Mineralogy, Crystallography and Structural Complexity of Natural Uranyl Silicates

Naturally occurring uranyl silicates are common constituents of the oxidized parts (i.e., supergene zone) of various types of uranium deposits. Their abundance reflects the widespread distribution of Si4+ in the Earth’s crust and, therefore, in groundwaters. Up to date, 16 uranyl silicate minerals are known. Noteworthy is that the natural uranyl silicates are not extremely diverse regarding their crystal structures; it is a result of possible concentrations (activity) of Si4+ in aqueous solutions derived from dissolution of primary Si minerals or the composition of late hydrothermal fluids. Therefore, in natural systems, we distinguish in fact among two groups of uranyl silicate minerals: uranophane and weeksite-group. They differ in U:Si ratio (uranophane, 1:1; weeksite, 2:5) and they form under different conditions, reflected in distinctive mineral associations. An overview of crystal-chemistry is provided in this paper, along with the new structure data for few members of the uranophane group. Calculations of the structural complexity parameters for natural uranyl silicates are commented about as well as other groups of uranyl minerals; these calculations are also presented from the point of view of the mineral paragenesis and associations.


Introduction
Uranyl silicates minerals are typical representatives of the oxidized parts of uranium deposits worldwide [1][2][3][4][5], forming during oxidizing weathering of uraninite, ideally UO 2 , or coffinite, ideally U(SiO 4 ).The abundance of occurrences reflects the widespread distribution of Si 4+ in the Earth's crust [5].Besides the important role that uranyl silicate minerals play in the evolution of U deposits and controlling the mobility of U in their oxide-zones [6], they were also identified as alteration products of spent nuclear fuel, which may have a significant impact upon the mobility of U and other radionuclides under geological repository conditions [7][8][9][10].
To date, the group of naturally occurring uranyl silicates comprises 16 mineral members (Table 1).However, some of them remain only poorly defined.The crystal-chemistry of uranyl silicates attracted much attention for a long time [11][12][13][14][15][16][17][18][19][20] namely in connection with the increased attention due to spent nuclear fuel waste management.Infrared and Raman spectroscopy data for natural uranyl silicates have been reported by [21] and also recently by a thorough review of Raman spectroscopy of uranyl minerals and phases [22].With increased attention many synthetic uranyl silicate compounds have been synthesized, usually with crystal chemistry divergent from the natural ones [23][24][25][26][27][28][29], and also both experimental [30][31][32] and theoretical studies [33,34] have been undertaken in order to determine the thermodynamic properties of uranyl silicate minerals.

Single-Crystal X-Ray Diffraction
New single-crystal X-ray data were collected for sklodowskite, cuprosklodowskite, oursinite, and soddyite.Crystals used in the study were retrieved from the collections of the National Museum in Prague (cuprosklodowskite) and Museum of Natural History in Luxembourg (sklodowskite, oursinite, and soddyite).
Intensity data were collected using two different X-ray diffractometers (XRD).( 1) Rigaku SuperNova single-crystal diffractometer connected with the Atlas S2 CCD detector; equipped with Mo micro-focus X-ray tube, working at 50 kV and 0.8 mA (40 W), providing a beam spot of ~120 µm; and (2) Oxford Diffraction Gemini single-crystal diffractometer equipped with the Atlas CCD detector and using monochromated MoKα radiation from a sealed X-ray tube, operating at 50 kV and 35 mA (1500 W), collimated with a fiber-optics Mo-Enhance collimator.Data processing was done using CrysAlis pro software [44].
For the structure solution, SHELXT software [45] was used and structures were subsequently refined by Jana2006 [46] against F 2 .Hydrogen atom positions were localized from the difference Fourier maps and the H atoms found were treated by the mixture of independent and constrained refinement.The resulting R values are given in overview in Table 2. Crystallographic information files were deposited as Supplementary files.

