The moonstone from Haumyan, North Korea, which belongs to sanidineanorthoclase cryptoperthite series (after Tuttle, 1952), was observed in the transmission electron microscope, and it was found that the angle σ, which defines the position of composition plane (in the pericline twin), of the pericline twinned Na-phase of the moonstone is minus 4.5 degrees, and that the value of the angle σ is nearly the same as that of an anorthoclase twinned after the pericline law.
This fact can be used to estimate the degree of Al, Si order in the crystal structure of the cryptoperthite having Na-phase twinned after the pericline law.
And it is proposed that, at the temperatures between the liquidus and solvus curves in the phase diagram of the alkali feldspar, anorthoclase may be twinned after the pericline law, that is, the exsolution may be preceded by the twinning in the formation process of anorthoclase.

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The crystal structure of a kaolin mineral Al₂Si₂O₅(OH)₄ from the northeast of Yaita city, Tochigi Prefecture has been investigated with the procedure of the X-ray powder pattern-fitting. Assuming a two-layer monoclinic cell, three models with different modes of kaolin-layer stacking were derived. The least-squares fitting of the calculated pattern to the observed one selected out a correct model, giving the final R-value (profile) of 0.067 for 1301 observed intensity data measured with CuKα radiation in the range 15°≤2θ≤80°. The present specimen, which was reported previously to bs a six-layer new polytype of kaolin minerals, has been proved to be an aggregate of nacrite crystals containing a small amount of α-quartz and an amorphous material.

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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.

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