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 I b) 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. This review of clay mineral transformation, precipitation and replacement reactions during burial diagenesis has attempted to gather together evidence that identifies iron as a particularly significant element. Detrital clays are dioctahedral and contain Fe $^{III}$. Authigenic clays are trioctahedral and contain Fe $^{III}$. Diagenesis amounts to elimination of Fe $^{III}$ from the former and precipitation of Fe $^{III}$ in the latter and in ferroan carbonate cements. Reduction of Fe $^{III}$ to Fe $^{III}$ is the critical step because solubility (potential mobility) and alkalinity are markedly affected. Organic matter pyrolysis seems to offer the only possibility as a source of both reducing agents and acids to effect transport in the zone of maximum clay mineral diagenesis. Relatively little seems to be known about low molecular mass polar pyrolysis products but their systematic investigation would seem to be eminently worthwhile. One of the reasons for holding this meeting, however, was the hope and expectation that answers to some at least of these questions would emerge. This paper is a consequence of a programme to obtain `more reliable' clay mineral analyses by analytical transmission electron microscopy of single crystals. Many people in Sheffield have been involved; J. A. Whiteman, B. J. Ireland, R. Mulvaney, C. R. Hughes, C. K. Whittle, M. Vesey and S. Curtis. The programme was funded by the Science and Engineering Research Council, the Natural Environment Research Council and the Amoco Production Company, Tulsa Research Center. M. L. Coleman and D. A. Spears contributed in a more general geochemical way and the manuscript was prepared by S. Forster.