Diagenetic modification of detrital muscovite: an example from the Great Limestone Cyclothem (Carboniferous) of Co. Durham, UK

Diagenetically modified muscovite grains occur as a frequent component of many Upper Palaeozoic and Mesozoic fluvial-deltaic sandstones and siltstones from Britain and the North Sea. Mineralogical, textural and chemical investigation of examples from the Great Limestone Cyclothem show that such grains consist simply of muscovite and kaolinite. No other sheet-silicate phases are involved. Diagenetic textures indicate that modified grains result from the displacive growth of kaolinite between opened cleavage sheets of detrital muscovite. Fabrics of this type should consequently be termed kaolinite-cemented. In contrast to similar sheet-silicate intergrowths reported from diagenetically altered biotite and chlorite, little or no alteration of muscovite is actually required to account for observed textures.