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Microstructures and interlayering in pyrophyllite from the Coastal Range of central Chile: evidence of a disequilibrium assemblage

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Abstract:

Pyrophyllite from a Triassic sedimentary formation from the Coastal Range of Chile has been investigated by transmission/analytical electron microscopy (TEM/AEM). The mineral assemblage includes pyrophyllite, muscovite, paragonite, a kaolin mineral, boehmite, rutile and hematite. The textures indicate that the protolith was a volcanoclastic rock. Petrographic evidence, chemistry, and the mineral assemblage suggest the intense leaching of the parent rock by a weathering process, before the metamorphic episode, to create the protolith for the pyrophyllite. Pyrophyllite always grows from the kaolin mineral, and both phases show close orientation relationships. The presence of parallel intergrowths of pyrophyllite and muscovite indicate that muscovite also grew from the kaolin mineral. Nevertheless, the composition of muscovite suggests that this phase must also form from another precursor, probably Al smectite. The AEM data and textural relationships between pyrophyllite and muscovite reveal the presence of two generations of muscovite and suggest that Na-rich muscovite recrystallized into a Na-free muscovite and paragonite.

Keywords: ANALYTICAL ELECTRON MICROSCOPY; ANDES; CHILE; DICKITE; METACLASTITE; MUSCOVITE; PYROPHYLLITE; STRUCTURAL DEFECTS; TRANSMISSION ELECTRON MICROSCOPY

Document Type: Research Article

DOI: http://dx.doi.org/10.1180/0009855043940146

Publication date: December 1, 2004

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