| Summary: | The present study deals with three main mineralogical and geochemical problems related to the behaviour of two common clay minerals---illite and chlorite---in the Lower Paleozoic sedimentary sequences of high-grade diagenetic and very low grade metamorphic (VLGM) zones in the Quebec Appalachians, Gaspe Peninsula. === In high-grade diagenetic to VLGM zones, illite and chlorite coexist, but as they are notoriously fine-grained and intimately intergrown, tracing the chemical changes in chlorite with conventional (wet chemical method or electron microprobe) methods is a difficult task. A refined XRD method for the determination of the structural formula of chlorites was developed in this study. Application of this program to high-grade diagenetic to VLGM chlorite from the Gaspe Peninsula revealed that these chlorites are of "ripidolitic" to "pycnochloritic" composition and show a compositional homogenization in the anchizone, as compared to chlorite of the diagenetic zone. === With increasing metamorphic grade, chlorites become less siliceous, have decreasing octahedral-site vacancies, and approach the ideal trioctahedral chlorite composition. Although Fe-asymmetry increases, the [Fe (total)] decreases, which may reflect a preferential decrease in [Fe (total)] in the brucite-like sheet. === At high (>80%) illite levels in mixed-layer illite smectite, the gradual improvement of illite crystallinity (IC), coupled with advancing VLGM conditions, suggests some modification of the clay mineral structure, possibly involving loss of the still existing expandable layers. Failure to detect expandable components in anchimetamorphic illitic materials upon glycolation has led previous investigators to rule out the existence of expandable components beyond the high-grade diagenetic zone. X-ray diffraction (XRD) patterns and high-resolution transmission electron images of n-alkylammonium-cation-treated clay minerals showed the presence of expandable layers in high-grade diagenetic to VLGM phyllosilicates and document the impact of these expandable layers on IC measurements. === With advancing diagenetic to VLGM grade, the intermediate metastable phases of a rectorite-like R1-ordered layer structure, either with an intermediate or high-charge vermiculite interface, and an R3-ordered structure are found in the fine fraction. (Abstract shortened by UMI.)
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