Origin of Name
From the Greek word meaning “oblique,” in reference to anorthite’s crystal shape.
Chemical Formula - CaAl2Si2O8
Hand Specimen Identification
Cleavage, hardness, luster, and association help identify anorthite, but it can be extremely difficult to tell from other feldspars. Several kinds of twinning are common. Albite twins, if present, may be difficult to see in hand specimen.
Physical Properties
| Hardness | 6 to 6.5 |
| Specific Gravity | 2.76 |
| luster/transparency | pearly, vitreous/translucent |
| Color | White, Grey |
| streak | white |
| Fracture | Uneven |
| Cleavage | Perfect (001), good (010), poor (110) |
Optical Properties
In thin section, anorthite is similar to other plagioclase feldspars, Biaxial (-), alfa = 1.577, Beta = 1.585, gama = 1.590, delta = 0.013, 2V = 78°
Crystallography
Triclinic, a = 8.17, b = 12.88, c = 14.16, alfa = 93.33°, Beta = 115.60°, gama = 91.22°, Z = 8; space group P1bar; point group 1bar.
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Habit
Anorthite is common as cleavable masses or irregular grains, Euhedral crystals are rare. They may be prismatic, tabular, or bladed and are frequently twinned according to the same laws as albite. When present, albite twins give calcic plagioclase characteristic polysynthetic twinning, but the width of the twins is usually greater than is common for albitic plagioclase.
Structure and Composition
Anorthite is the calcic end member of the plagioclase feldspar series, but the name is also used for any plagioclase containing > 90% CaAl2Si3O8. Its structure is similar to those of albite and orthoclase. As with the other feldspars, the ordering of AlO4 and SiO4 tetrahedra decreases with increasing temperature, leading to minor changes in structure. Complete solid solution with albite, NaAlSi3O8, is possible at all but very low temperatures. Minor solid solution with orthoclase, KAISi3O8, is common.
Occurrence and Associations
Anorthite, found primarily in mafic igneous rocks, is rarer than other plagioclase feldspars. In igneous rocks, it associates with amphibole, pyroxene, or Olivine. It is occasionally found in metamorphosed carbonates.
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Related Minerals
Anorthite is closely related to the other, more sodic plagioclase feldspars, and to the potassic feldspars (orthoclase, sanidine, and microcline).
3D Structure
Labradorite (variety of Anorthite) is a plagioclase feldspar with an albite : anorthite molar ratio ranging from 30 : 70 to 50 : 50. Labradorite can display an iridescent optical effect (or schiller) known as labradorescence, which is a strong play of iridescent blue, green, red, orange, and yellow colors.