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.

33,20 ct moonstone cabochon
34,80 ct moonstone cabochon
23,00 ct moonstone cabochon
4,91 ct moonstone cabochon

Adularia in the World

The largest crystals of adularia from alpine clefts reach 15 kg and come from the Habachtal (Tyrol, Austria), but the deposits in Switzerland (Gletsh, Airolo) or Austria (Zillertal), have also provided crystals with remarkable, actively sought after by collectors. The "Moonstone" variety exists in superb specimens with shimmering reflections in the Black Range (New Mexico) and in Tissamaharama (Sri Lanka). Particularly spectacular samples of 2 cm on 20 cm quartz crystals come from Nae Gun in South Korea.

Adularia in France

In France, crystals of adularia are common in the Alps, they are commonly found in the alpine clefts around Bourg d'Oisans (Isère) and Chamonix (Haute-Savoie).

Twinning

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

Fakes and treatments

No fakes known for this mineral species.



Hardness : 6
Density : 2.55 to 2.63
Fracture : Irregular to conchoidal
Trace : White



TP : Translucent to transparent
IR : 1.518 to 1.525
Birefringence : 0.004 to 0.005
Optical character : Biaxial -
Pleochroism : Low
Fluorescence : White or red


Solubility : Hydrofluoric acid

Magnetism : None
Radioactivity : None

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