Potassium Aluminum Sulfate Hydroxide
KAl3(SO4)2(OH)6
Alunite is the sodium substitute for potassium, and when it is high in sodium, it is called natroalunite.
The History Says
Historically extensive deposits were mined in Tuscany and Hungary, and at Bulladelah in New South Wales.
The Present Scenario
Currently it is found at Tolfa, Italy. In the United States it is found in the San Juan district of Colorado; Goldfield, Nevada; Marysvale, Utah; and Red Mountain in Patagonia, Arizona.
ALUNITE IS also known as alumstone. This mineral was first observed in the 15th century at Tolfa, a place near Rome. Here, it was mined to manufacture alum, KAl(SO4)2 - 12H2O.
In 1707, it was first named as aluminilite by J. C. Delametherie. In 1824, it was renamed to alunite by FS Beudant. The name originates from the Latin alunit, which means "alum."
Alunite is formed from the action of sulfuric acids upon potassium rich feldspars in a process which is called as "alunitization." The sulfuric acids accompany hydrothermal solutions. It is generally rich in certain ore metals. These solutions result in large bodies of alunite, making it a rock forming mineral.
Sometimes, alunite is mistaken for massive rock forming dolomite or limestone (calcite). An acid test proves the identification. Alunite does not bubble even when powdered.
The symmetry of alunite is same as the members of the Tourmaline Group but crystals of alunite do not form prismatic crystal like those of the typical tourmaline mineral. The crystals of alunite are more flattened and it resembles nearly cubic rhombohedrons. "Rhombohedrons" are the combination of two trigonal pyramids.
Composition of Alunite
| Molecular Weight = 414.21 gm | ||
| Potassium | 9.44 % (K) | 11.37 % (K2O) |
| Aluminum | 19.54 % (Al) | 36.92 % (Al2O3) |
| Hydrogen | 1.46 % (H) | 13.05 % (H2O) |
| Sulfur | 15.48 % (S) | 38.66 % (SO3) |
| Oxygen | 54.08 % (O) | |
| 100.00% | 100.00 % = TOTAL OXIDE |
Physical Properties of Alunite
| Color | White, gray, yellowish gray, reddish gray, yellowish white. |
| Density | 2.59 - 2.9, Average = 2.74 |
| Diaphaniety | Transparent to translucent |
| Fracture | Uneven - Flat surfaces (not cleavage) fractured in an uneven pattern |
| Cleavage | Good |
| Habit | Earthy - Dull, clay-like texture with no visible crystalline affinities, (e.g. howlite). Fibrous - Crystals made up of fibers. Massive - Granular - Common texture observed in granite and other igneous rock. |
| Hardness | 3.5-4 - Copper Penny-Fluorite |
| Specific Gravity | Approximately 2.7 - 2.8+ (average for translucent minerals) |
| Luminescence | None |
| Luster | Vitreous - Pearly |
| Streak | White |
Uses of Alunite
It is a good source of alum. Sometimes, it is also used to recover potassium and aluminum. Commercially it is used as alloys, tools, equipment.
Alunite occurs as a vein and replacement mass in trachyte, rhyolite, and similar potassium rich volcanic rocks. It is also formed at volcanic fumaroles. The white, fine granular mass of alunite closely resemble the fine granular limestone, dolomite, anhydrite, and magnesite in appearance. The compact kinds, which come from Hungary, due to its hardness and toughness, are used for millstones.
Presently, alunite is found at Tolfa in Italy. In the U.S., it is found in the San Juan district of Colorado; Goldfield, Nevada; Marysvale, Utah; and Red Mountain in Patagonia, Arizona. The occurrence in Arizona lies above a canyon named Alum Gulch. It is mined as an ore of both, potassium as well as aluminium at Marysvale.