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

Abstract

“Ovchinnikovite”, a unique iron oxysulfide from the burned dumps of the Chelyabinsk coal basin (Ural, Russia), is a product of high-temperature (1000–1200 °C) alteration of xenoliths of siderite and ankerite rocks by the sulfide-rich gases emanating from the interior of the dumps. According to chemical and crystal-structure analysis, the compound is orthorhombic, with the chemical formula Ca₄Fe²⁺₃Fe³⁺₂O₆S₄ (Z = 8). Among five Fe positions with octahedral coordination, two Fe³⁺ sites are bonded to four O and two S atoms each, while three Fe sites occupied by Fe²⁺ are bonded to two O and four S atoms each. The structure is based upon a three-dimensional framework of Fe-centered octahedra, with two types of layers alternating along the a axis. One layer type is built from corner-sharing Fe³⁺O₄S₂ octahedra (connected through O atoms). In the second layer, Fe²⁺O₂S₄ octahedra share faces and corners. S atoms connect the adjacent layers, and Ca atoms with 9-fold coordination are in between the layers. Density functional theory calculations indicate that Fe₄₀Ca₃₂O₄₈S₃₂ (i.e., Fe_2.5Ca₂O₃S₂) is a magnetic semimetal material. Other physical characteristics (spin magnetic moment; band structure; optical and dielectric properties) are presented. Crystal chemical relationships with synthetic Ca–Fe oxysulfides are discussed. Conditions of formation of “ovchinnikovite” are compared to those of rare oxysulfides in natural combustion metamorphic rocks as well as in metallurgical processes. The study of “ovchinnikovite” shows that man-made geochemical environments such as those occurring in burned coal dumps serve as another source of novel crystal chemistries almost unprecedented among synthetic systems.