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Perryite, natural Ni-silicide, is a minor but regular constituent of the metal phase in enstatite chondrite (aubrite) and enstatite chondrite meteorites. Its synthetic analog was shown to have promising catalytic properties. The first-time solution of the crystal structure of natural perryite was completed on the material from the Mount Egerton aubrite. The mineral is trigonal, R-3c, a = 6.6525(5), c = 37.998(5) Å, V = 1456.3(3) and Z = 6. The structure was refined to R1 = 0.0137 based on 457 independent observed reflections. The chemical formula obtained from the structure refinement, (Ni14.14Fe1.88)S16.02PSi5, agrees with that derived from the electron microprobe data, (Ni13.39Fe2.65Co0.01)S16.05P1.22Si4.74. This research showed that P and Si in perryite are ordered, resulting in the simplified formula (Ni,Fe)16PSi8, in contrast to the currently accepted variant (Ni,Fe)8(Si,P)3. The detailed results of EBSD study reveal previously unknown relationships between perryite, associated a-(Fe,Ni) metal (also known as kamacite) and schreibersite, (Fe,Ni)3P. Since enstatitic meteorites represent the early stages of nebular accretion, our results demonstrate that the crystal-chemical factor could affect the differentiation of chemical elements upon the onset of the Solar System formation. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) is used to determine positions of H atoms and to investigate hydrogen bonding in the crystal structures of two polymorphs of Cu3(AsO4)(OH)3: clinoclase and gilmarite. Hydrogen bonds in clinoclase involve interactions between hydroxyl groups and O atoms of arsenate tetrahedra, whereas the crystal structure of gilmarite features OH···OH bonding, which is rather uncommon in copper hydroxy-oxysalts. Information-based parameters of structural complexity for clinoclase and gilmarite show that the former is more complex (IG,total = 213.212 bits/cell) than the latter (IG,total = 53.303 bits/cell), which indirectly points out that gilmarite is metastable. This suggestion is supported by the lower density of gilmarite (4.264 g/cm3) compared to that of clinoclase (4.397 g/cm3). The hypothesis of metastable character of gilmarite is in agreement with the Goldsmith's simplexity principle and the Ostwald-Volmer rule. [ABSTRACT FROM AUTHOR]

A new uranyl sulfate, [Co(H2O) 6]3[(UO2)5 (SO4)8 (H2O)](H2O) 5, has been synthesized using mild hydrothermal methods. The structure (monoclinic, P21 /c, a = 27.1597(14), b = 9.9858 (5), c = 22.7803 (12) Å, β = 106.520(1)°, V= 5923.2(5) ų Z = 4) has been solved by direct methods and refined on the basis of F² for all unique reflections to R1 = 0.056, calculated for the 9124 unique observed reflections (|Fo| ≥ 4σ F). It contains five symmetrically distinct uranyl pentagonal bipyra-mids and eight sulfate tetrahedra that link via the sharing of vertices between uranyl polyhedra and sulfate tetrahedra, resulting in sheets parallel to (001). Adjacent sheets are linked by hydrogen bonds to Co2+ (H2O) 6 octahedra and H2O groups located in the interlayer. The uranyl sulfate sheet contains four- and five-connected uranyl pentagonal bipyramids and three- and four-connected sulfate tetrahedra. The sheet may be constructed using modules from related structures involving pentagonal bipyramids and tetrahedra, and is readily described using a nodal representation. In general, the uranyl sulfate sheets in [Co(H2O) 6]3 [(UO2)5(SO4)8 (H2O)](H2O) 5 are more rigid than the structural units typically found in comparable uranyl molybdates, which involve the sharing of vertices between uranyl pentagonal bipyramids and molybdate tetrahedra. [ABSTRACT FROM AUTHOR]

Orange platy crystals of K2Na8 (UO2)8Mo4O24 [(S,Mo)O4, the first known uranium molybdosulfate, have been prepa-red by high-temperature solid-state reactions. The stmcture (monoclinic, C2/c, a = 24.282(4), b = 12.1170(18), c = 13.6174(17) Å, β = 106.33(1)º, V= 3845.0(9) ų Z= 4) has been solved by direct methods and refined to R1 = 0.058 on the basis of 2837 unique observed reflections. The structure consists of a three-dimensional heteropolyhedral framework based upon the [(UO2)4O4(Mo2O10)] layers parallel to (010). The layers are formed by linkage of chains of edge-sharing UO7 bipyramids running along the c axis via the Mo2O10 dimers of edge-sharing MoO6 octahedra. Linkage of the layers into a three-dimensional framework is provided by the MoO4 and SO4 tetrahedra that share comers with the MoO6 octahedra. The Na+ and K+ cations reside in framework cavities in between the uranyl molybdate layers. The stmcture of K2Na8 (UO2)8Mo4O24 [(S,Mo)O4] is yet another complex derivative of the U2MoO8 structure type. [ABSTRACT FROM AUTHOR]

