gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1976.040.315.12 Compositional Range of P2/n Omphacite from the Eclogitic Rocks of Central Shikoku, Japan Yokoyama Kazumi Banno Shohei Geological Institute, University of Tokyo, Hongo, Tokyo 113, Japan Matsumoto Takeo Department of Earth Sciences, Kanazawa University, Kanazawa 920, Japan 9 1976 40 315 773 779 24 11 1975 Copyright © 1976, The Mineralogical Society 1976 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.