Complexity Calculations and a Bond-Valence Approach
The structural complexity, the Shannon information content per atom (I G ) and per unit cell (I G,total ), were calculated using an approach proposed in [47,48], as follows: the complexity of a crystal structure can be characterized quantitatively by the amount of Shannon information, which is measured in bits (binary digits) per atom (bits/atom) and per unit cell (bits/cell), respectively.The concept used herein of Shannon information, also known as Shannon entropy, originates from information theory.The amount of Shannon information reflects the diversity and relative proportion of different objects e.g., the number and relative proportion of different sites in an elementary unit cell of a crystal structure.The corresponding equations can be found in the references given above.The information based structural-complexity parameters for hereby studied uranyl silicates were calculated using the crystal-structure data by the software package TOPOS [49].The surrogate H atoms were used in calculations to calculate also the contribution of the H atoms to the overall complexity of the minerals, where structure data lack information about H atoms.
The chemical complexity was estimated by considering chemical formula as a message, where symbols correspond to different chemical elements, and it was calculated equivalently to the structural complexity following the approach given by [50].

Mineralogy and Crystallography of Natural Uranyl Silicates
Based on the U:Si ratio, uranyl silicates may be subdivided into three major groups: the uranophane group (1:1), weeksite and haiweeite (2:5), and soddyite (2:1).The vast majority of natural uranyl silicate structures are based upon sheets topologies, with a few minerals containing heteropolyhedral frameworks.The higher the U:Si ratio (i.e., the higher Si concentration) the higher is the degree of polymerization of silicate tetrahedra.An increase of the U:Si ratio is also related to a change of linkage mode between U and Si polyhedra, from edge-to corner-sharing.The high U:Si is characteristic for the synthetic compounds [23][24][25][26][27][28][29], which will not be mentioned in the following text.The approach concerning secondary-building units (SBUs)is a useful tool for highlighting differences in mineral groups mentioned above.This approach has been recently successfully applied to a group of synthetic uranyl silicates and germanates, mostly having framework structures [29].In the case of natural uranyl silicates the two SBUs can be identified, both of them with five-connected U atoms (Figure 1).The first type, with U 2 Si 2 tetramers, is typical for the uranophane group of minerals, while the second type, where three additional Si tetrahedra are linked to U 2 Si 2 to form U 2 Si 5 heptamers, is characteristic of the weeksite group of minerals.

Complexity Calculations and a Bond-Valence Approach
The structural complexity, the Shannon information content per atom (IG) and per unit cell (IG,total), were calculated using an approach proposed in [47,48], as follows: the complexity of a crystal structure can be characterized quantitatively by the amount of Shannon information, which is measured in bits (binary digits) per atom (bits/atom) and per unit cell (bits/cell), respectively.The concept used herein of Shannon information, also known as Shannon entropy, originates from information theory.The amount of Shannon information reflects the diversity and relative proportion of different objects e.g., the number and relative proportion of different sites in an elementary unit cell of a crystal structure.The corresponding equations can be found in the references given above.The information based structural-complexity parameters for hereby studied uranyl silicates were calculated using the crystal-structure data by the software package TOPOS [49].The surrogate H atoms were used in calculations to calculate also the contribution of the H atoms to the overall complexity of the minerals, where structure data lack information about H atoms.
The chemical complexity was estimated by considering chemical formula as a message, where symbols correspond to different chemical elements, and it was calculated equivalently to the structural complexity following the approach given by [50].

Mineralogy and Crystallography of Natural Uranyl Silicates
Based on the U:Si ratio, uranyl silicates may be subdivided into three major groups: the uranophane group (1:1), weeksite and haiweeite (2:5), and soddyite (2:1).The vast majority of natural uranyl silicate structures are based upon sheets topologies, with a few minerals containing heteropolyhedral frameworks.The higher the U:Si ratio (i.e., the higher Si concentration) the higher is the degree of polymerization of silicate tetrahedra.An increase of the U:Si ratio is also related to a change of linkage mode between U and Si polyhedra, from edge-to corner-sharing.The high U:Si is characteristic for the synthetic compounds [23][24][25][26][27][28][29], which will not be mentioned in the following text.The approach concerning secondary-building units (SBUs)is a useful tool for highlighting differences in mineral groups mentioned above.This approach has been recently successfully applied to a group of synthetic uranyl silicates and germanates, mostly having framework structures [29].In the case of natural uranyl silicates the two SBUs can be identified, both of them with five-connected U atoms (Figure 1).The first type, with U2Si2 tetramers, is typical for the uranophane group of minerals, while the second type, where three additional Si tetrahedra are linked to U2Si2 to form U2Si5 heptamers, is characteristic of the weeksite group of minerals.

