Occurrence and origins of minerals in mixed-layer illite/smectite-rich coals of the Late Permian age from the Changxing Mine, eastern Yunnan, China

Abstract Minerals in coals from the Changxing Mine of eastern Yunnan, southwestern China, were investigated using low temperature ashing plus X-ray diffraction (LTA + XRD), optical microscopy, and scanning electron microscopy with an energy dispersive X-ray spectrometer (SEM-EDX). The results show that minerals in the Changxing coals include mixed-layer illite/smectite (I/S), quartz, kaolinite, dolomite, Mg-bearing siderite, anatase, ammonium illite, K-illite, calcite, and chamosite, as well as minor amounts of hematite, paragonite, K-feldspar, and an Al-hydroxide mineral (gibbsite?). The Changxing coals are typically rich in mixed-layer I/S, accounting for an average of 47.2% in their low temperature ashes. Mixed-layer I/S occurs mainly in two types: (1) lentoid as thin beds, and (2) fractures- or cells-fillings. The Type 1 is of terrigenous detrital origin and probably related to the weathering products of the Emeishan basalt, while the Type 2 is of epigenetic origin and precipitated from hydrothermal fluids of a mafic magma intrusion. The high K and poor Na in the Type 1 mixed-layer I/S indicate that it is in a higher evolution stage than the Type 2. Ammonium illite, K-illite, and paragonite are probably products of dissolution and recrystallization of mixed-layer I/S, while Mg-bearing siderite and partly chamosite are related to the Fe, Mg, Al, and Si released from the evolution of mixed-layer I/S.