Anorthite

Anorthite is a calcium-rich feldspar mineral with the formula CaAl₂Si₂O₈. It is the calcium endmember of the plagioclase solid-solution series.

Plagioclase: A continuous series of feldspar minerals ranging from sodium-rich albite to calcium-rich anorthite. Anorthite defines the calcium-rich end of this compositional series.
Solid solution: A crystalline phase in which elements substitute for one another across a range of compositions. Plagioclase accommodates changing sodium and calcium proportions between its endmembers.
Anorthosite: A coarse-grained intrusive rock composed predominantly of plagioclase, commonly labradorite or bytownite. Anorthite-rich plagioclase occurs in some anorthosites and records their crystallization history.
Bytownite: A calcium-rich plagioclase feldspar composition between labradorite and anorthite. It approaches anorthite in composition without reaching the endmember.
Feldspar: A major group of rock-forming aluminosilicate minerals containing potassium, sodium, or calcium. Anorthite belongs to this mineral group.
Albite-anorthite series: The plagioclase compositional series between NaAlSi₃O₈ and CaAl₂Si₂O₈. Anorthite is one endpoint of this substitution series.
Basalt: A fine-grained volcanic rock formed from low-viscosity, iron- and magnesium-rich lava. Calcium-rich plagioclase, including anorthite-rich compositions, commonly crystallizes in basaltic magma.
Anorthosite complex: A large intrusive body dominated by anorthosite and associated rocks. Such complexes provide extensive natural records of plagioclase-rich magmatism.
Albite: The sodium-rich feldspar endmember of the plagioclase series, with formula NaAlSi₃O₈. It contrasts with anorthite at the opposite compositional end of plagioclase.
Silicon–aluminium ordering: The arrangement of silicon and aluminum atoms among tetrahedral sites in a mineral structure. Ordering among anorthite’s tetrahedral sites varies with its formation and thermal history.

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