Uranophane Group (1:1)
The best-known representatives of the group, and also the most frequently occurring, are uranophane-α (commonly called uranophane) (Figure 2a) and uranophane-β (Figure 2b).They are both monoclinic polymorphs of Ca[(UO 2 ) 2 (SiO 3 OH) 2 ]•5H 2 O.While uranophane-α crystallizes in the space group P2 1 , the structure of uranophane-β adopts P2 1 /c.All minerals of this group are based upon layered structures, with sheets of so-called uranophane topology (Figure 3a) that contain (UO 7 ) pentagonal bipyramids and acid [SiO 3 (OH)] groups (except of kasolite, which contains SiO 4 groups), occupying triangles of the topology.The pentagonal bipyramids form edge-sharing chains that are connected by Si-tetrahedra.In minerals with the uranophane type of sheets there are several orientational isomers, differing in the orientation of the free vertices of the SiO 4 tetrahedra pointing up (u) or down (d) relative to the plane of the sheets [17].For example, both uranophane-α and uranophane-β have PT (pentagon-triangles) chains with vertices of tetrahedra pointing all up or all down (Figure 3b,c), while for an ST (square-triangles) chain there is a . . .ududud . . . the sequence for uranophane-α (Figure 3b) and . . .uudduudd . . .sequence for uranophane-β (Figure 3c) [36].Adjacent sheets of U-Si polyhedra are linked through the O yl -Metal-O yl bonds (yl = O that belongs to uranyl) as in the case of both uranophanes or cuprosklodowskite, and O Si -Metal-O Si (Si = O atoms that are linked to Si) as in the case of sklodowskite.The additional linkage is provided through H-bonds that secure transfer of the charge from the interstitial cations to the sheets of polyhedra, the anionic part of the structure.Such a linkage is done by the action of transformer H 2 O groups (O is 3-fold coordinated), linked to metal cation sites, through non-transformer H 2 O groups (O is 4-fold coordinated), "free" H 2 O not-bonded directly to any metal cation site, as well as by OH groups.These are localized, except for kasolite that does not contain any of OH, either at the free vertex (not linked to the Me site) of the Si-tetrahedron (cuprosklodowskite, uranophane-α, uranophane-β), or at the same vertex of the Si-tetrahedron, but linked to the interstitial Me site (sklodowskite, oursinite).In the case of sklodowskite, the position of the (OH) group was inferred mistakenly by [12]; however, in the case of oursinite, which is the derivative structure of sklodowskite, it has been localized properly by [17].
, and cuprosklodowskite (Figure 1d), Cu[(UO 2 )(SiO 3 OH)] 2 •6H 2 O, have tightly related structures, differing in symmetry due to the presence of cations with distinct stereochemistry.The lowering from the C2/m (sklodowskite) (Figure 4b) to P-1 (cuprosklodowskite) is due to Jahn-Teller effect on Cu 2+ in the octahedral crystal-field and a consequent change of coordination from O h to C 4v (4 + 2).Therefore, the Cu in cuprosklodowskite is linked by the two longer bonds to O yl atoms instead of the bonds towards O Si atoms (Figure 4a).
Oursinite (Figure 2e), (Co 0.82 Mg 0.18 ) Σ1.00 , is a derivative of the sklodowskite structure, where Co is substituting for Mg at the interlayer octahedrally coordinated Me site (Figure 4c).Interestingly, the current refinement provided practically the same Co:Mg ratio as reported in [17].Moreover, few additional crystals were tested for the chemical composition on the electron microprobe; all preliminary results showed practically the same Co:Mg ratio for all checked crystals.It seems that Mg is essential there for the stabilization of the structure.Interestingly, the current structure refinement provided essentially the same peak in the difference Fourier map as [17]; this maximum is located ~1.6 Å from O yl atom.A trial to refine this residual maximum as a partially occupied O site lowered R values considerably (drop down about 0.75%) and a goodness-of-fit (GOF) (about 0.5), however, yielded only a low population fraction and negative values of U eq .Check for possible twinning, even considering oursinite as monoclinic with a halved unit-cell volume (twinned by the two-fold axis in 001*) did not lead to a better fit (many correlations among atom fractional coordinates and atomic displacement parameters) and or removal of this peak in the difference Fourier map.
Kasolite (Figure 2f), a Pb-dominant member of the uranophane group with the composition Pb[(UO 2 )(SiO 4 )]•H 2 O, is the only one known member of this class of minerals that do not contain protonated structural sheets.It is an effect of the incorporation of the large Pb 2+ cation into the dense structure.Based on the structure refinement [19] it seems that the H 2 O group in kasolite is a non-transformer, instead of the inverse transformer as suggested by [51].Sklodowskite (Figure 2c), Mg[(UO2)(SiO3OH)]2•6H2O, and cuprosklodowskite (Figure 1d), Cu[(UO2)(SiO3OH)]2•6H2O, have tightly related structures, differing in symmetry due to the presence of cations with distinct stereochemistry.The lowering from the C2/m (sklodowskite) (Figure 4b) to P-1 (cuprosklodowskite) is due to Jahn-Teller effect on Cu 2+ in the octahedral crystal-field and a consequent change of coordination from Oh to C4v (4 + 2).Therefore, the Cu in cuprosklodowskite is linked by the two longer bonds to Oyl atoms instead of the bonds towards OSi atoms (Figure 4a).
Oursinite (Figure 2e), (Co0.82Mg0.18)Σ1.00[(UO2)(SiO3OH)]2•6H2O, is a derivative of the sklodowskite structure, where Co is substituting for Mg at the interlayer octahedrally coordinated Me site (Figure 4c).Interestingly, the current refinement provided practically the same Co:Mg ratio as reported in [17].Moreover, few additional crystals were tested for the chemical composition on the electron microprobe; all preliminary results showed practically the same Co:Mg ratio for all checked crystals.It seems that Mg is essential there for the stabilization of the structure.Interestingly, the current structure refinement provided essentially the same peak in the difference Fourier map as [17]; this maximum is located ~1.6 Å from Oyl atom.A trial to refine this residual maximum as a partially occupied O site lowered R values considerably (drop down about 0.75%) and a goodness-of-fit (GOF) (about 0.5), however, yielded only a low population fraction and negative values of Ueq.Check for possible twinning, even considering oursinite as monoclinic with a halved unit-cell volume (twinned by the two-fold axis in 001*) did not lead to a better fit (many correlations among atom fractional Boltwoodite (Figure 2g), K 2 [(UO 2 )(SiO 3 OH)] 2 •3H 2 O, usually with a particular portion of K substituted by Na.The Na-rich variety is usually called natroboltwoodite (or earlier also sodiumboltwoodite).The reliable structure data were given by [35], documenting that boltwoodite is monoclinic (space-group P2 1 /m), with the formula (K 0.56 Na 0.42 )[(UO 2 )(SiO 3 OH)](H 2 O) 1.5 , obtained from the refinement of the structure and supported by the results of an electron microprobe analysis.The structure is based upon sheets with the uranophane anion-topology (ud stereoisomer) and interlayer with four distinct atom sites that are partially occupied by one K (in seven-fold coordination), one Na (in six-fold coordination), and two O (of the H 2 O groups) atoms.The structure and electron-microprobe study evidenced that there are not any H 3 O + groups in the structure as assumed previously by [14].Positions of H atoms in the structure of boltwoodite have not been determined so far.
Swamboite-(Nd) (Figure 2h), Nd 0.333 [(UO 2 )(SiO 3 OH)]•2.41H 2 O, is a particularly interesting member of the uranophane group in particular; its relation to this class of minerals has been established and elucidated recently [36].Swamboite-(Nd) is one of only a few uranyl minerals with super-structural behavior.The effect of incorporation of highly charged Nd 3+ cations between the [(UO 2 )(SiO 3 OH)] - uranophane-type of sheets and a low fraction of H 2 O lead to both positional and occupational ordering of Nd 3+ in the interlayer resulting in the (6, 3, 2) supercell of swamboite-(Nd).Kasolite (Figure 2f), a Pb-dominant member of the uranophane group with the composition Pb[(UO2)(SiO4)]•H2O, is the only one known member of this class of minerals that do not contain protonated structural sheets.It is an effect of the incorporation of the large Pb 2+ cation into the dense structure.Based on the structure refinement [19] it seems that the H2O group in kasolite is a nontransformer, instead of the inverse transformer as suggested by [51].
Boltwoodite (Figure 2g), K2[(UO2)(SiO3OH)]2•3H2O, usually with a particular portion of K substituted by Na.The Na-rich variety is usually called natroboltwoodite (or earlier also sodiumboltwoodite).The reliable structure data were given by [35], documenting that boltwoodite is monoclinic (space-group P21/m), with the formula (K0.56Na0.42)[(UO2)(SiO3OH)](H2O)1.5, obtained from the refinement of the structure and supported by the results of an electron microprobe analysis.The structure is based upon sheets with the uranophane anion-topology (ud stereoisomer) and interlayer with four distinct atom sites that are partially occupied by one K (in seven-fold coordination), one Na (in six-fold coordination), and two O (of the H2O groups) atoms.The structure and electron-microprobe study evidenced that there are not any H3O + groups in the structure as

