Feldspaattia

Feldspaattia is a common group of rock-forming silicate minerals that make up about 41% of the Earth's continental crust by weight. They are essential components of most igneous and metamorphic rocks, and are also found in many sedimentary rocks. The name feldspar comes from the German words "Feld" (field) and "Spatt" (spar), meaning field spar, reflecting their abundance in agricultural fields and their cleavage properties resembling spar.

Feldspars are tectosilicates, meaning their structure consists of a framework of SiO4 tetrahedra. Aluminum atoms substitute for silicon atoms within this framework, creating a negative charge that is balanced by alkali metal cations, typically sodium (Na+) or potassium (K+), or by divalent cations like calcium (Ca2+). This substitution leads to the two main series of feldspars: the alkali feldspars and the plagioclase feldspars.

Alkali feldspars include minerals like orthoclase, microcline, and sanidine (all potassium feldspars) and albite (sodium feldspar), which can form solid solutions with each other. Plagioclase feldspars form a continuous solid solution series between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). The relative proportions of sodium and calcium determine the specific plagioclase mineral. Feldspars are characterized by their hardness (6-6.5 on the Mohs scale) and their two directions of cleavage at nearly right angles. They are important in the ceramics industry, as glassmaking ingredients, and are sometimes used as gemstones.