PlagioclaseFestlösung

PlagioclaseFestlösung is a term used in mineralogy to describe the complete solid solution series of plagioclase feldspars. This series encompasses the minerals from albite (NaAlSi3O8) to anorthite (CaAl2Si2O8). These two end-members form a continuous range of compositions where sodium and calcium ions can substitute for each other in the crystal structure. The name "plagioclase" itself derives from the Greek words "plagios" meaning oblique and "klasis" meaning fracture, referring to its characteristic cleavage angles.

The composition of plagioclase feldspars is typically expressed by the mole fraction of anorthite, denoted as An, or albite, denoted as Ab. For example, a plagioclase with 30% anorthite and 70% albite would be written as An30Ab70 or An30. The properties of plagioclase, such as its refractive indices, specific gravity, and optical angles, vary systematically across the solid solution series. This variation is crucial for mineral identification in hand samples and thin sections.

Plagioclase feldspars are abundant rock-forming minerals found in a wide variety of igneous, metamorphic, and even some sedimentary rocks. They are particularly common in mafic and intermediate igneous rocks like basalt, andesite, and diorite, and are also significant components of many metamorphic rocks, especially granulites and amphibolites. The presence and composition of plagioclase can provide valuable information about the origin and evolution of rocks.