Is the rarity of glauconite in Precambrian Bhima Basin in India related to its chloritization?

An integrated textural, mineralogical and chemical investigation reveals the origin of authigenic glauconite within the Mesoproterozoic Rabanpalli Formation in the Bhima Basin and its subsequent alteration to chlorite. Glauconite forms within the transgressive shallow shelf deposits in the Rabanpalli Formation. It primarily occurs in two modes, as thoroughly replaced form of detrital grains, and as altered zones along cleavages and fractures of quartz, K-feldspar and mica. All varieties of glauconites exhibit high contents of K2O. The Fe2O3 (T) content is highest in glauconite formed within mica, while it is least in glauconite formed by the replacement of feldspars. Pseudomorphic replacement of substrates by glauconite best explains the origin of glauconite in the Rabanpalli Formation. Diagenetic chlorite, mostly occurring as thick rims on grains and along the grain margin fractures replaces glauconite. Chloritization predominantly takes place in the glauconite variety forming within the mica. The mineral chemistry of chlorite is characterized by high MgO and Fe2O3 (T). Chloritization involves the simultaneous removal of Fe, Mg and Si from the glauconite structure to form brucite type sheets. The high contents of Fe2O3 (T) and MgO, reducing porewater and the high thermal maturation of sediments facilitate the formation of chlorite within the Rabanpalli Formation. This study, therefore, indicates that the rarity of glauconite in the Precambrian is related to its alteration to chlorite.