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Anorthite Crystals on Matrix

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 species.
Chemical Formula CaAl2Si2O8
Composition Calcium aluminum silicate. May contain some sodium replacing the calcium, but that amount must be less than 10 percent to be strictly Anorthite.
Variable Formula (Ca,Na)Al2Si2O8
Color Colorless, white, cream, gray, brown, pink, pale yellow, pale green
Streak White
Hardness 6 - 6.5
Crystal System Triclinic
3D Crystal Atlas
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Crystal Forms
and Aggregates
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.
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
Complex Tests Soluble in hydrochloric acid
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
Popularity (1-4) 3
Prevalence (1-3) 3
Demand (1-3) 2


Anorthite ON EBAY




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

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.


anorthite PHOTOS
 
ADDITIONAL RESOURCES
 
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