Anorthite is one of the plagioclase feldspars, an important group of minerals abundant in the Earth's crust. Anorthite's chemical formula is CaAl₂Si₂O₈.

Mineralogy

---

Anorthite is the calcium-rich endmember of the plagioclase system, the other endmember being albite (sodium-rich). The range of intermediate compositions have varying ratios of calcium and sodium ions.

Occurrence

---

Anorthite occurs in igneous and metamorphic rocks. Its type localities are Monte Somma and Valle di Fassa, Italy. It also makes up much of the lunar highlands, and was discovered in samples from comet Wild 2.

Details

---

Crystallographic system	Triclinic
Colours	Normally colourless or white, colourless in thin section.
Lustre	Vitreous
Hardness	6-6.5
Refractive index	1.51
Cleavage	3: 1 perfect, 1 good, 1 poor
Twinning	Multiple lamellar and Carlsbad twins
Pleochroism	Yellow to dark green to transparent to opaque

See also

---

• Feldspars
• List of minerals

References

---

• WebMineral entry

Tectosilicates | Calcium minerals

Anorthit | אנורתיט | Anorthiet | Anortyt | Anortita | Анортит | Anortit