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 (see albite). Albite twins, if present, may be difficult to see in hand specimen.
Chemical Composition : CaAl2Si2O8
Hardness : 6 to 61 2
Cleavage/Fracture : perfect (001), good (010),
Luster/Transparency : pearly, vitreous/translucent
Color : white, gray
Streak : white
Optical Properties : In thin section, anorthite is similar to other plagio-clase feldspars. Biaxial 1- 2, a = 1.577, b = 1.585, g = 1.590, d = 0.013, 2V = 78°.
Crystallography : Triclinic, a = 8.17, b = 12.88, c = 14.16, a = 93.33°, b = 115.60°, g = 91.22°, Z = 8; space group P1;
point group 1.
Habit : Anorthite is common as cleavable masses or irregular grains. Euhedral crystals are rare. They may be pris-matic, 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 plagio-clase feldspar series, but the name is also used for any plagioclase containing 7 90% CaAl2Si2O8. 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 tem-perature, 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, KAlSi3O8, 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.
Related Minerals : Anorthite is closely related to the other, more sodic plagioclase feldspars, and to the potassic feldspars (orthoclase, sanidine, and microcline).