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Occurs in Pinal County.

Chemical Formula: CaAl2Si2O8

Specific Gravity: 2.72-2.75

Luster: Vitreous

Hardness: 6-6.5

Anorthite is a plagioclase feldspar mineral and is the calcium end-member of the plagioclase series. It is a common constituent of many igneous and metamorphic rocks.

Next Pinal County mineral: Antigorite

MinDat Anorthite

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:

  1. 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.
  2. 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.
  3. Cleavage: Anorthite exhibits two cleavage directions that intersect at approximately 90 degrees. These cleavage planes can lead to the formation of tabular crystals.
  4. 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.
  5. 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.
  6. 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.