Fluorine mineralisation from burning coal spoil-heaps in the Russian Urals

Summary

Anhydrous iron, aluminum and fluorine-rich paralavas were found in the burned spoil-heaps of the Chelyabinsk coal basin, Russia. The rocks contain tridymite, anorthite, ferroan fluorine-bearing cordierite, fluorine-bearing mullite, periclase, fluorapatite, micas of the F-biotite–F-phlogopite series, fluortopaz, fluorite, and sellaite. The fluorine-rich minerals formed as a result of local thermal reactions of sedimentary carbonates and silicates with gaseous fluorine. During coal combustion fluorine concentrates in the annealed ankeritic marls where the increase of F is hundreds of times over its concentration in the initial sedimentary rocks. The formation of MgF₂ and CaF₂ promotes local melting at relatively low temperatures (T < 1000 °C) with the residuum consisting of two immiscible liquids. One crystallises as the fluorides, the other as fluorine-substituted analogues of the hydrosilicates, which under the extremely dry conditions, produce minerals containing extremely high F-contents.