plagioclase

Plagioclase refers to a group of feldspar minerals that forms a continuous solid solution between the sodium-rich endmember albite (NaAlSi3O8) and the calcium-rich endmember anorthite (CaAl2Si2O8). Intermediate members include oligoclase, andesine, labradorite, and bytownite, arranged by increasing anorthite content. The general formula is (Na,Ca)AlSi3O8, with calcium substituting for sodium in the crystal lattice.

Plagioclase belongs to the plagioclase feldspar group within the tectosilicates. It has two good cleavages at or near 90 degrees and commonly shows polysynthetic twinning in thin sections. It is typically vitreous to pearly and colorless to white in pure forms, with a hardness around 6 to 6.5 on the Mohs scale. In polarized light it exhibits birefringence and characteristic extinction patterns.

The mineral is a major constituent in many igneous rocks, especially granites, diorites, and basaltic rocks, and is also found in some metamorphic rocks. It forms during crystallization of silica-rich magmas and persists in crustal rocks as temperatures and pressures fall. Its presence helps distinguish rock types and provides clues about magmatic history.

A notable variety is labradorite, which can display labradorescence, an iridescent play of colors caused by the internal lamellar structure. Other plagioclases include oligoclase, andesine, and bytownite. The name derives from Greek plagios, meaning oblique, in reference to characteristic twinning and cleavage features.