Weeksite Group (2:5)
Weeksite group is characteristic for the structures with chains of SiO 4 tetrahedra.The mineral weeksite (Figure 5a), K 2 [(UO 2 ) 2 (Si 5 O 13 )]•4H 2 O, contains chains of edge-sharing UO 7 bipyramids that are linked by chains and sheets of vertex-sharing SiO 4 tetrahedra (Figure 6a).The sheets consist of 6-membered and 14-membered SiO 4 rings (Figure 6c) that are oriented perpendicular to the approximate plane formed by the equatorial ligands of the bipyramids.Therefore, these sheets both serve to link the chains of UO 7 into sheets and to link adjacent sheets together.Weeksite was considered to be an orthorhombic mineral in the past [52], but more recent investigations proved that crystals are usually twinned, and the real symmetry is monoclinic.
Haiweeite (Figure 5b), ideally Ca[(UO 2 ) 2 (SiO 3 OH) 2 (Si 3 O 6 )]•6H 2 O, is another naturally occurring uranyl silicate with 2:5 ratio.Noteworthy, its structure contains uranophane-type chains of edge-sharing UO 7 bipyramids with edge-sharing SiO 4 tetrahedra.Nevertheless, the linkages between these chains differ much from that of the uranophane group.In the structure of haiweeite, a complex network of SiO 4 tetrahedra links these chains of UO 7 bipyramids and SiO 4 tetrahedra, whereas, in the uranophane group, the chains are linked directly by sharing vertices.Adjacent sheets are then linked via Ca-O bonds and H-bonds [20] (Figure 6b).are linked by chains and sheets of vertex-sharing SiO4 tetrahedra (Figure 6a).The sheets consist of 6membered and 14-membered SiO4 rings (Figure 6c) that are oriented perpendicular to the approximate plane formed by the equatorial ligands of the bipyramids.Therefore, these sheets both serve to link the chains of UO7 into sheets and to link adjacent sheets together.Weeksite was considered to be an orthorhombic mineral in the past [52], but more recent investigations proved that crystals are usually twinned, and the real symmetry is monoclinic.Haiweeite (Figure 5b), ideally Ca[(UO2)2(SiO3OH)2(Si3O6)]•6H2O, is another naturally occurring uranyl silicate with 2:5 ratio.Noteworthy, its structure contains uranophane-type chains of edgesharing UO7 bipyramids with edge-sharing SiO4 tetrahedra.Nevertheless, the linkages between these chains differ much from that of the uranophane group.In the structure of haiweeite, a complex network of SiO4 tetrahedra links these chains of UO7 bipyramids and SiO4 tetrahedra, whereas, in the uranophane group, the chains are linked directly by sharing vertices.Adjacent sheets are then linked via Ca-O bonds and H-bonds [20] (Figure 6b).The structure of coutihnoite, ThxBa1-2x(H2O)y[(UO2)2(Si5O13)]•3H2O (with 0 < x < 0.5, 0 < y< (2 + x)), remains unknown; however, the suggestion that it is related to the structure of weeksite has been made by [37], based on the stoichiometry and similarities in diffraction pattern.
down [001].Color scheme same as previous, except of Ca atoms (lavender).(c) Silicate sheet resulting from sharing of six-membered rings of Si-tetrahedra, which is present in the structure of both minerals.

