Significance and Genesis of Early Diagenetic Glauconite-Pyrite Assemblage in the Quilon Limestone, Kerala

Pyrite and glauconite are the only lauthigenic iron minerals present in Quilon Limestone. Pyrite occurs as fossil infiIlings and as disseminated grains having shapes like cubes and framboids. Four morphologically distinct types of glauconite have been identified, viz., capsulated pellets, globular pellets, microfossil casts and moulds. Glauconite occurs as fine sand sized aggregate of clay particles. The colour varies between different shades of green to greyish green.

The XRD pattern of glauconite pellets shows the presence of a mixed layer clay mineral assemblage of chlorite and montrnorillonite. The pyrite-, glauconite- and micrite-fabrics of the limestone show evidence of their formation in early diagenetic stages. Pyrite has formed in a neutral to alkaline pH condition in a localised reducing condition; glauconite also formed in a similar environment. However, overall environment was oxidising in a shallow and quiet inner shelf area.