Origin of a kaolinite-NH4-illite-pyrophyllite-chlorite assemblage in a marine-influenced anthracite and associated strata from the Jincheng Coalfield, Qinshui Basin, Northern China

The coal from the Jincheng Coalfield, Qinshui Basin, is a medium- to low-ash yield anthracite. The mineral assemblage in the anthracite is dominated by NH4-illite and kaolinite; with varying proportions of pyrophyllite, chlorite, pyrite, and calcite; and minor anatase, rutile, diaspore, florencite, and synchysite. Kaolinite formed during early diagenetic and epigenetic processes has been largely altered to other phyllosilicates by processes associated with the different stages of coal rank advance. The formation of NH4-illite in the Jincheng coals is the result of illitization of an early diagenetic kaolinite precursor by interaction of kaolinite with nitrogen derived from organic matter. This reaction may have occurred at the latest stage of bituminization (subbituminous to bituminous transition). Pyrophyllite was produced by the interaction of kaolinite and quartz formed by metamorphic processes associated with anthracitization. Cookeite is relatively common in the studied coals. It may have been derived from the reaction of kaolinite with Li ions, although the source of Li is not yet clearly understood. Minerals in the non-coal samples (partings and floor strata) appear to have responded differently to the metamorphic conditions than those in the adjacent coals. Possible mechanisms for the formation of NH4-illite in the Jincheng non-coal rocks at least involve isomorphous substitution of NH4 for K in the K-illite structure. The well-crystallized K-illite in the non-coal rocks probably formed from detrital K-illite, and wasrecrystallized with the coal rank advance. Absence or minor proportions of pyrophyllite in non-coal samples may be due to a lack of epigenetic kaolinite in the non-coal horizons. Pyrophyllitization occurred during a late stage of metamorphism. The general absence of chlorite in the thick non-coal interval is probably due to the low permeability of that horizon, which resulted in a lack of infiltrating Fe- and Mg-bearing fluids.