Abstract
The author has recently discovered numerous examples of natural high-tem-perature plagioclases more sodic than An 35 from some Miocene rhyolite lavas in Japan. The optical properties of these high-temperature sodic plagioclases differ distinctly from those of what are regarded as low-temperature plagioclases and are similar to those of synthetic high-temperature albite reported by O. F. Tuttle and N. L. Bowen. In this paper, the author presents new data on the refractive indices, axial angles, extinction angles, and Köhler-angles of these high-temperature plagioclases, and revised stereograms showing migration of optical indicatrices and twinning-axes in them.