Structural and Chemical Complexity of Uranyl Silicate Minerals
Complexity (information) measures for uranyl silicate minerals as calculated are given in Table 3; calculations were done taking into of account the H atoms.The median of I G,total values is 281 bits/cell (average is 1593 bits/cell); the distribution of the complexity values is highly asymmetric, showing a positive skewness (=3.161).Crystal structures of naturally occurring uranyl silicates can be characterized as simple (<100 bits/cell) to intermediate (100-500 bits/cell) regarding the information content.Nevertheless, there are few exceptions, reaching values of complexity making them complex, or very complex.Namely, it goes about uranophane-β (692 bits/cell), haiweeite (2314 bits/cell) and swamboite-(Nd).The last of those three minerals mentioned is worth more detailed comment.Spgr.-space group; v-the number of vertices in the corresponding quotient graph; I G -structural complexity contribution per atom (bits/atom); I G,total -structural complexity of the whole structure (bits/cell); I Chem -chemical information measures (bits/formula).# -average structure; Super-superstructure, space-group P2 1 /m(a1, b1, g1)00(-a1, b1, g1)00(a2, 0, g2)0s.
Minerals 2018, 8, 551 10 of 15 The modulation in the structure of swamboite-(Nd) is a commensurate one; therefore, it can also be described by the super-cell.In Table 3, two values can, therefore, be found for swamboite-(Nd), one for average structure and a second for a superstructure.It is clear that the one for the average structure is a rough approximation only.The difference between those two numbers reflects the "amount of frustration".The calculation of complexity measures for modulated structures has not been established so far.However, it seems that for the commensurate case the super-cell approach should work.The number of 12,836.18bits/cell makes swamboite-(Nd) extraordinarily complex, one of the most complex minerals in Nature, besides ewingite [53], Mg 8 Ca 8 (UO 2 ) 24 (CO 3 ) 30 O 4 (OH) 12 (H 2 O) 138 (25,882.01bits/cell; including H atoms-surrogated), ilmajokite, (Na,Ca,Ba) 10 Ti 5 Si 14 O 22 (OH) 44 •nH 2 O (7106.51 bits/cell; H atoms not considered) [54], and paulingite, K 6 Ca 16 (Al 38 Si 130 O 336 )(H 2 O) 113 (6767.00bits/cell) [47].From the point of view of chemistry, uranyl silicates are relatively simple minerals (Table 3), none of which exceed 100 bits/formula.However, both weeksite and haiweeite, which also possess an increased structural complexity, belong to chemically less simple uranyl silicate minerals (Figure 8).

