| Popis: |
Fayalite is a widespread mineral in high-Fe paralavas, which arose as a result of combustion metamorphism (CM) of coal-bearing sediments during artificial and natural coal fires. Fayalite as a rock-forming mineral is most common in natural CM-rocks of the Kuznetsk coal basin (Kuzbass) and the Ravat area in Tajikistan. The optimum for fayalite crystallization from CM-melts in Kuznetsk and Ravat fires are resulted from combination of suitable composition of fused metasedimentary rocks (pelitic and high-Fe sediments) and redox conditions (fO2 lower than the OFM buffer) of deep fire foci. In Kuznetsk basin fayalite-bearing paralavas are commonly totally fused sandstone/siltstone with admixture of siderite. Bulk compositions of these paralavas are characterized by high SiO2 (62.6–67.5 wt.%) and Al2O3 (10.9–14.2 wt.%), moderate amounts of iron (13.4–14.2 wt.% Fe2O3) and K2O (1.7–3.4 wt.%), and low CaO (∼0.6 wt.%). In Kuznetsk paralavas fayalite associates with Fe-cordierite (sekaninaite), tridymite, hercynite–magnetite, cristobalite, aluminous clinoferrosilite, and high-silica Al-K-glass. The composition of fayalites is similar to that of the Fe2SiO4 end-member. The mineral contains low CaO and enriched by MnO and P2O5 relative to fayalites from the paralavas of other CM complexes of the world. In the Kuznetsk paralavas, fayalite is a late phase that crystallized immediately before CM-melt quenching. |
