Composition and mode of occurrence of minerals in Late Permian coals from Zhenxiong County, northeastern Yunnan, China

Abstract Minerals in the Late Permian coals from the Niuchang-Yigu mining area, Zhenxiong County, northeastern Yunnan, China, were investigated using optical microscopy and low temperature ashing plus X-ray diffraction (LTA + XRD). The results showed that minerals in the coal LTAs are mainly quartz, kaolinite, chamosite, mixed-layer illite/smectite (I/S), pyrite, and calcite, with trace amounts of marcasite, dolomite, and bassanite. The authigenic quartz generally occurs in collodetrinite or as a filling in cleats or cell cavities. This silica was mainly derived from aqueous solutions produced by the weathering of basaltic rocks in the Kangdian Upland and from hydrothermal fluids. The presence of β-quartz paramorph grains in collodetrinite probably indicates that these grains were detrital and came from a volcanic ash. Clay minerals are generally embedded in collodetrinite and occur as cell-fillings. Pyrite occurs as framboidal, anhedral, and euhedral grains and a cell-filling. The coals are high in pyrite and the high pyrite content probably results from seawater invading during the stage of peat accumulation. Calcite generally occurs as vein-fillings, indicating an epigenetic origin