Rapidcreekite of anthropogenic origin - ’korkinoite’ from burnt mine dump in the Chelyabinsk coal basin, South Urals, Russia: crystal structure refinement, thermal behavior and spectroscopic characterization
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  (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.
IF (WoS, 2021): 1.778
5 YEAR IF (WoS, 2021): 1.848
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