alunite

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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.

Environment

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

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.

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.

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).

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.

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.

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.

• Mineral compounds

Alunite
Alunite from Utah - USGS
General
Category	Sulfate minerals
Chemical formula	KAl3(SO4)2(OH)6
Strunz classification	07.BC.10
Crystal symmetry	Trigonal 3m or 32/m
Unit cell	a = 6.98 Å, c = 17.32 Å; Z=3
Identification
Color	Yellow, red, to reddish brown, colorless if pure; may be white, pale shades of gray,
Crystal habit	fibrous to columnar, porcelaneous, commonly granular to dense massive
Crystal system	Trigonal
Cleavage	On {0001}, perfect
Fracture	Conchoidal
Tenacity	Brittle
Mohs scale hardness	3.5 - 4
Luster	Vitreous, somewhat pearly on {0001} for crystals, earthy if massive
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.6 - 2.9
Optical properties	Uniaxial (+)
Refractive index	nω = 1.572 nε = 1.592
Birefringence	δ = 0.020
Other characteristics	Strongly pyroelectric
References	[1][2][3][4]

Alunite is a sulfate mineral that was first observed in the 15th century in Monti della Tolfa, north of Rome, where it was mined for the manufacture of alum. First called aluminilite by J.C. Delamétherie in 1797, this name was contracted by François Beudant in 1824 to alunite.

Distinct crystals of alunite are rarely found in cavities in the massive material. Alunite crystallizes in the hexagonal system with crystals forming trigonal pyramids that resemble rhombohedra with interfacial angles of 90° 50', so that they resemble cubes in appearance. Minute glistening crystals have also been found loose in cavities in altered rhyolite. Alunite varies in color from white to yellow gray. The hardness is 4 and the specific gravity is between 2.6 and 2.8. The mineral is a hydrated aluminium potassium sulfate, KAl₃(SO₄)₂(OH)₆. Sodium substitutes for potassium and when high in sodium it is called natroalunite. It is insoluble in water or weak acids, but soluble in sulfuric acid. Jarosite is an iron analogue in which Fe³⁺ replaces the aluminium. It occurs as a secondary mineral on iron sulfate ores.

Alunite from Slovakia

Alunite Marysvale, Utah

Alunite occurs as veins and replacement masses in trachyte, rhyolite, and similar potassium rich volcanic rocks. It is formed by the action of sulfuric acid bearing solutions on these rocks during the oxidation and leaching of metal sulfide deposits. Alunite also is found near volcanic fumaroles. The white, finely granular masses closely resemble finely granular limestone, dolomite, anhydrite, and magnesite in appearance. The more compact kinds from Hungary are so hard and tough that they have been used for millstones.

Historically extensive deposits were mined in Tuscany and Hungary, and at Bulahdelah, Australia. Currently found at Tolfa, Italy. In the United States it is found in the San Juan district of Colorado; Goldfield, Nevada; the ghost town of Alunite, Utah near Marysvale; and Red Mountain near Patagonia, Arizona. The Arizona occurrence lies appropriately above a canyon named Alum Gulch. Alunite is mined as an ore of both potassium and aluminium at Marysvale.^[4] Some of the ore deposits were located by airborne and satellite multispectral imaging.

See also

• Jarosite
• Acid mine drainage

References

Wikimedia Commons has media related to: Alunite

1. ^ http://rruff.geo.arizona.edu/doclib/hom/alunite.pdf Handbook of Mineralogy
2. ^ http://www.mindat.org/min-161.html Mindat.org
3. ^ http://webmineral.com/data/Alunite.shtml Webmineral data
4. ^ ^{a b} Hurlbut, Cornelius S.; Cornelis Klein (1985). Manual of Mineralogy (20th ed. ed.). ISBN 0-471-80580-7. 

• Hurlbut, Cornelius S. (1966). Dana's Manual of Mineralogy (17th ed. ed.). ISBN 0-471-03288-3. 

External links

• Mineral Galleries

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