Abstract
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.
