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Mafic Phyllosilicates in Low-Grade Metabasites. Characterization Using Deconvolution Analysis

Published online by Cambridge University Press:  09 July 2018

D. Robinson  and
R. E. Bevins
D. Robinson
Affiliation:
Department of Geology, University of Bristol, Bristol BS8 1RJ, and Department of Geology, National Museum of Wales, Cardiff CF1 3NP, UK
R. E. Bevins
Affiliation:
Department of Geology, University of Bristol, Bristol BS8 1RJ, and Department of Geology, National Museum of Wales, Cardiff CF1 3NP, UK
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Abstract

The <2 µm clay fractions from low-grade metabasalts of eastern North Greenland are mixtures of mafic phyllosilicates and celadonite that show complex X-ray diffraction (XRD) patterns. Interpretation of such patterns is difficult and subjective using only visual examination of peak positions and shapes. Deconvolution analysis offers a less subjective means of identifying peak positions for overlapping peaks and is applied to several composite peaks in a pattern; the peaks identified are then rationalized in terms of specific mixed-layer phases. Comparison against NEWMOD patterns of the identified clay mixture provides an additional constraint on the phases identified. Three mafic phyllosilicates, discrete chlorite and two chlorite-smectites with 60:40 and 80:20 proportions are demonstrated in the same XRD pattern. Modelling using NEWMOD of a mechanical mixture between these mineral shows an excellent match to the observed XRD pattern. The recognition in low-grade metabasalts of chlorite and random and different varieties of regular mixed-layer chlorite-smectite offers support for the classical model of a tri-smectite to chlorite transition. This is in contrast to an alternative model proposing smectite and chlorite end-members with corrensite as a discrete phase crystallizing directly without intervening mixed-layer chlorite-smectite.

Type
Research Article
Information
Clay Minerals , Volume 29 , Issue 2 , June 1994 , pp. 223 - 237
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1994

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