Mineralogical Journal
Online ISSN : 1881-4174
Print ISSN : 0544-2540
ISSN-L : 0544-2540
 
Birefringence and composition of grandite garnet from the Shinyama ore deposit of the Kamaishi mine, Iwate Prefecture, Japan
Tadashi MARIKOYasuhiro NAGAI
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1980 Volume 10 Issue 4 Pages 181-191

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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.
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© 1980 Japan Association of Mineralogical Sciences
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