CaAl2Si2O8

CaAl2Si2O8, commonly known as anorthite, is the calcium end-member of the plagioclase feldspar group. It forms a continuous solid solution series with albite (NaAlSi3O8) and together these minerals are major constituents of the continental crust.

Composition and structure: Anorthite consists of a framework of corner-sharing SiO4 and AlO4 tetrahedra. Substitution of Al3+ for Si4+ in the framework creates a negative charge that is balanced by calcium ions in large interstitial sites. The end-member composition is An100 (CaAl2Si2O8); the plagioclase series ranges toward albite as An0 (NaAlSi3O8).

Occurrence and formation: Anorthite occurs in a variety of igneous rocks, particularly mafic to intermediate rocks such as basalt and gabbro, where it often forms as part of the plagioclase feldspar assemblage. It is also found in certain high-grade metamorphic rocks formed from calcium- and aluminum-rich protoliths. Crystallization from magma and metamorphic recrystallization of plagioclase-bearing rocks are common pathways for its formation.

Physical properties: Anorthite is typically white to gray with a vitreous luster. It has a Mohs hardness around 6 to 6.5 and exhibits good cleavage in two directions, commonly with characteristic striations on cleavage surfaces. The mineral’s density is roughly 2.7 g/cm³.

Name and significance: The name derives from Greek and reflects aspects of its crystal appearance. As a major plagioclase component, anorthite is an important mineral for petrographic analysis and helps characterize the origin and evolution of igneous and metamorphic rocks.