Anthropogenic rapidcreekite, Ca2(SO4)(CO3)·4H2O, from burnt dumps of coal mines in the Chelyabinsk coal basin, South Urals, Russia, previously described as 'korkinoite', has been studied by single-crystal X-ray diffraction, high-temperature powder X-ray diffraction, infrared (IR) and Raman spectroscopy. The mineral is orthorhombic, Pcnb, a = 15.5334(6) Å, b = 19.2379(7) Å, c = 6.1625(3) Å, V = 1841.54(13) Å3. The crystal structure is based upon heteropolyhedral sheets parallel to (100) and can be described as produced from the crystal structure of gypsum by chemical twinning. The Ca1 and Ca2 sites are coordinated by six O atoms and two H2O molecules each. The CaO8 polyhedra form chains parallel to the с axis, connected via CO3 groups and SO4 tetrahedra to form a two-dimensional arrangement. The sheets are linked to each other by means of the hydrogen bonding network. The main bands of the IR and Raman spectra correspond to symmetric and asymmetric vibrations in the SO42- tetrahedra and CO32- groups. The OH-stretching region possesses high-intensity peaks. 'Korkinoite' is stable up to 200 °C. Thermal expansion has a strong anisotropic character in the ab and bc planes and is almost isotropic within the ac plane. The thermal expansion coefficients are approximately the same at room temperature and 200 °C (×10-6 °C-1): (αa = 53.7, αb = 11.6, αc = 52.0, αv = 117.3 (25 °C); αa = 53.2, αb = 11.6, αc = 51.6, αv = 116.4 (200 °C)). The maximal thermal expansion is along [100] (perpendicular to the plane of the layers). Both 'korkinoite' (anthropogenic rapidcreekite) and rapidcreekite are structurally related to gypsum due to the unit-cell twinning induced by replacing one-half of sulfate groups in gypsum by carbonate ions in rapidcreekite. These structural relations determine the similarity of thermal behavior of gypsum and rapidcreekite with strongly anisotropic expansion within the sheet of Ca polyhedra and sulfate or sulfate and carbonate groups. The current study confirms that 'korkinoite' is identical to rapidcreekite, except for the insignificant differences in the chemical composition. [ABSTRACT FROM AUTHOR]

The crystal structure of magnesian halotrichite has been refined for two samples collected as white efflorescences from the surface of geothermal fields associated with the Koshelevsky (sample VK4-09) and Centralny Semyachik (sample SC2-20) volcanoes (both Kamchatka peninsula, Russia). Halotrichite and its Mg-rich varieties are common products of the acid leaching of rocks, both volcanic and technogenic. The crystal structures of two halotrichite crystals were refined in the P21/n space group (vs. P21/c used previously) with the unit-cell parameters a = 6.1947(2)/6.1963(5) Å, b = 24.2966(8)/24.2821(14) Å, c = 21.0593(8)/21.063(2) Å, β = 96.512(4)/96.563(9) º, V = 3149.2(2)/3148.3(5) Å3, Z = 4 to R1 = 0.055 and 0.067 for 5673 and 3936 reflections with I > 2σI reflections, respectively. The crystal structure consists of isolated Al(H2O)6 octahedra, SO4 tetrahedra, H2O molecules and [X(SO4)(H2O)5]0 clusters (X = Fe, Mg). The chemical analyses of both samples show their enrichment of Mg at the Fe2+ site. The analysis of geometrical parameters of the crystal structures of halotrichite and its Mg-analogue pickeringite suggests that the localization of O atoms carried out in this work is more accurate and the single-crystal X-ray diffraction data for the first time allowed localization of hydrogen atom positions. The refined number of H2O molecules agrees with the ideal chemical formula. The crystal structure complexity of halotrichite is estimated as IG, total = 2305 bits/cell, which belongs to the family of very complex mineral structures. The contribution of hydrogen bonding system plays a significant role in the overall bonding scheme and the overall complexity of the crystal structure, increasing the Shannon information amount more than twice from IG, total(noH) = 988 bits/cell (no hydrogen atoms) to IG, total = 2305 bits/cell (all atoms including hydrogen). The comparative distribution of halotrichite relative to other Fe-Al hydrated sulfates from the standpoint of structural complexity is considered. [ABSTRACT FROM AUTHOR]

