Vermiculite-like minerals in low-grade metasediments from the Coastal Range of central Chile
Mixed-layer minerals with optical properties similar to metamorphic vermiculite were identified in rocks belonging to a Palaeozoic and a Triassic formation separated by an angular unconformity and exposed in the Coastal Range of central Chile. Both formations are affected by low-grade metamorphism. The mixed-layer minerals were studied by optical microscopy, X-ray diffraction (XRD), electron microprobe (EMPA), and transmission/analytical electron microscopy (TEM/AEM). Two types of phyllosilicates were identified: chlorite-vermiculite and mica-chlorite, which are present in the Palaeozoic and Triassic rocks respectively. Chlorite-vermiculite mixed layers form packets with well-defined boundaries and mainly show 1:1 ordered sequences. On the contrary, mica-chlorite mixed layers show, in most cases, random sequences evolving laterally toward chlorite. The AEM data indicate compositions close to that of chlorite in the ternary Si-Al-(Fe+Mg+Mn) diagrams for both types of mixed-layer chlorite, they have lower (Fe+Mg) contents, and a higher Si/Al ratio.They are interpreted as products of the transformation of chlorite, developed during prograde metamorphism, and probably represent intermediate, metastable phases, in the chlorite to biotite transformation. phyllosilicates. Relative to the coexisting
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Document Type: Research Article
Publication date: 2002-06-01