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

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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.