THE VARIABILITY OF MICACEOUS MINERALS IN SEDIMENTARY ROCKS

SUMMARY Relationships between crystal chemistry and genetic conditions are presented for a series of dioctahedral micaceous minerals. Using a ferruginicity index, three groups of micaceous minerals are distinguished: Al‐micas, Fe‐illites, and glauconites. The degree of Al‐for‐Si substitution in the tetrahedral sheets of micaceous minerals depends on their thermodynamic genetic conditions. If pressure and temperature increase, the Al‐IV content increases and the octahedral sheet is “cleaned” from divalent cations. This process is accompanied by structural alteration with a polytype sequence 1M d ‐1M‐2M 1 . Tetrahedral and octahedral charges are the same for micaceous minerals formed at the surface (sedimentogenesis/diagenesis) and are independent of the iron content. For most illites of sedimentary rocks an authigenic origin is suggested.