Questionable, Poorly Defined or Unnamed Natural Uranyl Silicates
There are some questionable or poorly defined or even unnamed natural uranyl silicates, usually with a lack of detailed structural data.A short overview, which is given below, aims to provide a short review of available data for those phases that are either poorly defined minerals or related to the known minerals.
Calcioursilite, Ca4(UO2)4(Si2O5)5(OH)6•15H2O, was described as a new mineral from the Oktyabr'skoye uranium deposit in Tajikistan by Chernikov in 1957 [40], but under the original name "ursilite".In 2018 [42] and references therein discredited "ursilite" as an old name and instead two mineral names were established: calcioursilite and magnioursilite.However, there is a particular controversy, because calcioursilite is considered as pentadecahydrate while magnioursilite as eikosahydrate, Mg4(UO2)4(Si2O5)5(OH)6•20H2O.The difference in water content makes sense from the point of view of the crystal chemistry; nevertheless, it should be probably reflected by the nomenclature as well.
Metahaiweeite, Ca[(UO2)2(SiO3OH)2(Si3O6)]•nH2O (where n < 6) has been mentioned in the original description [55] as a lower hydrate of haiweeite, being a part of partially dehydrated aggregates.There are neither any other relevant data on natural metahaiweeite nor any hightemperature X-ray studies available.
Orlite, Pb3(UO2)3(Si2O7)2•6H2O, mentioned from an unknown locality in the former USSR by [56] Figure 8.The relationship between the structural and chemical complexity of natural uranyl silicates (value of swamboite-(Nd) taken for the average structure; the value of the superstructure is taken as an outlier and is not shown).

