Fluorellestadite from burned coal dumps: crystal structure refinement, vibrational spectroscopy data and thermal behavior

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) A, c = 6.906(2) – 6.938(8) A, V = 530.3(4) – 538.41(9) A3. 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.