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".

Twins are common and commonly polysynthetic.

No fake identified for this mineral species.