Feldspar: major rock-forming tectosilicate minerals

Appearance and physical properties

Feldspars commonly occur as tabular or blocky crystals and typically show two directions of perfect cleavage that meet at right angles. Typical colors include white, gray, pink, and salmon; some varieties are green or brown. Feldspars have a Mohs hardness near 6, making them about as hard as window glass. Under the microscope they show characteristic twinning and optical properties that help distinguish species.

Composition and major groups

All feldspars are framework silicates built from silicon and aluminum tetrahedra; these tetrahedral frameworks are balanced by alkali and alkaline earth cations. They are commonly divided into two principal groups:

• Alkali (K-) feldspars: potassium-bearing species whose ideal chemical formula is KAlSi3SiO8 (often written KAlSi3O8). Common minerals in this group include orthoclase, microcline and sanidine. Sodium can substitute for potassium in some compositions and micro-textures such as perthite record later exsolution.
• Plagioclase feldspars: a continuous solid-solution series between NaAlSi3O8 (albite) and CaAl2Si2O8 (anorthite). Individual plagioclase minerals are classified by their sodium-to-calcium ratio and are abundant in basaltic and gabbroic rocks.

Formation, occurrence and alteration

Feldspars crystallize from cooling magma in both intrusive and extrusive rocks, forming essential constituents of granite, syenite, basalt and andesite. Large bodies composed largely of plagioclase are called anorthosite. Feldspars are also present in many sedimentary rock types as sand grains or detrital fragments and occur in metamorphic rocks where original feldspars have been recrystallized.

At Earth's surface feldspars weather chemically to form clay minerals such as kaolinite and release cations that contribute to soil chemistry. Microtextures produced by cooling histories, pressure and alteration are important clues used by geologists to reconstruct rock histories.

Uses, economic importance and scientific value

Industrially, feldspars are used as a flux in glass and ceramic production, as fillers in paints and plastics, and as raw material in some enamel and frit formulations. Because potassium-bearing feldspars contain radioactive 40K, they are commonly used in geochronology (K–Ar and Ar–Ar dating) to determine the age of igneous rocks. Petrologists and mineralogists rely on feldspar compositions to classify igneous rocks and to interpret magmatic processes.

Notable distinctions and facts

Feldspars are distinguished from other rock-forming minerals by their framework silicate structure and by the range of chemical substitution between sodium, potassium and calcium. Textural phenomena such as perthitic intergrowths (exsolution of Na- and K-rich phases) and characteristic optical twinning are diagnostic in thin section. Because of their abundance and variety, feldspars play a central role in understanding crustal composition and geologic history.

For further reading on mineral classification, crystal chemistry and uses, see linked resources: tectosilicate overview, mineral classification, Earth composition, crystal habits, magma and igneous processes, rock types, sedimentary contexts, potassium chemistry, silicon chemistry, oxygen in minerals, sodium chemistry, and calcium compounds.