Occurs in Pinal County.
Chemical Formula: CaAl2Si2O8
Specific Gravity: 2.72-2.75
Anorthite is a plagioclase feldspar. Plagioclase feldspars are important constituents of many igneous and metamorphic rocks. Anorthite is the calcium end-member of the plagioclase series, with varying amounts of sodium (Na) and aluminum (Al) ions substituting for the calcium ions in its crystal structure.
Key characteristics of anorthite include:
- Crystal Structure: Anorthite crystallizes in the triclinic crystal system. Its crystal structure consists of three-dimensional frameworks of linked silicon-oxygen (SiO4) tetrahedra. Aluminum and calcium ions occupy various positions within this framework.
- Twinning: Anorthite is often twinned, meaning that multiple crystals grow together in a symmetrical pattern. The most common twinning law for anorthite is the Carlsbad twin, which results in repeated mirror-image intergrowths of crystal faces.
- Cleavage: Anorthite exhibits two cleavage directions that intersect at approximately 90 degrees. These cleavage planes can lead to the formation of tabular crystals.
- Color and Luster: Anorthite’s color varies from white to gray, and it can sometimes exhibit a bluish hue. It has a vitreous (glassy) to pearly luster.
- Occurrence: Anorthite is commonly found in igneous rocks, particularly in plutonic (intrusive) rocks like gabbro and anorthosite. It can also be present in certain metamorphic rocks and lunar basalts.
- Rock-Forming Mineral: Anorthite, along with other plagioclase feldspars, is an important rock-forming mineral. It contributes to the composition of various igneous and metamorphic rocks, influencing their mineralogical and textural characteristics.