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The Mineral anorthite

Anorthite Crystals on Matrix

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.

Chemical Formula

CaAl₂Si₂O₈

Color

Colorless, white, cream, gray, brown, pink, pale yellow, pale green

Crystal System

Triclinic

Properties

Streak
White

Hardness
6 - 6.5

Transparency
Transparent to translucent

Specific Gravity
2.74 - 2.76

Luster
Vitreous. Pearly on cleavage surfaces.

Cleavage
2,1 - basal ; 2,1 - prismatic ; 3,1 - pinacoidal. The cleavage angle is about 90º.

Fracture
Conchoidal to uneven

Tenacity
Brittle

Crystal Habits

Crystals are prismatic or tabular, and are usually twinned. Also occurs grainy and massive.

Click here for more detailed information on the crystal structure of the Feldspars.

3D Crystal Atlas

Additional Information

Composition
Calcium aluminum silicate. May contain some sodium replacing the calcium, but that amount must be less than 10 percent to be strictly Anorthite.

In Group
Silicates; Tectosilicates; Feldspar Group

Striking Features
Crystal habits, cleavage, and hardness.

Environment
In igneous environments of recent volcanic activity, and in contact metamorphic rocks.

Rock Type
Igneous, Metamorphic

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

Quartz, Muscovite, Biotite, Hornblende, Augite

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.