Questionable, Poorly Defined or Unnamed Natural Uranyl Silicates
There are some questionable or poorly defined or even unnamed natural uranyl silicates, usually with a lack of detailed structural data.A short overview, which is given below, aims to provide a short review of available data for those phases that are either poorly defined minerals or related to the known minerals.
Calcioursilite, Ca 4 (UO 2 ) 4 (Si 2 O 5 ) 5 (OH) 6 •15H 2 O, was described as a new mineral from the Oktyabr'skoye uranium deposit in Tajikistan by Chernikov in 1957 [40], but under the original name "ursilite".In 2018 [42] and references therein discredited "ursilite" as an old name and instead two mineral names were established: calcioursilite and magnioursilite.However, there is a particular controversy, because calcioursilite is considered as pentadecahydrate while magnioursilite as eikosahydrate, Mg 4 (UO 2 ) 4 (Si 2 O 5 ) 5 (OH) 6 •20H 2 O.The difference in water content makes sense from the point of view of the crystal chemistry; nevertheless, it should be probably reflected by the nomenclature as well.
Metahaiweeite, Ca[(UO 2 ) 2 (SiO 3 OH) 2 (Si 3 O 6 )]•nH 2 O (where n < 6) has been mentioned in the original description [55] as a lower hydrate of haiweeite, being a part of partially dehydrated aggregates.There are neither any other relevant data on natural metahaiweeite nor any high-temperature X-ray studies available.
Orlite, Pb 3 (UO 2 ) 3 (Si 2 O 7 ) 2 •6H 2 O, mentioned from an unknown locality in the former USSR by [56] differs in U:Si ratio from kasolite; however, the Pb:U ratio remains the same.If it goes about a self-standing mineral species or not, is hard to decide.Nevertheless, no material is available for the study and electron-microprobe studies of kasolite specimens from distinct localities did not show any significant deviations in U:Si ratio.
There are two different Ca-uranyl-silicate-titanate phases mentioned from the Dog mine, Ambrosia Lake, New Mexico, described under the codes UM1982-17-SiO:CaTiU and UM1982-19-SiO:TiU [57].It goes about unique chemistry.Unfortunately, no additional or newer information is available.
Uranosilite, UO 3 •7SiO 2 , has been described by [43] from the Krunkelbach Valley Uranium deposit, Menzenschwand, the Black Forest Mountains, Baden-Württemberg, Germany.It is noteworthy that it has an extraordinary Si:U ratio among known phases, which does not resemble any known minerals or synthetic compounds.
A rather interesting unnamed mineral phase with the XRD pattern matching that of the synthetic Ca 2 (UO 2 ) 2 (Si 2 O 5 ) 3 •10H 2 O was reported from Jáchymov [58].Nevertheless it has never been verified.

Laboratory Synthesis of Uranyl Silicates of the Uranophane-Group
Only few research papers report on the synthesis of analogs of naturally occurring uranyl silicate minerals.An interesting paper by Cesbron et al. [59] describes attempts to synthesize uranophane-β.They performed about fifty hydrothermal syntheses (at 150 • C) in steel autoclaves in order to obtain the uranyl silicates found at Nopal I uranium deposit in Mexiko, and to understand their conditions of formation.They reported that the products of the synthesis are weeksite, natroboltwoodite, its K-and (NH 4 )-analogues and uranophane-α.Despite adopting various pH and temperatures, uranophane-β could not have been synthesized.The explanation has been given recently by [36].Uranophane-α was obtained using aqueous solutions of uranyl acetate, calcium acetate, and sodium metasilicate as reagents.Interestingly, uranophane-α may crystallize from solutions of a broad range of Ca:Na ratios ranging from 0.5:2 to 4:2.At low concentrations of Ca, crystallization of boltwoodite and natroboltwoodite takes place, respectively.According to the experiment the acid pH (<5) is crucial to obtain good crystallinity of products; with the higher Si activity, crystallinity decreases.
An alternative way to obtain minerals of the uranophane group has been described in the paper by Vochten et al. [60] focused primarily onto a synthesis of boltwoodite.That was obtained by mixing a solution of uranyl nitrate and KCl in water at pH of 11.5 maintained by adding KOH.This solution was placed into a Teflon vessel in a Parr reaction bomb with a portion of synthetic hydrothermal quartz chunks.The reaction bomb was heated at 185 • C for seven days (under the pressure of ca.1.3 MPa).After cooling to room temperature, the reaction mixture was found to be consisting of quartz fragments, a colloidal precipitate, and a crystalline phase of pale yellow needles and fine prismatic yellow crystals.Those were identified as boltwoodite.Time-dependent experiments showed that the yield of boltwoodite increases to a maximum after seven days, and then it decreased along with the formation of soddyite.The experiments that continued for 40 days or more provided crystals of soddyite only.The slow transformation of boltwoodite into soddyite was confirmed in separate experiments under the same conditions, later on, in which both natural and synthetic boltwoodite were suspended in distilled water.When the synthetic boltwoodite was placed into 2 M solution of Ca(NO 3 ) 2 , MgSO 4 , Pb(NO 3 ) 2 and NaCl enclosed in the Parr reaction autoclave for 24 h at 185 • C, uranophane-α, sklodowskite, kasolite and natroboltwoodite were obtained, based on identification by powder X-ray diffraction.Experiments running for more than fourty days yielded yellow prismatic crystals identified later as Na 2 (UO 2 ) 2 (SiO 4 )F 2 [61].

