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Nine different samples of fluorellestadite from Chelyabinsk, Kizel and Kuznetsk coal basins were studied by single-crystal X-ray diffraction analysis, thermal X-ray diffraction (25–800 °C), Infrared (IR) and Raman spectroscopy. Fluorellestadite is hexagonal, space group P63/m, the unit-cell parameters for the nine samples studied vary within rather small ranges: a = 9.415(5) – 9.4808(7) Å, c = 6.906(2) – 6.938(8) Å, V = 530.3(4) – 538.41(9) Å3. The mineral is isotypic with apatite, the structure is based upon isolated TO4 tetrahedra, where the T position is statistically occupied by Si4+ and S6+ with the ideal ratio Si:S equal to 1:1. The fluorine atoms are located in channels of the Ca4[(S,Si)O4]6 framework oriented parallel to the c axis. The thermal expansion of fluorellestadite is almost isotropic in the temperature range 25–800 °C (for ambient temperature: αa = 12.0·10−6 °C−1, αc = 11.9·10−6 °C−1; for 800 °C: αa = 18.2·10−6 °C−1, αc = 18.6·10−6 °C−1). A similar thermal behavior had been observed for fluorapatite. Despite the same structure motifs and close conditions of formation, the samples of fluorellestadite show different S/Si/P occupancies for T site and the F/Cl/OH (X-position) ratios. [ABSTRACT FROM AUTHOR]

The crystal structure of dmisteinbergite has been determined using crystals from the type locality in Kopeisk city, Chelyabinsk area, Southern Urals, Russia. The mineral is trigonal, with the following structure: P312, a = 5.1123(2), c = 14.7420(7) Å, V = 333.67(3) Å3, R1 = 0.045, for 762 unique observed reflections. The most intense bands of the Raman spectra at 327s, 439s, 892s, and 912s cm −1 correspond to different types of tetrahedral stretching vibrations: Si–O, Al–O, O–Si–O, and O–Al–O. The weak bands at 487w, 503w, and 801w cm−1 can be attributed to the valence and deformation modes of Si–O and Al–O bond vibrations in tetrahedra. The weak bands in the range of 70–200 cm−1 can be attributed to Ca–O bond vibrations or lattice modes. The crystal structure of dmisteinbergite is based upon double layers of six-membered rings of corner-sharing AlO4 and SiO4 tetrahedra. The obtained model shows an ordering of Al and Si over four distinct crystallographic sites with tetrahedral coordination, which is evident from the average bond lengths (T = Al, Si), equal to 1.666, 1.713, 1.611, and 1.748 Å for T1, T2, T3, and T4, respectively. One of the oxygen sites (O4) is split, suggesting the existence of two possible conformations of the [Al2Si2O8]2− layers, with different systems of ditrigonal distortions in the adjacent single layers. The observed disorder has a direct influence upon the geometry of the interlayer space and the coordination of the Ca2 site. Whereas the coordination of the Ca1 site is not influenced by the disorder and is trigonal antiprismatic (distorted octahedral), the coordination environment of the Ca2 site includes disordered O atoms and is either trigonal prismatic or trigonal antiprismatic. The observed structural features suggest the possible existence of different varieties of dmisteinbergite that may differ in: (i) degree of disorder of the Al/Si tetrahedral sites, with completely disordered structure having the P63/mcm symmetry; (ii) degree of disorder of the O sites, which may have a direct influence on the coordination features of the Ca2+ cations; (iii) polytypic variations (different stacking sequences and layer shifts). The formation of dmisteinbergite is usually associated with metastable crystallization in both natural and synthetic systems, indicating the kinetic nature of this phase. Information-based complexity calculations indicate that the crystal structures of metastable CaAl2Si2O8 polymorphs dmisteinbergite and svyatoslavite are structurally and topologically simpler than that of their stable counterpart, anorthite, which is in good agreement with Goldsmith's simplexity principle and similar previous observations. [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]