Flint Clay
Another clay associated with coal beds is a smooth, flintlike refractory clay or mudstone composed dominantly of kaolin, called "flint clay". Flint clay breaks with a pronounced conchoidal fracture and resists slaking in water.
Flint clay can be either detrital or authegenic in origin. Detrital flint clays consist of kaolinite-rich sediments eroded and transported from uplands deeply weathered under tropical climates and redeposited within the coastal plains, in which coal-bearing strata accumulated. Authegenic flint clays consist of sediments altered in place after deposition as beds within acid, organic sediments, i.e. peat, accumulating within swamps and mires.
Flint clays associated with coal typically occur as thin, laterally continuous layers (bands), called "tonsteins", found within coal beds. At least, in case of tonsteins found within coal, the formation of flint clays resulted from the alternation of glass comprising volcanic ash by acid waters after it accumulated as thin beds within peat swamps or mires.
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