Any of a group of aluminosilicate minerals whose crystal structures are composed of corner-sharing (AlO₄) and (SiO₄) tetrahedra linked in an infinite three-dimensional array, with charge-balancing cations (primarily sodium (Na), potassium (K), and calcium (Ca)) occupying large, irregular cavities in the framework of the tetrahedra. Collectively, the feldspars constitute about 60% of the outer 8–10 mi (13–17 km) of the Earth's crust. They are nearly ubiquitous igneous and metamorphic rocks, and are a primary constituent of arkosic sediments derived from them. The importance of the many feldspars that occur so widely in igneous, metamorphic, and some sedimentary rocks cannot be underestimated, especially from the viewpoint of a petrologist attempting to unravel earth history.