alunite

(ăl'yə-nīt')
n.
A gray mineral, chiefly K₂Al₃(OH)₆(SO₄)₃, used in making alum and fertilizer.

[French, from alun, alum, from Latin alūmen.]

A mineral of composition KAl₃(SO₄)²(OH)₆. Alunite occurs in white to gray rhombohedral crystals or in fine-grained, compact masses. Alunite is produced by sulfurous vapors on acid volcanic rocks and also by sulfated meteoric waters affecting aluminous rocks. Alunite is used as a source of potash or for making alum. Alum has been manufactured from the well-known alunite deposits at Tolfa, near Civita Vecchia, Italy, since the mid-15th century. In the United States alunite is widespread in the West. See also Alum.

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KAl
Hexagonal -- Ditrigonal pyramidal

Environment

A secondary rock-making mineral where acid, often ore-bearing, solutions altered orthoclase feldspar-rich rocks such as rhyolite.

Crystal description

Usually massive; crystals are rare and most often pseudocubic rhombohedrons. When seen they are likely to be coating the walls of a fissure in massive alunite.

Physical properties

White, light gray, or flesh red. Luster vitreous to pearly; hardness 3Ɖ-4; specific gravity 2.6-2.9; fracture flat conchoidal to uneven; cleavage fair basal and poor rhombohedral. Brittle; translucent to transparent; sometimes fluorescent orange in longwave ultraviolet light.

Composition

Alkaline hydrous sulfate of aluminum and potassium (11.4% K ₂ O, 36.9% Al ₂ O ₃ , 38.7% SO ₃ , 13.0% H ₂ O).

Tests

High heating in closed tube gives water that tests acid with litmus paper (a blue paper that turns pink in acid). Colors flame purple (potassium). Infusible, but after blowpiping becomes soluble in nitric acid.

Distinguishing characteristics

Closely resembles, in its massive form, limestone and dolomite. A test is essential for identification if alunite is suspected. Its eventual solubility, without effervescence, is a favorable sign.

Occurrence

A great mountain of alunite is to be found at Marysvale, Utah, as well as elsewhere in the West, where hydrothermal alteration has alunitized rhyolitic formations. Several attempts, so far unsuccessful, have been made to work the Utah deposit commercially, to extract potash for fertilizer and use the balance as an ore of aluminum. Similar large alunite deposits are known in Colorado, at Red Mountain, and associated with the ores at Goldfield, Nevada. Although uninteresting from the specimen standpoint, it is potentially valuable as a raw material. White and colorless crystals are found in Civatavecchia, Italy and elsewhere.

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