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

Kazumi Yokoyama, Shohei Banno, Takeo Matsumoto
1976 Mineralogical magazine  
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
more » ... bined 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.
doi:10.1180/minmag.1976.040.315.12 fatcat:ukhxcaengrhabakin2hzqktpe4