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.

No twin known for this mineral species.

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.