Crystal structures of two uranyl selenite minerals derriksite, Cu4[(UO2)(SeO3)2](OH)6, and demesmaekerite, Pb2Cu5[(UO2)2(SeO3)6(OH)6](H2O)2, which structures are based on uranyl selenite 1D structural units, were studied employing single-crystal X-ray diffraction analysis at various temperatures. The refinement of their crystal structures reveals the detailed dynamics of the interatomic interactions during the heating process, which allows describing the thermal behavior. Uranyl selenite chains and their mutual arrangement mainly provide the rigidity of the crystal structure. Thus the lowest expansion in the structure of derriksite is observed along the direction of uranyl selenite chains, while the largest expansion occurs in the direction normal to chains, with the space occupied by lone electron pairs of Se4+ atoms and low covalent bond distribution density. The maximal expansion in the crystal structure of demesmaekerite is manifested approximately along the [100], which matches the direction of chains of less strongly bonded Cu-centered octahedra, and gaps between Cu chains occupied by the Pb cations. The crystal structure of demesmaekerite undergoes contraction in the direction of the space between the U-bearing chains with the deficiency of strong covalent bonds. Contraction of the structure can also be attributed to the orthogonalization of the oblique triclinic angles of the unit cell. It is demonstrated that the assignment of U-bearing units during structure description is reasonably justified since, regardless of their dimensionality, these substructural units are one of the most stable and rigid blocks in the structural architecture, and they govern the thermal behavior of the entire structure. [ABSTRACT FROM AUTHOR]

Magnesiovesuvianite (IMA 2015-104), ideally Ca19Mg(Al,Mg) 12Si18O69 (OH) 19, a new vesuvianite-group member, was found in an old museum specimen labelled as vesuvianite from the Tuydo combe, near Lojane, Republic of Macedonia. It occurs as radiating aggregates up to 2 cm across consisting of acicular tetragonal crystals (up to 7 mm long and 5-40 μm thick) with distinct fibre-optic effect. Associated minerals are calcite, grossular--andradite and clinochlore. Single crystals of magnesiovesuvianite are transparent, light pink with a silky lustre. Dominant crystal forms are {100}, {110}, {101} and {001}. The Mohs' hardness is 6. The measured and calculated densities are 3.30(3) and 3.35 g/cm³, respectively. Mineral is optically uniaxial, negative, ω = 1.725(2), ε = 1.721(2). The chemical composition (wt. % electron-microprobe data) is: SiO2 36.73, Al2O3 20.21, CaO 36.50, MgO 1.80, MnO 0.18, FeO 0.04, Na2O 0.01, H2O 3.10, total 98.37. The empirical formula based on 19 (Ca + Na) apfu is: (Ca18.99Na0.01)Σ19.00(Mg0.60Al0.40)Σ1.00(Al11.05Mg0.70Mn0.07Fe0.02)Σ11.84Si17.84O68.72(OH)9. Absorption bands in the IR spectrum are: 393, 412, 443, 492, 576, 606, 713, 802, 906, 968, 1024, 3200, 3450, 3630 Cm-1. The eight strongest lines of the powder X-ray diffraction pattern are (I-d(Å)-hkl): 23-10.96-110, 22-3.46-240, 33-3.038-510, 100-2.740-432, 21-2.583-522, 94-2.365-620, 19-2.192-710, 25-1.6165-672. Magnesiovesuvianite is tetragonal, space group P4/n, unit-cell parameters refined from the powder data are a 15.5026(3), c 11.7858(5), V 2832.4(2) Ų, Z = 2. The crystal structure has been refined to R1 = 0.027 for 3266 unique observed reflections with Fo ≥ 4σF. The structure refinements provided scattering factors of the Y1A,B sites close to 12 e- that supports predominant occupancy of these sites by the Mg2+ cations, in perfect agreement with the 27Al MAS NMR data. Magnesiovesuvianite is a member of the vesuvianite group with Mg2+ as a dominant cation at the Y1 site. The name magnesiovesuvianite is given to highlight the species-defining role of Mg. [ABSTRACT FROM AUTHOR]

Single crystals of two new uranyl selenates K3 (H3O)[(UO2)4 (SeO4)6 (H2O) 4]·5H2O (I) and K2.5(no)(NO3)0.5[(UO2)2(SeO4)3 (H2O)]·4H2O (II) have been prepared by room-temperature evaporation from aqueous solution of uranyl nitrate, selenic acid, potassium carbonate and (for the compound I) carbamide. The crystal structure of I has been solved by direct me-thods [monoclinic, P21/m, a = 12.001(3), b = 13.613(3), c = 13.753(3) Å, β = 109.187(4)°, V = 2122.0(8) ų= and Z = 2] and refined to R1 = 0.029 (wR2 = 0.084) for 4865 reflections with |Fo| ≥ 4σF using least-square methods. The crystal structure of II has been solved by direct methods [monoclinic, C2/c, a = 20.290(4), b = 10.380(2), c = 21.436(4) Å, β = 103.446(3)°, V= 4391.0( 13) ų and Z=4] and refined to R1 = 0.027 (wR2, = 0.066) for 7944 reflections with |Fo| ≥ 4σF using least-square techniques. The structures of I and II are based upon the [(UO2)2 (SeO4)3 (H2O) 2]2- and [(UO2)2 (SeO4)3 (H2O)]2-layers, respectively, consisting of UO7 pentagonal bipyramids sharing comers with SeO4 tetrahedra. Potassium cations induce curvature of the uranyl selenate layers, which is mediated by the interlayer water molecules, hydronium ions and nitrate groups. The topology of the 2D units in the structtire of I is novel for the structural chemistry of uranyl selenates. [ABSTRACT FROM AUTHOR]