Fayalite from Fe-rich paralavas of ancient coal fires in the Kuzbass,Russia

Fayalite is a common mineral of Fe-rich paralavas related to spontaneous combustion of coal seams. Fayalite has also been found in parabasalts from burned coal waste piles of the Chelyabinsk coal basin. Among paralavas from different combustion metamorphic (CM) complexes of the world, fayalite is the most widespread in the fused rocks of the Kuznetsk coal basin (Kuzbass) and the Ravat area in Tajikistan. The optimal conditions for fayalite formation as products of coal fires in the Kuzbass and Ravat resulted from a favorable combination of the composition of fused protolith (parental rocks) composed of pelitic and Fe-rich sediments and the redox conditions of the deep subsurface ($ f_{O_2 } $ f_{O_2 } is lower than the QFM buffer). In the Kuzbass, fayalite is commonly hosted in high-silica aluminous Fe-rich paralavas composed of Fe-cordierite (sekaninaite), tridymite, hercynite-magnetite, cristobalite, aluminous clinoferrosilite, and Al-K silicic glass. The composition of all Kuzbass fayalites is close to the Fe₂SiO₄ end member. Kuzbass fayalites are characterized by a negligibly low CaO content and higher MnO and P₂O₅ contents like fayalites from burned rocks of other CM complexes. In Kuzbass paralavas, Fe-olivine is the late phase that crystallized after sekaninaite and tridymite, immediately before melt quenching.