Integrated TEM, XRD and electron microprobe investigation of mixed‐layer chlorite–smectite from the Point Sal ophiolite, California

Abstract

Five basalt samples from the Point Sal ophiolite, California, were examined using HRTEM and AEM in order to compare observations with interpretations of XRD patterns and microprobe analyses. XRD data from ethylene-glycol-saturated samples indicate the following percentages of chlorite in mixed-layer chlorite-smectite identified for each specimen: (i) L2036 = 50%, (ii) L2035 = 70 and 20%, (iii) 1A-13 = 70%, (iv) 1B-42 = 70%, and (v) 1B-55 = 100%. Detailed electron microprobe analyses show that 'chlorite' analyses with high Si, K, Na and Ca contents are the result of interlayering with smectite-like layers. The Fe/(Fe + Mg) ratios of mixed-layer phyllosilicates from Point Sal samples are influenced by the bulk rock composition, not by the percentage of chlorite nor the structure of the phyllosilicate.Measurements of lattice-fringe images indicate that both smectite and chlorite layers are present in the Point Sal samples in abundances similar to those predicted with XRD techniques and that regular alternation of chlorite and smectite occurs at the unit-cell scale. Both 10-and 14-A layers were recorded with HRTEM and interpreted to be smectite and chlorite, respectively. Regular alternation of chlorite and smectite (24-A periodicity) occurs in upper lava samples L2036 and 1A-13, and lower lava sample 1B-42 for as many as seven alternations per crystallite with local layer mistakes. Sample L2035 shows disordered alternation of chlorite and smectite, with juxtaposition of smectite-like layers, suggesting that randomly interlayered chlorite (<O.S)-srnectite exists. Images of lower lava sample 1B-55 show predominantly 14-A layers. Units of 24 A tend to cluster in what may otherwise appear to be disordered mixtures, suggesting the existence of a corrensite end-member having thermodynamic significance.

Chlorite, Corrensite, and Chlorite-Mica in Late Jurassic Fluvio-Lacustrine Sediments of the Cameros Basin of Northeastern Spain

“…Higher substitutions of octahedral Fe 2+ for Mg 2+ in the chlorites may compensate for the distortions introduced by the substitution of the larger A13+ cation for Si 4+ in the tetrahedral sheet (Shau et al, 1990). However, Hillier (1993), Bettison-Varga et al (1991), andMackinnon (1997) found no changes in the Fe/Mg ratio of coexisting chlorites and corrensites. Our results indicate a clear difference between chlorite and corrensite compositions, with well-defined ranges for each phase (Figures 4 and 5).…”

“…However, corrensite and chlorite may also form by replacement of biotite Peacor, 1994a, 1994b;Li et al, 1998) and amphibole (Meunier et al, 1988), or by reaction between Mg-rich carbonate and dioctahedral clay minerals (Hutcheon et al, 1980;Hillier, 1993). In recent years it has been debated whether the transition from smectite to chlorite occurs as a continuous progressive transformation characterized by random and regular interlayering of different proportions of the end-member phases, as described by Bettison-Varga et al (1991), or if such transition is more accurately described as discontinuous steps between discrete sincerite, corrensite, and chlorite (Shau et al, 1990;Hillier, 1993Hillier, , 1995Schiffman and Staudigel, 1995;Roberson et al, 1999). Shau and Peacor (1992) suggested that the continuous transition occurs within incompletely recrystallized samples (i.e., those affected by relatively low fluid/rock interactions), whereas for higher fluid/ rock ratios the discontinuous transformation is favored, and only the discrete phases are present.…”

Prograde Transitions of Corrensite and Chlorite in Low-Grade Pelitic Rocks from the Gaspé Peninsula, Quebec

“…The smectite-like interlayer of corrensite is defined by Si-rich tetrahedral sheets that give rise to a small interlayer net negative charge, which requires that the octahedral sheet have minimum dimensions as consistent with low Fe occupancy to minimize misfit. The analyses of corrensite and chlorite by , Bettison-Varga et al (1991), and Inoue and Utada (1991), however, show decreased or nearly constant Fe/(Mg + Fe) ratios concomitant with an increase of A1/(Si + A1) ratios in conversions of smectite to corrensite. Bettison-Varga et al (1991) suggested that such an inverse chemical relation may be due to a lack of ample fluid, resulting in diverse local chemical environments and formation processes.…”

“…The analyses of corrensite and chlorite by , Bettison-Varga et al (1991), and Inoue and Utada (1991), however, show decreased or nearly constant Fe/(Mg + Fe) ratios concomitant with an increase of A1/(Si + A1) ratios in conversions of smectite to corrensite. Bettison-Varga et al (1991) suggested that such an inverse chemical relation may be due to a lack of ample fluid, resulting in diverse local chemical environments and formation processes. The wide distribution of data points in Figure 17 may be a reflection of local environments, but the general crystal-chemical relations between corrensite and chlorite remain consistent for most analyses obtained for the diagenetic sample.…”

“…Although it is difficult to characterize C/S by XRD, it has recently been shown that analytical and transmission electron microscope (AEM and TEM) studies can characterize textures, crystal-chemical relations, and formation processes of C/S in metamorphosed and hydrothermally altered mafic rocks (Shau et al 1990;Bettison-Varga et al 1991;Shau and Peacor 1992). Although there have been reports of occurrences of expandable chloritic minerals in pelitic rocks based on XRD data, no detailed TEM characterization of C/S in such rocks has been carded out before.…”