Formation of columnar/tabular and vermicular kaolinite in the crystal tonsteins within the Late Carboniferous Coal in the Datong Coalfield, Shanxi Province, North China

The Datong Coalfield, Shanxi Province, North China, is rich in high-grade coal-measure kaolin, primarily occurring as partings within the Late Carboniferous–Early Permian coal seams. There is a type of kaolinite-dominant coarse-grained parting similar to sandstone in appearance, whereas the phanerocrystalline minerals are columnar/tabular and vermicular kaolinite rather than terrigenous detrital minerals. The origins of the coarse-grained partings and the formation mechanism of the columnar/tabular and vermicular kaolinite remain ambiguous. Herein, petrographical, mineralogical, whole-rock geochemical, and zircon U–Pb–Hf isotopic studies of the coarse-grained partings were conducted to explore these issues. The zircons in the coarse-grained partings show typical features of magmatic origin and yield a single age group of very latest Carboniferous at ca. 300 Ma, indicating they were transformed from air-fall volcanic ash. Therefore, the coarse-grained partings are essentially crystal tonsteins. The wide range of negative εHf(t) values and arc-related tectonic affinity of the zircons genetically link the primary volcanic ash to the Late Paleozoic continental arc volcanoes that existed in the Inner Mongolia Paleo-Uplift, related to the southward subduction of the Paleo-Asian oceanic plate beneath the North China Block. Two orthogonal cleavages are generally observed on the columnar/tabular and vermicular kaolinite in the crystal tonsteins, contradicting the crystalline structure of kaolinite that belongs to a 1:1 type phyllosilicate with only one cleavage along its (0 0 1) lattice plane. These columnar/tabular and vermicular kaolinite are considered to be pseudomorphic kaolinite after plagioclase in the crystal-rich volcanic ash. When the crystal-rich volcanic ash fell into open and acidic peat-forming environments (mires), the low-pH water containing abundant organic acids and good water circulation conditions facilitate dissolution of plagioclase along the chemically unstable (0 0 1) lattice planes and leaching out of Na +, Ca 2+, and much of silica. Simultaneously, the Al 3+ or hydroxides of aluminum and silica recrystallized along the (0 0 1) lattice planes of plagioclase as the substrate, constraining layer-by-layer growth of large kaolinite platelets to eventually form pseudomorphic kaolinite. The cleavages perpendicular to the exclusive (0 0 1) cleavage of pseudomorphic kaolinite should be inherited from the twin planes and (0 1 0) cleavage cracks (if present) of plagioclase.