Birefringence and composition of grandite garnet from the Shinyama ore deposit of the Kamaishi mine, Iwate Prefecture, Japan

Abstract

Grandite garnet from the Kamaishi mine are subdivided into three types “A”, “B₁” and “B₂”. The garnet “A”, crystallized in the earlier stage, is always isotropic and has the composition ranging from And₂₈Gro₇₂ to And₆₇Gro₃₃. Birefringence of the garnet “B₁”, which is commonly zoned, decreases from about 0.01 to zero with increasing of the andradite mol.% ranging from 46 to 100. The garnet “B₂” whose compositional range almost agrees with that of the garnet “A” is characterized by a maximum birefringence (about 0.004) at And₄₀Gro₆₀. It is suggested that the garnet “B₂” have been formed during a stage between those of the garnet “A” and “B₁”, or recrystallized from the garnet “A” during the crystallization of garnet “B₁”. The refractions and birefringence of garnet And₅₀Gro₅₀ calculated using the rhombic cell model and Lorentz formula are α=1.806, β=1.811, γ=1.819 and γ–α=0.013; the optical orientation is a=X, b=Y and c=Z.