FELDSPAR GROUP IN THIN SECTION

Feldspar Group

introduction

Common feldspars are tenary solid solution of three cpmpouds: NaAlSi3O8 - albite (Ab), KAlSi3O8 - orthoclase (Or) and CaAlSi3O8 - anorthite (An). Solid solution with a predominance of An + Ab are called plagioclase feldspars, those predominance composed of Ab + Or are called alkali feldspars. At stable equilibrium a rock can contain only two feldspar phases, a plagioclase phase (called below PL) and an alkali feldspar phase (AF), but in practice more than two phases are often present metastable. In the alkali feldspar field the intergrown are commonly visible in an optical microscope, and are known generically as perthite. In the plagioclase field intergrowths are almost always sub-optical.