Concluding Remarks
Uranyl silicates that occur in nature do not represent an extremely populated (about 16 members), but rather a widespread mineral group (uranophane-α has been reported from more than 900 localities all around the world).Among them, those of the uranophane topology are dominating, and furthermore, uranophane-α, the Ca 2+ -dominant species, is by far the most common.This is probably a consequence of preferential precipitation of uranophane-type sheets from aqueous solutions due to their thermodynamic stability [32].Nevertheless, the stability range of uranophanes (both -α and -β) is connected with more alkaline conditions than of soddyite, but approximately with the same activity of SiO 2 in the solution.Soddyite, similarly to a "simple" uranyl carbonate rutherfordine, (UO 2 )(CO 3 ), forms from the elementally-poor solutions of the relatively low pH.The formation of uranophanes is, therefore, connected with the later stages of alteration connected with the leaching of alkalis and alkaline earth elements from surrounding rocks.Unfortunately, there are not any available thermodynamic data for weeksite group of minerals; however, theoretical predictions based on the approach described in [62] show that both weeksite (∆G 0f,298 ~−8825 kJ/mol) and haiweeite (∆G 0f,298 ~−8993 kJ/mol) are both thermodynamically stable phases; the values of their Gibbs free energies of formation are even lower than of uranophane-α, −6193 kJ/mol [32].Both minerals form under entirely distinct conditions than the uranophane group of minerals; the activity of Si should be rather high, which is reflected in a higher Si:U ratio.The typical association for weeksite and haiweeite is a silicate-dominating matrix.
In general, uranyl silicates have been observed as being formed under less oxidizing conditions compared to, e.g., uranyl phosphates and arsenates [6].Although this can depend on local conditions at the particular locality, there are many documents that such a scenario is valid in general [63][64][65][66].

Figure 1 .
Figure 1.Connectivity in secondary-building-units (SBUs) in natural uranyl silicates shown in ball-and-stick and polyhedral representation (U = yellow, Si = green).

Minerals 2018, 8 ,
x FOR PEER REVIEW 7 of 16 coordinates and atomic displacement parameters) and or removal of this peak in the difference Fourier map.

Figure 4 .
Figure 4. Crystal structures of minerals of the uranophane group (including H atoms). (a) Cuprosklodowskite.(b) Sklodowskite.(c) Oursinite.UO7 bipyramids are in yellow; SiO4 tetrahedra are in green, O atoms of the H2O groups are in red, unit-cell edges outlined as red solid-lines.

Figure 4 .
Figure 4. Crystal structures of minerals of the uranophane group (including H atoms). (a) Cuprosklodowskite.(b) Sklodowskite.(c) Oursinite.UO 7 bipyramids are in yellow; SiO 4 tetrahedra are in green, O atoms of the H 2 O groups are in red, unit-cell edges outlined as red solid-lines.

Figure 6 .
Figure 6.Crystal structures of weeksite-group minerals.(a) Weeksite viewed down [001].UO 7 bipyramids are yellow, SiO 4 in green, K atom in violet, O atoms of the H 2 O in red.(b) Haiweeite viewed down [001].Color scheme same as previous, except of Ca atoms (lavender).(c) Silicate sheet resulting from sharing of six-membered rings of Si-tetrahedra, which is present in the structure of both minerals.

Minerals 2018, 8 , 16 Figure 8 .
Figure 8.The relationship between the structural and chemical complexity of natural uranyl silicates (value of swamboite-(Nd) taken for the average structure; the value of the superstructure is taken as an outlier and is not shown).

Table 2 .
Crystallographic data for uranyl silicate minerals studied by single-crystal X-ray diffraction (XRD).

Table 3 .
Uranyl silicate minerals and their complexity measures including H atoms.