Abstract
Diopside in olivine gabbro from Horoman area, Hokkaido, has exsolution lamellae arranged parallel to the (100) diallage partings. These (100) exsolution lamellae are composed of two different lattices, i. e., a monoclinic pyroxene with space group P21/c, and an orthorhombic pyroxene with space group Pbca. These two phases may presumably be grown sub-microscopically side by side. Estimating the chemical composition of these (100) lamellae by refractive indices and lattice constants, the monoclinic pyroxene with space group P21/c is a closer member to clinoenstatite in the compositional range from clinoenstatite to pigeonite.
