Transmission Electron Microscopy Study of Conversion of Smectite to Illite in Mudstones of the Nankai Trough: Contrast with Coeval Bentonites

Abstract

Abstract~lay minerals in shales from cores at Site 808, Nankai Trough, have been studied using X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and analytical electron microscopy (AEM) to compare the rates and mechanisms of illitization with those of coeval bentonites, which were described previously. Authigenic K-rich smectite having a high Fe content (--7 wt. %) was observed to form directly as an alteration product of volcanic glass at a depth of --500 meters below seafloor (mbsf) with no intermediate precursor. Smectite is then largely replaced by Reichweite, R, (R = 1) illitesmectite (I-S) and minor illite and chlorite over depths from --550 to --700 mbsf. No further mineralogical changes occur to the maximum depth cored, -1300 m. Most smectite and I-S in shales are derived from alteration of glass, rather than being detrital, as is usually assumed. Discrete layer sequences of smectite, I-S, or illite coexist, indicating discontinuities of the transformation from smectite to (R = 1) I-S to illite. Authigenic Fe-rich chlorite forms concomitantly with I-S and illite, with the source of Fe from reactant smectite.Smectite forms from glass with an intermediate precursor in coeval bentonites at approximately the same depth as in shales, but the smectite remains largely unchanged, with the exception of exchange of interlayer cations (K ---) Na ~ Ca) in response to formation of zeolites, to the bottom of the core. Differences in rates of illitization reflect the metastability of the clays. Temperature, structure-state, and composition of reactant smectite are ruled out as determining factors that increase reaction rates here, whereas differences in water/rock ratio (porosity/permeability), Si and K activities, and organic acid content are likely candidates.

Thermal models and clay diagenesis in the Tertiary-Cretaceous sediments of the Alava block (Basque-Cantabrian basin, Spain)

“…It seems to be closely related instead to the smectite-to-illite transformation as this chlorite is present when R0 is absent and shows a clear parallel with illite-rich (R1 and R3) mixed-layer clays. Chlorite could be a by-product originating from Fe, Mg and even Si from the smectite-to-illite transformation (Ahn & Peacor, 1985;Drief & Nieto, 2000;Masuda et al, 2001). …”

Illite-Smectite Mixed-Layer Minerals in the Hydrothermal Alteration of Volcanic Rocks: I. One-Dimensional XRD Structure Analysis and Characterization of Component Layers

“…inferred from the TEM investigation of both diagenetic and hydrothermal I-S samples (Dong et al, 1997;Bauluz et al, 2000;Masuda et al, 2001;Tillick et al, 2001;Yan et al, 2001;Bauluz et al, 2002). In their model, the transformation of smectite illitization is characterized by discontinuity from smectite → R1 I-S → illite without R0, R2, and R3 I-S, using the Reichweite parameter.…”

Alteration, adsorption and nucleation processes on clay–water interfaces: Mechanisms for the retention of uranium by altered clay surfaces on the nanometer scale

“…Tardy and Nalon, 1985;Masuda et al, 2001). For example, one of the best studied alteration processes is the transformation of smectite into illite, a process that will be also of great importance for this study.…”

“…For example, one of the best studied alteration processes is the transformation of smectite into illite, a process that will be also of great importance for this study. The latter process occurs commonly during diagenesis of argillaceous sediments and mainly depends on temperature, pressure, fluid-rock ratio, the activity of H 4 SiO 4 and the activity ratio of K/H (Li et al, 1997;Masuda et al, 2001;and references therein). Three fundamental mechanisms have been proposed for the replacement of smectite by illite (Altaner and Ylagan, 1997): (1) a solid-state-transformation with a layer by layer replacement of smectite by illite (Inoue et al, 1987); (2) a dissolution-(transportation)-crystallization process (Li et al, 1997) and (3) a combination of the processes (1) and (2) (Whitney and Northrop, 1988).…”