A group of silicate minerals, the most abundant minerals in the earth’s crust. They have a structure in which (Si,Al)O₄ tetrahedra are linked together with potassium, sodium, and calcium and very occasionally barium ions occupying the large spaces in the framework. The chemical composition of feldspars may be expressed as combinations of the four components: anorthite (An), CaAl₂Si₂O₈; albite (Ab), NaAlSi₃O₈; orthoclase (Or), KAlSi₃O₈; celsian (Ce), BaAl₂Si₂O₈. The feldspars are subdivided into two groups: the alkali feldspars (including microcline, orthoclase, and sanidine), in which potassium is dominant with a smaller proportion of sodium and negligible calcium; and the plagioclase feldspars, which vary in composition in a series that ranges from pure sodium feldspar (albite) through to pure calcium feldspar (anorthite) with negligible potassium. Feldspars form colourless, white, or pink crystals with a hardness of 6 on the Mohs scale.