The Mineral anorthite
Anorthite belongs to the Plagioclase Feldspar group, an isomorphous solid solution series. Albite is one end member, containing sodium and no calcium. The other end member, Anorthite, contains calcium and no sodium. The intermediary members are
Oligoclase, Andesine, Labradorite, and Bytownite. Labradorite and Bytownite are considered by some to be a variety of Anorthite rather then a separate mineral. The acclaimed Dana's System of
Mineralogy lists these intermediary members as individual minerals,
whereas the IMA does not recognize them as individual mineral
Colorless, white, cream, gray, brown, pink, pale yellow, pale green
6 - 6.5
Transparent to translucent
2.74 - 2.76
Vitreous. Pearly on cleavage surfaces.
2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal. The cleavage angle is about 90º.
Conchoidal to uneven
Anorthite is a rare member of the Feldspar group. Italian Localities include Monte Somma, Mount Vesuvius; Val Schiesone, Sondrio; and Val Di Fassa, Trento. Japan produces excellent crystals thinly coated with a dark layer of lava at Miyaki Jima (Miyaki Island), Tokyo Prefecture. Other localities are Grass Valley, Nevada Co., California; and Franklin, Sussex Co., New Jersey.
Distingushing Similar Minerals
Potassium Feldspar group - Don't exhibit striations on
twinned crystal surfaces, whereas the Plagioclase feldspars sometimes
do. Otherwise can be difficult to distinguish.
Other Plagioclase Feldspars - Usually cannot be
determined by practical means.
Rhodonite - Although crystals are generally more elongated, it is difficult to distinguish pink Anorthite from Rhodonite, though Rhodonite often has characteristic black veins running through it that are lacking in Anorthite.
Spodumene - Has a splintery fracture.
Calcite - Much lower hardness.