TEM study of the mechanism of transformation of detrital kaolinite and muscovite to illite/smectite in sediments of the Salton Sea Geothermal Field

Abstract

The reaction by which kaolinite transforms to I-S has been studied by SEM and TEM using core samples from the Salton Sea Scientific Drilling Project. Kaolinite is abundant from the surface to a depth of ∼300 m (∼115°C), decreasing in abundance and becoming undetectable at a depth of ∼500 m (∼160°C), with a concomitant increase in abundance of I-S. In a 256-m deep sample, kaolinite occurs as large (∼100 μm) detrital grains, in the fine-grained (< 1 μm) matrix, and as packets forming stacks in detrital biotite and muscovite. In sharp contrast, no kaolinite was detected in the sample from a depth of 477 m but illite and minor I-S occur in the matrix. Authigenic chlorite occurs as subhedral crystals in the matrix with subparallel illite crystals, and as muscovite-chlorite stacks. I-S occurs in three ways: (1) Interlayered within kaolinite as a few, curved layers with 10- and 20-A periodicities with the characteristics of collapsed, dehydrated I-S. Along-layer transitions with change in (001) d-value from 7 to 10 A occur. (2) As coalescent, randomly oriented packets filling pore space in the matrix. (3) Interlayered with detrital muscovite. These observations show that R1 T-S in Salton Sea-area sediments forms primarily through alteration of kaolinite and muscovite of detrital origin over a temperature interval of ∼100 to < 200°C, with I-S serving as a precursor to illite. Reaction occurs both by direct layer-by-layer replacement of kaolinite and muscovite, and by dissolution, ion transport and crystallization in pore space. Both processes involve dissolution and crystallization, with only the scale of transport varying from that of along-layer interfaces to the dimensions of pore space. The pH value and activity of K+ in the hydrothermal brines play important roles in the alteration process and control the simultaneous transformation of both kaolinite and muscovite to I-S.