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Smectite formation in rhyolitic obsidian as inferred by microscopic (SEM-TEM-AEM) investigation

Published online by Cambridge University Press:  09 July 2018

S. Fiore*
Affiliation:
Istitutodi Ricerca sulle Argille, CNR, C/da S. Loja, 85050 Tito Scalo (PZ), Italy
F. J. Huertas
Affiliation:
Departamento de Cienciasde la Tierra y Química Ambiental, Estación Experimental del Zaidín, CSIC, C/ Prof. Albareda 1, 18008 Granada, Spain
F. Huertas
Affiliation:
Departamento de Cienciasde la Tierra y Química Ambiental, Estación Experimental del Zaidín, CSIC, C/ Prof. Albareda 1, 18008 Granada, Spain
J. Linares
Affiliation:
Departamento de Cienciasde la Tierra y Química Ambiental, Estación Experimental del Zaidín, CSIC, C/ Prof. Albareda 1, 18008 Granada, Spain

Abstract

Experimental alteration of a rhyolitic obsidian by solutions containing variable concentrations of Mg was carried out at 100, 150 and 200°C, for 30, 60 and 90 days, to investigate the mechanism of smectite formation. The smectite exhibits two distinct morphologies: (1) small flakes (aggregates of a few crystals); and (2) large flakes (massive groups). The small flakes are the earlier alteration products. Both morphological types have distinct chemical compositions: the smallest laminae are dioctahedral and contain more Fe, whereas the particles from the rose-shaped aggregates contain Mg and are trioctahedral. Intermediate compositions have been observed between the extreme compositions.

It is suggested that the two morphologies are the result of two distinct genetic processes: (1) the small flakes (Fe-rich smectite) form by solid state rearrangements of the hydrated external layer of glass and/or via the formation of domains within the glass; (2) the rose-shaped aggregates (Mg-rich smectite) form by precipitation from solution.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2001

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