oligoklaas

Oligoclase is a mineral belonging to the plagioclase feldspar series, a solid solution between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). Specifically, oligoclase is characterized by a sodium content of roughly 70-90% and a calcium content of 10-30%. It is an important rock-forming mineral found in a variety of igneous and metamorphic rocks. Its chemical formula is typically represented as (Na,Ca)Al(Al,Si)Si2O8.

This mineral commonly occurs as irregular grains or as tabular crystals. Its color can range from white and gray to yellowish or greenish. Oligoclase exhibits cleavage in two directions, approximately at right angles, a characteristic feature of all feldspars. It is relatively hard, ranking between 6 and 6.5 on the Mohs hardness scale, and typically possesses a vitreous luster.

Oligoclase is a significant component of many granitoids, including granites and syenites, as well as in some metamorphic rocks such as gneisses and schists. It can also be found in some volcanic rocks. Gem varieties of oligoclase, such as sunstone, are known for their aventurescence, a shimmering effect caused by the presence of tiny, reflective mineral inclusions, typically hematite or goethite. Understanding the composition and occurrence of oligoclase is crucial for petrological studies and mineral exploration.