Potassium Tobelite As Indicator of the Postsedimentary Transformation of Coal-Bearing Sedimentary Complexes

The rare mineral, potassium tobelite being the NH4-bearing mica, was first described in Lower Carboniferous coal-bearing rocks of the Egorshin–Kamensk district on the eastern slope of the Urals. It was formed from kaolinite and pyrophyllite at temperature less than 500°C and pressure about 2000 atm. The formation of tobelite was probably favored by postsedimentary stresses. Owing to sharp increase in pressure, gases were released from organic matter of the coal-bearing sequence and transformed into ammonia, which was subsequently incorporated as NH+4in the structure of clay minerals. The potassium tobelite is an indicator of high-grade metamorphism (up to anthracite rank) of the organic matter in coal-bearing sequences.