Calcium Fluorosilicate Ca5[SiO4]2F2 from Burnt Dumps of the Chelyabinsk Coal Basin (South Urals): Crystal Chemistry, Spectroscopy, Thermal Behavior

The anthropogenic calcium fluorosilicate “kutyukhinite” Ca₅[SiO₄]₂F₂ from burnt dumps of the Chelyabinsk coal basin has been studied by single crystal and powder X-ray diffraction analyses in a wide temperature range. “Kutyukhinite” (P2₁/a, a = 11.4985(4) Å, b = 5.0535(2) Å, c = 8.7848(3) Å, β = 109.008(4)°, V = 482.63(3) ų, R₁ = 0.0183) is an anthropogenic analogue of kumtyubeite, which belongs to the structural type of chondrodite. The empirical formula of “kutyukhinite” is Ca_5.02[Si_1.99O_7.98]F_2.04. Upon heating, its crystal structure expands anisotropically, with the direction of maximum thermal expansion close to the [100] direction. The relative change in bond lengths in silicon–oxygen tetrahedra with increasing temperature (27–927 °C) is less than 0.6%, which is within the margin of error. At the same time, the relative increase in the average bond length 〈Ca–O〉 varies from 1% (〈Ca1–O〉) to 1.5% (〈Ca3–O〉). The largest relative change of  2% was found for the average bond length 〈Ca3–F1〉.