Potash felspars in some metamorphic rocks

Abstract

Potash felspars in the Akka metamorphics (contact metamorphic rocks), several other metamorphics and granites are studied optically and by the x-ray powder method. Triclinicity values (Δ) in potash felspars of the Akka metamorphics show monoclinic symmetry, not only in orthoclase zone (higher part of amphibolite facies), but also in cordierite zone. Potash felspars in hornfelses close to granite mass seem to show diffuse reflection and large optic angle. In other metamorphic terrains, potash felspars show extremely low triclinicty values, while those in the Hida gneiss, as well as the Hida granite, show large triclinicity values. From these results, the relations between symmetry change of potash felspar and metamorphism are discussed. From the albite molecular percent in some alkali felspars, the temperature of potash felspar formation is determined by the Barth's method.