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A rectorite-pyrophyllite-chlorite-illite assemblage in pelitic rocks from Colombia

Published online by Cambridge University Press:  09 July 2018

B. Brattli
B. Brattli*
Affiliation:
Department of Geology and Mineral Resources Engineering, Norwegian University of Scienee and Technology, 7034 Trondheim, Norway
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Abstract

In samples of slate from the Fomeque Formation near Bogota, Colombia, pyrophyllite was found to occur together with mixed-layered illite-smectite, chlorite and illite. Other minerals were quartz, K-feldspar, dolomite and pyrite. X-ray diffraction patterns revealed that the mixed-layer represents an R1 ordered rectorite with 80–90% illite layers. The microfabric is developed as a closely spaced cleavage in the phyllosilicate-rich rocks, and grades into a fracture cleavage with coarsening of the grain size. No cleavage was observed in the interbedded siltstones. It is suggested that the microfrabrics developed in these rocks correspond to high diagenetic to anchizonal conditions. The illite crystallinity from the slate has been measured on glycolated samples and ranges from 0.47 to 0.55°Δ2θ with a mean of 0.52°Δ2θ Based on the stability of R1 ordered rectorite, the illite crystallinity and the microfabric development, it is proposed that the rocks have been subjected to a temperature of ∼200°C at low pressure. At this temperature, pyrophyllite can only be stabilized at the expense of kaolinite and quartz if aH2O ≪ 1.

Type
Research Article
Information
Clay Minerals , Volume 32 , Issue 3 , September 1997 , pp. 425 - 434
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1997

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