Fe‐Illites in A Proterozoic Deep Marine Slope Deposit in the Penganga Group of the Pranhita Godavari Valley: Their Origin and Environmental Significance

Ferric illite (Fe‐illite) developed extensively in a siliciclastic submarine fan sequence within a lime‐mud succession in the Proterozoic Penganga Group at Adilabad, South India. The minerals formed as de novo by preferentially replacing high‐K minerals like alkali feldspar and mica. It is compositionally intermediate between glauconite and illite in a continuously varying chemical spectrum. The mineral exhibits an R1 or R3 type stacking sequence, with low (<10%) interlayering of illite and smectite (I/S). All the analyzed grains are characterized by high potash content, and it has been inferred that the maturity is dictated by high ak in the microenvironment. This occurrence strongly calls into question the environmental significance that has so far been attributed to glauconitic minerals.