feldsparplagioclase

Feldsparplagioclase, often shortened to plagioclase, is a group of tectosilicate minerals. These minerals are the most common minerals in the Earth's crust, making up about 41% by weight of the continental crust. Plagioclase feldspars are a solid solution series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). This means that any composition between pure albite and pure anorthite can exist within the plagioclase series. The name plagioclase itself comes from the Greek words plagios meaning oblique and klasma meaning fragment, referring to its two cleavage directions which are not at right angles to each other.

The chemical formula of plagioclase can be represented as (Na,Ca)(Al,Si)AlSi2O8, where the ratio of sodium to calcium varies. This variation in composition is accompanied by changes in physical properties, such as refractive index and specific gravity. Plagioclase minerals typically exhibit a vitreous luster and are often white or gray in color, though they can also be found in pink, yellow, or green hues depending on impurities. They are relatively hard, with a Mohs hardness of 6 to 6.5.

Plagioclase feldspars are important rock-forming minerals found in a wide variety of igneous, metamorphic, and sedimentary rocks. In igneous rocks, they are abundant in basalt, gabbro, andesite, diorite, granite, and rhyolite. In metamorphic rocks, they are common in gneisses and schists. Their presence and composition are key indicators for classifying and understanding the origin of many geological formations. Industrially, plagioclase is used in the manufacturing of glass, ceramics, and as a filler in paints and plastics.