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Vermiculite-like minerals in low-grade metasediments from the Coastal Range of central Chile

Published online by Cambridge University Press:  09 July 2018

M. D. Ruiz Cruz ,
E. Puga ,
L. Aguirre ,
M. Vergara  and
D. Morata
M. D. Ruiz Cruz*
Affiliation:
Departamento de Química Inorgánica, Cristalografía y Mineralogía, Facultad de Ciencias, Universidad de Málaga, 29071 Málaga, Spain
E. Puga
Affiliation:
Instituto Andaluz de Cienciasde la Tierra C.S.I.C., Avda. Fuentenueva, 18002 Granada, Spain
L. Aguirre
Affiliation:
Departamento de Geología, Universidad de Chile, Plaza Ercilla803, Santiago, Chile
M. Vergara
Affiliation:
Departamento de Geología, Universidad de Chile, Plaza Ercilla803, Santiago, Chile
D. Morata
Affiliation:
Departamento de Geología, Universidad de Chile, Plaza Ercilla803, Santiago, Chile
*
*E-mail: [email protected]
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Abstract

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 phyllosilicates. Relative to the coexisting 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.

Keywords

Coastal RangeChilechloritemixed-layer mineralsTEMvermiculite
Type
Research Article
Information
Clay Minerals , Volume 37 , Issue 2 , June 2002 , pp. 221 - 234
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2002

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