Origin of the Permo-Triassic clay mica assemblage

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Abstract
Clay mica is the predominant component of the fine-grained siliciclastic sediments of the Western European Permo-Trias and it may occur as the sole component of the clay assemblage. Its characteristics have been studied by chemical analysis, radioisotope (K/Ar) data, X-ray diffraction and electron microscopy in the clay assemblages from Triassic and Permian sediments in Spain, Western Approaches, South Devon and East Yorkshire. The clay mica is a ferric dioctahedral mineral containing on average 6.5% Fe2O3 and 7.5% K2O. Crystal thickness ranges from 8 × 10 Å to 115 × 10 Å, and varies with geological, stratigraphical and grain-size factors. Radioisotope data and geological considerations suggest that much of the Permo-Triassic clay mica was formed originally in coeval desert soils rather than being derived from pre-existing rocks. It was then eroded, sometimes mixed with much older material, and deposited as fine-grained detritus in adjacent areas. Upon deep burial, this detrital mica assemblage underwent recrystallization with the development of euhedral crystals and the alteration of the K/Ar values.