Minerals.net
Anorthite Crystals on Matrix
SILICATES; TECTOSILICATES; FELDSPAR GROUP · TRICLINIC

Anorthite

CaAl2Si2O8

Hardness
6–6.5
Streak
White
Spec. Gravity
2.74–2.76
System
Triclinic

About

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 species.

Crystal Forms & Aggregates

Complex Tests

Striking Features

Environment

Noteworthy Localities

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.

Common Mineral Associations

Distinguishing 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.

Photos

Crystal forms

Drag to rotate, scroll to zoom.

Modified Triclinic
Complex Triclinic