Alunite - Encyclopedia

    Class : Sulphates, chromates, molybdates
    Subclass : Anhydrous sulphates
    Crystal System : Trigonal
    Chemistry : KAl3(SO4)2(OH)6
    Rarity : Fairly frequent

Alunite is a crystallizing sulphate by the action of sulfuric acid on aluminous silicate rocks. The acid can come from volcanic fumaroles or hydrothermal fluids ; in the second case, the alunite is arranged in aureoles around the hydrothermal deposits. Its name comes from the Latin alumen, in reference to its chemical composition. Alunite is an acidic pH mineral that very rarely gives small rhombohedral, pseudocubic or tabular crystals, usually implanted on massive alunite. Usually alunite occurs in fine grained, earthy and sometimes fibrous masses, chalky in appearance and dull in luster. Its theoretically white color frequently evolves towards red or greenish blue by the addition of iron or copper compounds. Alunite is sometimes an important aluminum ore, when it is blue-green in color it has sometimes been used in jewelry : the "turquoise" of Navajo jewelry is thus often alunite.

Alunite from Tambo Mine, El Indio deposit, Coquimbo, Chile -
© Rob Lavinsky

Alunite from Ravin de la Craie, Mont-Dore, France - Collection E. Médard © Pascal Chollet
Alunite from Allumiere, Lazio, Italy - © Domenico Preite
Alunite from the Zadní Hrobce quarry, Czech Republic - © Lubos Vrtiska

Alunite in the World

The best-known alunite deposit is that of the solfataras of Tolfa (Lazio, Italy), mined for alum for centuries. Alunite is also deposited in the hot springs of Lassen (California). It is common in several copper-bearing porphyry deposits in the USA where it appears in clusters in the fissures of hydrothermalized rocks. It is often the copper variety, blue-greenish, reminiscent of turquoise. It is also found in centimetric crystals at the old gold and copper mine of Chinkuashih in Taiwan, which undoubtedly produced the most beautiful specimens in the world (main photo © Rob Lavinsky).

Alunite in France

In France, masses of alunite of fumarolic origin are known in the Massif Central, at the Ravin de la Craie (Mont-Dore) and in the Oligocene marls affected by the fumaroles of St-Germain-Lembron (Puy-de-Dôme).


No twin known for this mineral species.

Fakes and treatments

Alunite cannot be synthesized, on the other hand we find on the market crystals of various colors sold under this name, most often purple, but which can also be colorless, red, green, etc... These crystals are alums of potassium crystallized in the laboratory on pieces of natural rock. Often there is unfortunately no mention of the synthetic nature of the object and these deceptions trap neophytes quite easily because of an attractive appearance. It is easy to produce potassium alum crystals at home by hot dissolving this powdered product in distilled water and then allowing the solution to cool. See our article on the alums.

Hardness : 3.5 to 4
Density : 2.6 to 2.9
Fracture : Irregular to conchoidal
Trace : White

TP : Translucent to transparent
RI : 1.572 to 1.592
Birefringence : 0.020
Optical character : Uniaxial +
Pleochroism : None
Fluorescence : White to orange

Solubility : Sulfuric acid

Magnetism : None
Radioactivity : None


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