On the perthitic structure of moonstone (III)

A moonstone having sodium phase twinned according to pericline law

Abstract

The moonstone examined is anorthoclase cryptoperthite (Or₄₀, Ab₅₀, An₁₀) having a bluish purple iridescence which may be caused by fine lamellae in parallel to rhombic section (near (^-601)). The moonstone is composed of one monoclinic potassium phase, one monoclinic sodium phase and three triclinic sodium phases, all these four sodium phases having same chemical composition. The iridescence disappeared on heating the moonstone at temperatures above 618°C at which the moonstone lost the lamellar structure.
In the light of the mutual orientation of those constituent five phases, the perthitic structure of the moonstone is a new type, a model of which is proposed in Fig. 7.