Adularia - Encyclopedia

    Class : Silicates
    Subclass : Tectosilicates
    Crystal system : Monoclinic
    Chemistry : KAlSi₃O₈
    Rarity : Common

Adularia is a colorless variety of orthoclase (and sometimes microcline) which crystallizes at low temperatures. It comes in pseudorhombohedral or pseudo-orthorhombic looking crystals, often well formed, with a pearly luster. It is called "moonstone" when it is translucent to transparent and has shimmering bluish reflections, a term which however also designates other feldspars with these aesthetic attributes. It owes its name to its place of discovery : Mount Adula (Grisons, Switzerland). Adularia is often associated with chlorite and albite in the famous alpine clefts of the French, Swiss, Austrian and Italian Alps, which have provided the best crystals of this variety. It is also abundant in certain low-temperature hydrothermal veins (fluorite and baryte, gold, etc...) and in the alteration aureoles of metallic deposits. Adularia from hydrothermal veins only rarely occurs in collection samples, but it is a very useful mineral because it allows geochronological measurements to be made (potassium-argon and argon-argon methods) and therefore to date these veins.

The Carlsbad, Baveno and Manebach twins are common for the species