plagioklaasreeks

Plagioklaasreeks refers to a series of solid solutions of feldspar minerals composed of varying proportions of albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). These two end-members are miscible at all compositions under geological conditions, forming a continuous range of minerals. The plagioclase series is a fundamental component of many igneous and metamorphic rocks. Its composition is typically expressed as the mole percentage of anorthite, denoted as An. For example, labradorite is a plagioclase with a composition between An50 and An70. The general formula for plagioclase is (Na,Ca)Al(Si,Al)Si2O8, where the ratio of sodium to calcium varies. Plagioclase is characterized by its hardness, typically between 6 and 6.5 on the Mohs scale, and its cleavage angles, which are not at 90 degrees, distinguishing it from alkali feldspars. The optical properties, such as refractive indices and birefringence, also change systematically across the plagioclase series, allowing for identification under a petrographic microscope. Variations in plagioclase composition can provide valuable information about the crystallization conditions and evolutionary history of the rocks in which they are found.