Anorthite - Encyclopedia

    Class : Silicates
    Subclass : Tectosilicates
    Crystal System : Triclinic
    Chemistry : (Na,Ca)(Al,Si)4O8
    Rarity : Very common

Anorthite constitutes the calcium pole of plagioclase feldspars (An90 to An100 and Ab0 to Ab10), the sodium pole corresponding to albite. It is a rare constituent of basic igneous rocks (basalts and calc-alkaline gabbros) and of certain metamorphic limestones. Its name comes from the Greek anorthos (oblique) in reference to its triclinic crystal system. In geodes, anorthite often occurs as almost perfect and very richly shaped crystals, prismatic or elongated, colorless or sometimes weakly tinted (yellowish, grayish, reddish).

Anorthite in the World

The most beautiful samples are found in a volcanic environment. Magnificent crystals come from blocks of limestone rejected by Somma and Vesuvius (Italy) and from Miyashe (Japan). Very beautiful crystals are also reported in Val di Fassa, in the Alps of Trentino (Italy).
Main photo : Mount Somma anorthite, Vesuvius, Italy © Gianfranco Ciccolini

Anorthite in France

In France, anorthite is reported in micro-crystals in certain lavas of Haute-Loire and Cantal. It is also found in gem crystals at the Lassur mine (Ariège) in dolomites (photo on the right © Patrice Queneau).


Twins are common and commonly polysynthetic ; many other single and multiple contact twins exist.

Fakes and treatments

No fake identified for this mineral species.

Hardness :  6 to 6.5
Density : 2.75
Fracture : Irregular to conchoidal
Trace : White

TP : Translucent to transparent
RI : 1.573 to 1.590
Birefringence : 0.012 to 0.013
Optical character : Biaxial -
Pleochroism : Low
Fluorescence : None

Solubility : Concentrated hydrochloric acid

Magnetism : None
Radioactivity : None


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