The observations recorded in paper 1 of this series have been extended to cover 37 specimens of orthoclase and microcline perthites. The exsolved sodium phase of the perthitic specimens is usually a low temperature albite or oligoclase, although some specimens falling near the composition Or80(Ab+An)20 and with 2V between 50° and 70° contain an anorthoclase instead. The anorthoclase occurs as a pericline twin-type superstructure. Most of the albite-oligoclases are dominantly albite twinned: four specimens show both pericline and albite twinning, while seven specimens with compositions ~Or70 and 2Vα between 60° and 80° show mainly albite twin-type superstructure.

The potassium phase is monoclinic in 14 specimens, triclinic in 4 and partly each in 15 specimens. The triclinic components give reflections that vary from sharp to diffuse, with lattice angles α*, γ* varying from 90°, 90° to about 90°24′, 92°20′. All the triclinic components are composed of two or more units whose angular relations vary between the extremes of albite and pericline twinning, with the “diagonal” association fairly common.

The optic axial angles fall either in or close to a triangle formed by the three extreme varieties: low albite, Or0 2Vα 100°; orthoclase, Or100 2Vα 35°; and maximum microcline, Or100 2Vα 80°. There is a moderate, but not good, correlation between the position of the specimens in this triangle and the nature of the potassium-rich component. Specimens which either contain anorthoclase or consist predominantly of the albite twin-type superstructure of low albite-oligoclase fall into two small areas in the 2V versus composition diagram. The preferential occurrence of the superstructure in the more potassium-rich compositions supports the hypothesis that strain favors its existence in orthoclase and microcline perthites. The anorthoclase is thought to occur because the containing specimens have compositions that lie close to a phase boundary, a condition known to favor metastability.

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