatively little Ca2+ (Table 1). Ca-richmetaschoepite may be more likely to occur in carbonate-richassemblages, such as those at the Markey mine to the nearby
southwest. Hydrolysis reactions with abundant uranyl carbonates
found there may lead to the formation of Na or Ca-metaschoepite.
Samples of Na-rich metaschoepite from Jáchymov were shownto contain small amounts of Cu2+ and Al3+, and we can expect
these cations will occupy sites that maximize coordination bytransformer (H2O) groups (Sejkora et al. 2013).

Renewed interest in paraschoepite?
In a dry environment, schoepite slowly loses interlayer water(Kubatko et al. 2006; O’Hare et al. 1988), leading to a decreasein the interplanar spacing between sheets as water molecules
rearrange and relocate, forming a new H-bonded array in metaschoepite (Weller et al. 2000). Schoep and Stradiot (1947) noted
an opaque lemon yellow orthorhombic phase within altered
crystals of schoepite, which was indistinguishable from crystalsof schoepite except upon determination of its optical properties.
A combination of optical and chemical analyses indicated thematerial is unique from other schoepite family members, withthe formula UO3·1.9H2O, and was thus designated paraschoepite.
Christ and Clark (1960) report a large interplanar distance (c =15.22 Å) for single crystals of paraschoepite, and state “Becauseof the distinctive X‑ray pattern given by the yellow crystals andthe excellent agreement of the optical measurements obtainedin the present study with those originally given by Schoep andStradiot in 1947, there can be little doubt as to the validity ofparaschoepite.” Subsequent descriptions have attributed paraschoepite to a mixture of metaschoepite, dehydrated schoepite,
and ianthinite (Brugger et al. 2011; Finch et al. 1992, 1997);
however, the similar large interplanar spacing (a = 14.87 Å)
and arrangements found in leesite may be in part related toparaschoepite. Leesite contains a predominance of inversetransformer (H2O) groups, which maximize interactions withthe sheet in the presence of bulky K+ cations. In paraschoepite,
interlayer water remaining after partial dehydration would likely
rearrange to maximize bonding interactions with the sheet. Thisis best achieved by the inclusion of more inverse-transformer(H2O) group interactions, which are capable of transferringbond strength at longer distances. Finch et al. (1992) argue thatparaschoepite represents a metastable structure where localized
expansion is associated with the collapse of layers as “dehydrated
schoepite” forms. Local expansion of layers may be related toreorganization of interlayer (H2O) groups from transformer toinverse-transformer roles, and future X‑ray or neutron studies
exploring this metastable state may be supplemented by ourobservations of the structure of leesite.

Implications
With the description of leesite, we are better able to recognize
the conditions and crystal-chemical features that drive formation

OLDS ET AL.: LEESITE, A NEW MINERAL FROM UTAH, U.S.A.

of specific minerals in the schoepite family. Details of the cation
arrangement, water content, and H-bonding array can be compared
for this series of minerals, and predictions can be made towardpossible compositions not yet observed. Our observations of
natural samples from several localities reveal that the fourmarierite
sheet anion topology is capable of accommodating an interlayer
with a range of heterovalent cations and unique configurations.
Recognizing how and where large K+ cations incorporate into thisfamily reveals how short-lived radionuclides like 137Cs or 90Sr will
behave during the initial alteration stages of irradiated nuclear fuel(Giammar and Hering 2004). Long-lived radionuclides, such as
135
Cs or 237Np, may also be incorporated, albeit under different
circumstances. Low-valence cations (Cs+ and Sr2+) will accumulate within interlayer space during formation, or through cationexchange within the interlayer. The incorporation mechanism forhigh-valence cations (e.g., Np5+,6+, Pu5+,6+) depends heavily on theoxidation state, and whether the structure can support substitutionof actinyl ions (An5+,6+O2)1+,2+ for uranyl ions (UO2)2+ within the
sheet of polyhedra (Burns et al. 1997b). Incorporation of (Np5+O2)+
must be accompanied by an appropriate charge-balancing mechanism; through protonation of the sheet or inclusion of cations(Burns et al. 2004; Klingensmith et al. 2007). Schoepite family
phases formed during the initial alteration stages will readily incorporate the elements listed above through these processes, but
may be subsequently altered or undergo structural rearrangement.
For example, Sandino and Grambow (1994) observed the complete
conversion of metaschoepite into compreignacite in the presenceof excess K+ at room temperature. Leesite from the Jomac mine is
intimately associated with compreignacite, and facile conversion
to compreignacite may explain the rarity of leesite. In this case,the crystal-chemical predictability afforded by the bond-valence
approach is very powerful due to the penchant for UOH minerals
to form, rearrange, and redistribute U or cations (Finch et al. 1992).

Acknowledgments
This research is funded by the Office of Basic Energy Sciences of the U.S.
Department of Energy as part of the Materials Science of Actinides Energy Frontier
Research Center (DE-SC0001089). The Element2 HR-ICP-MS instrument used
for chemical analyses is housed within the Midwest Isotope and Trace Element
Research Analytical Center (MITERAC) at the University of Notre Dame. Electron
microscopy was carried out in the Applied Chemical and Morphological Analysis
Laboratory at Michigan Technological University. The John Jago Trelawney
Endowment to the Mineral Sciences Department of the Natural History Museumof Los Angeles County funded a portion of this study. Jakub Plášil is thankful forthe support from the project GACR 15-12653S.

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Manuscript received January 19, 2017
Manuscript accepted September 16, 2017
Manuscript handled by Martin Kunz

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