Ca3SiO4Cl2—An Anthropogenic Phase from Burnt Mine Dumps of the Chelyabinsk Coal Basin: Crystal Structure Refinement, Spectroscopic Study and Thermal Evolution

Abstract

The mineral-like phase Ca3SiO4Cl2, an anthropogenic anhydrous calcium chlorine-silicate from the Chelyabinsk coal basin has been investigated using single-crystal and high-temperature powder X-ray diffraction and Raman spectroscopy. The empirical formula of this phase was calculated as Ca2.96[(Si0.98P0.03)Σ1.01O4]Cl2, in good agreement with its ideal formula. Ca3SiO4Cl2 is monoclinic, space group P21/с, Z = 4, a = 9.8367(6) Å, b = 6.7159(4) Å, c = 10.8738(7) Å, β = 105.735(6) °, V = 691.43(8) Å3. The crystal structure is based upon the pseudo-layers formed by Ca–O and Si–O bonds separated by Cl atoms. The pseudo-layers are parallel to the (100) plane. The crystal structure of Ca3SiO4Cl2 was refined (R1 = 0.037) and stable up to 660 °C; it expands anisotropically with the direction of the strongest thermal expansion close to parallel to the [−101] direction, which can be explained by the combination of thermal expansion and shear deformations that involves the ‘gliding’ of the Ca silicate layers relative to each other. The Raman spectrum of the compound contains the following bands (cm–1): 950 (ν3), 848 (ν1), 600 (ν4), 466 (ν2), 372 (ν2). The bands near 100–200 cm−1 can be described as lattice modes. The compound had also been found under natural conditions in association with chlorellestadite.

Burned Coal Dumps as a Source of New Compounds: The Novel Mixed-Valent Iron Oxysulfide Ca4Fe2+3Fe3+2O6S4 from the Chelyabinsk Coal Basin, South Ural

“…The ChCB burned coal dumps are a unique object of technogenic mineralization due to the wide variety of mineral-like phases found there and the unusual and diverse manner of their crystallization. About 240 mineral-like technogenic phases have been described there so far, including around 50 new phases. − The formation of crystalline phases at burned coal dumps occurred under a wide range of temperatures (up to extremely high temperatures above 1200 °C), various redox conditions, metastability, and a high intensity of gas–transport reactions. As a result, many unique mineral-like phases (technogenic minerals), including metastable polymorphs, have been described within the dumps from ChCB.…”