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The chemistry and cell parameters of omphacites and related pyroxenes

Published online by Cambridge University Press:  05 July 2018

A. D. Edgar ,
A. Mottana  and
N. D. Macrae
A. D. Edgar
Affiliation:
Department of Geology, University of Western Ontario, London, Canada
A. Mottana
Affiliation:
Department of Geology, University of Western Ontario, London, Canada
N. D. Macrae
Affiliation:
Department of Geology, University of Western Ontario, London, Canada
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Summary

In an attempt to correlate the chemical compositions and cell sizes of omphacites and related pyroxenes, the cell dimensions of fifty-five analysed pyroxenes have been determined, or taken from the literature. Twenty-two of the chemical analyses are new, nineteen of them being done by electron microprobe. Approximately two-thirds of the total number of analyses may be considered first class, the remainder are of doubtful or unknown quality. Cell parameters, determined by X-ray powder diffraction methods, have errors of 0·1 % for the majority of samples, although for some samples taken from the literature errors are unknown.

The majority of methods of recalculating omphacite analyses into their end-member molecules are unsuitable for correlation of cell constants with chemistry, mainly due to the impossibility of graphical representation of more than three end-member molecules, and to the non-stoichiometry of these molecules. Using a modification of Tröger's (1962) method of recalculating chloromelanite analyses the present analyses have been recalculated into the diopside-jadeite-acmite and diopside-jadeite-hedenbergite molecules and compared with their determined cell parameters. Because of the gradations in all parameters between these end-member molecules, determination of compositions based on the cell parameters (a, b, c, vol, or β) can only be made within wide limits. However, using a method of projection of compositions from the acmite and hedenbergite apices to the diopside-jadeite join the ratios of diopside to jadeite can be determined for most samples to within ±5 mol%. As there are the most important constituents of most omphacites, this method permits an approximate estimation of omphacite compositions. From a knowledge of the cell sizes of the omphacite a rough indication of the conditions of formation of its host rock may also be obtained.

Type
Research Article
Information
Mineralogical Magazine , Volume 37 , Issue 285 , March 1969 , pp. 61 - 74
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1969

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