Calcium fluorosilicate Ca<sub>5</sub>(SiO<sub>4</sub>)<sub>2</sub>F<sub>2</sub> from burnt dumps of the Chelyabinsk coal basin (South Ural

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), b = 5.0535(2), c = 8.7848(3) Å, β = 109.008(4)°, V = 482.63(3) Å3, R1 = 0.0183) is an anthropogenic analogue of kumtyubeite which belongs to the structural type of chondrodite. The empirical formula of “kutyukhinite” is Ca5.02(Si1.99O7.98)F2.04. Upon heating, its crystal structure expands anisotropically, with the direction of maximum thermal expansion close to the direction [100]. 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 of the varies from 1 () to 1.5% (). The largest relative change of 2% was found for the average bond length .