plagioklaasit

Plagioklaasit, also known as plagioclase, is a major group of feldspar minerals defined by a continuous solid solution between albite (NaAlSi3O8) and anorthite (CaAl2Si2O8). The series includes oligoclase, andesine, labradorite, bytownite, and the end-members albite and anorthite.

Plagioklaasit feldspars are tectosilicates with a framework of aluminosilicate tetrahedra. Most plagioclase minerals crystallize in the triclinic system and commonly show twinning, notably Carlsbad twins. They have two good cleavage directions at about 90 degrees and commonly display fine striations on cleavage surfaces due to exsolution of Na- and Ca-rich phases.

Plagioklaasit is among the most abundant minerals in Earth's crust and occurs in felsic to mafic rocks, including granites, diorites, gabbros, basalts, and andesites, as well as in high-grade metamorphic rocks. They form by crystallization from cooling magmas and through differentiation of basaltic liquids, recording magmatic evolution. Labradorite, a calcium-rich member, often exhibits labradorescence, an iridescent play of color on polished surfaces.

Physical properties include a hardness of about 6–6.5, a specific gravity around 2.7, and two good cleavages near 90 degrees. In hand sample they are typically colorless to white, but may appear gray, pink, or green. Economically, plagioclase is used in ceramics and glassmaking and serves as a key petrographic indicator in geology.