Clay mineral precipitation and transformation during burial diagenesis

Abstract

Detrital clay minerals alter systematically during burial diagenesis. Smectites evolve via intermediate ‘illite-smectites’ to illite. Trioctahedral ‘smectite-chlorites’ or ‘vermiculite-chlorites’ evolve towards true (polytype Ib) chlorites. In other reactions, authigenic clays (kaolinite, illite, chlorite) precipitate directly from aqueous solution rather than by continuous modification of some precursor lattice. In yet others, one mineral replaces another. Many of these reactions are influenced by organic matter and the various low molecular mass soluble products of its diagenetic and thermal maturation. Carbon dioxide and organic acids influence pore-water pH and this, in turn, affects the solubility of clay minerals. Trioctahedral phyllosilicates are particularly sensitive. In contrast, organic matter also acts as a reducing agent when iron (III) oxides are destabilized to produce Fe 2+ and a marked increase in alkalinity. This reaction stabilizes trioctahedral clays such as chamosite. The balance between these different reactions affects the course of clay mineral diagenesis and itself varies systematically with burial depth and temperature.