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Compositional range of P2/n omphacite from the eclogitic rocks of central Shikoku, Japan

Published online by Cambridge University Press:  05 July 2018

Kazumi Yokoyama
Affiliation:
Geological Institute, University of Tokyo, Hongo, Tokyo 113, Japan
Shohei Banno
Affiliation:
Geological Institute, University of Tokyo, Hongo, Tokyo 113, Japan
Takeo Matsumoto
Affiliation:
Department of Earth Sciences, Kanazawa University, Kanazawa 920, Japan

Summary

The lattice types of omphacite-diopside series clinopyroxenes were determined for 9 Japanese, 2 Norwegian, and 1 South African samples. The boundary between the compositional ranges of P and C lattice clinopyroxenes lies at about 14·5 wt. % CaO in Japanese and Norwegian samples, and the compositional gap between them is very small, if it exists. The South African omphacite has C-lattice in the compositional range of P lattice omphacite in metamorphic eclogites. These data combined with the reported electron microscope studies suggest that the omphacite—diopside series clinopyroxene has a C lattice structure above 3 to 400°C and very slow cooling after regional metamorphism is responsible for the transition to the ordered structure with P lattice symmetry.

Type
Research Article
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1976

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