Anorthoclase - Encyclopedia

    Class : Silicates
    Subclass : Tectosilicates
    Crystal System : Triclinic
    Chemistry : (Na,K)AlSi3O8
    Rarity : Fairly common

Anorthoclase refers to the triclinic sodium terms of the sanidine-albite series. It is in a way an intermediate feldspar between the alkaline potassium and sodium feldspars (sanidine) and the sodium plagioclases (albite). Its name comes from the Greek anorthos (oblique) in reference to its triclinic crystal system. It is an alkali feldspar which closely resembles microcline and occurs in prismatic to tabular crystals, whitish to grayish in color, sometimes slightly yellowish. Anorthoclase is found in transitional environments between sodic alkaline volcanic rocks and nepheline syenites. The most famous deposit is larvikite from Norway, whose anorthoclase crystals show shimmering blue highlights (main photo). This rock is intensely mined in the Larvik region under the misleading name from a mineralogical point of view of "labradorite".

Anorthoclase in the World

Beautiful crystals of anorthoclase are known at Obsidian Cliff, in Yellowstone Park (Wyoming) and especially at Cripple Creek (Colorado). The andesitic lavas of Lipari, Ustica and Pantelleria (Sicily, Italy) also have good specimens. Famous deposits for the collection of anorthoclase are the sodium trachyte of Ropp (Nigeria), and, more difficult to access, the craters of Erebus (Antarctica) and Kilimanjaro (Tanzania).

Photo on the right : Anorthoclase from Pili Mine, Chihuahua, Mexico © Peter Megaw

Anorthoclase in France

In France, anorthosis is reported in certain lavas of Haute-Loire, Cantal, Puy-de-Dôme and Loire.


Twins are common and commonly polysynthetic.

Fakes and treatments

No fake identified for this mineral species.

Hardness :  6 to 6.5
Density : 2.60
Fracture : Irregular
Trace : White

TP : Translucent to transparent
RI : 1.519 to 1.536
Birefringence : 0.008
Optical character : Biaxial -
Pleochroism : Low
Fluorescence : None

Solubility : Hydrofluoric acid

Magnetism : None
Radioactivity : None