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Plagioclase is a major group of feldspar minerals within the plagioclase series, tectosilicates characterized by a complete solid solution between albite and anorthite. The series ranges from albite (NaAlSi3O8) to anorthite (CaAl2Si2O8), with intermediate members such as oligoclase, andesine, labradorite, and bytownite.

Crystal structure and properties: Plagioclases crystallize in a tectosilicate framework and show two good cleavages at near 90 degrees. They are commonly colorless to white; many specimens show characteristic polysynthetic twinning, especially Albite twins, and some display labradorite iridescence in labradorite. They have a hardness around 6–6.5 and a specific gravity about 2.6–2.8.

Occurrence: Plagioclase is a dominant mineral in many igneous rocks, particularly granites, diorites, and basalts. It forms by crystallization from magmas and can also recrystallize during metamorphism. It coexists with potassium feldspar, quartz, pyroxenes, and amphiboles, among others. The plagioclase composition in a rock can reflect magma differentiation and cooling history.

Identification and significance: The plagioclase composition is used to classify igneous rocks along a felsic–intermediate spectrum. Labradorite is notable for its iridescent labradorescence in some samples. The name plagioclase derives from Greek plagios, oblique, referring to its cleavage and twinning.

Notes: Plagioclase is a common crustal mineral and an essential mineral in petrology for understanding rock formation and magmatic processes.