plagioclases

Plagioclases are a major group of feldspar minerals that form a continuous solid solution series between the sodium-rich albite (NaAlSi3O8) and the calcium-rich anorthite (CaAl2Si2O8). The series includes intermediate members such as oligoclase, andesine, labradorite, and bytownite, distinguished mainly by their relative calcium (anorthite) content. They are framework silicates (tectosilicates) and are among the most abundant minerals in Earth's crust, occurring in a wide range of igneous and metamorphic rocks, as well as some unaltered sedimentary rocks.

Crystallography and properties: Plagioclases crystallize in triclinic (albite-rich) or monoclinic (anorthite-rich) systems, and display two good cleavages intersecting near 90 degrees. A diagnostic feature is fine horizontal striations on cleavage surfaces, caused by exsolution or twinning. Color is typically white to gray, but intermediate members can show blue or iridescent sheen when weathered; labradorite varieties exhibit labradorescence due to irregular optical lamellae. Common twinning is Carlsbad twinning.

Geologic significance and uses: In igneous rocks, the calcium–sodium ratio of plagioclase records melt evolution and crystallization history, helping to distinguish basaltic from granitic suites. In metamorphic rocks, plagioclase stability informs pressure–temperature conditions and metamorphic grade. Weathering converts plagioclase to clays and oxides. Practically, plagioclase is a major constituent of many rocks and is widely used in ceramics, glass manufacture, and as a building stone; its optical properties aid mineral identification in hand specimen and thin section.