Mineralogic characteristics of pyrophyllite mudstone in Mentougou, western Beijing

The associated mineral deposits in coal-bearing strata in China have many different types and large amount of resources. The pyrophyllite mudstone in Carboniferous-Permian coal-bearing strata in Mentougou of western Beijing is distinctive and rare, and it is of great significance for exploitation and utilization. The mineralogical features of pyrophyllite in Hongmiaoling Formation in Mentougou, western Beijing, were studied by use of polarizing microscopy, X-ray diffraction (XRD), infrared absorption spectra, thermal analysis and electron probe (EPM), etc. The results indicated that the pyrophyllite mineral is co-existed with diasporite and chloritoid. The pyrophyllite under polarizing microscope shows aphanitic or microcrystalline flakes with directional distribution. Based on chemical analysis and calculation of electron probe in micro area the chemical formula for crystal structure of pyrophyllite was calculated as:Al_1.9965 (Si_3.9982 O₁₀)(O₂H_2.0180). According to the characteristics of sedimentary geologic background and minerals association, it was concluded that the formation temperature of pyrophyllite was about 300, and it was℃ transformed from kaolinite under the condition of dynamic metamorphism by tectonic stress.