Alunite
Alunite is a fairly common potassium aluminium sulphate mineral, typically occurring as pale, chalky masses in volcanic and hydrothermally altered rocks.
It has a long history of practical use in the manufacture/production of potash and alumina, and well-crystallised specimens are much sought after by collectors.
Information about Alunite
Alunite most commonly occurs as fine-grained, earthy or chalky masses that can closely resemble limestone or chalk in appearance. Distinct, well-formed crystals are considerably rarer and are prized accordingly.
It is typically white, grey, or yellowish-white, though iron impurities can push the colour towards reddish or yellowish tones, and copper impurities can produce a distinctive blue-green variety that has historically been mistaken for turquoise. It has a vitreous to pearly lustre on crystal faces, becoming dull in massive material.
Alunite forms when sulphuric acid – derived from volcanic gases or from the weathering of iron sulphide minerals such as pyrite – reacts with aluminium-bearing rocks. It is commonly found alongside quartz, pyrite, kaolinite, and gypsum.
It is the aluminium equivalent of jarosite, where iron plays the equivalent structural role, and belongs to the large alunite supergroup.
A note for collectors: Synthetic potassium alum crystals – which can be grown at home from a powder dissolved in water – are frequently sold as natural alunite, sometimes without any indication that they are man-made. These tend to be larger, perfectly formed, and often tinted with dyes. If a specimen looks unusually perfect or comes in an unlikely colour, it is worth investigating before purchase.
Uses and History
Alunite has been mined and used by humans for centuries, primarily as a source of alum – a compound with a wide range of historical applications including dyeing and tanning textiles, preserving food, and producing paper.
The deposits at Tolfa in Lazio, Italy, were amongst the most significant in Europe and were mined intensively from the 15th century onwards, providing a commercially important source of alum to much of the continent. During the First World War it was exploited as a source of potash, and during the Second World War as a source of alumina.
In some parts of North America, blue-green alunite has been used in jewellery in place of turquoise, with which it can be easily confused.
The name alunite is a contraction of the earlier name aluminilite, itself derived from the Latin alumen – the historical term for alum.
Notable localities include Tolfa in Italy, Rodalquilar in southern Spain, Marysvale in Utah USA, the Lassen Volcanic National Park in California, Decazeville in France, and Chinkuashih in Taiwan – the last of which has produced some of the finest crystallised specimens in the world.
Mineralogy
Typically white, grey, or yellowish-white; reddish or yellowish due to iron impurities; blue-green in copper-bearing varieties.
Hazards and Warnings
Mineral collectors should wash their hands after handling specimens, to avoid any exposure to potential toxins.
Almost all rocks, minerals (and, frankly, almost all other substances on earth) can produce toxic dust when cutting, which can cause serious respiratory conditions including silicosis. When cutting or polishing rocks, minerals, shells, etc, all work should be done wet to minimise the dust, and a suitable respirator or extraction system should be used.
Translations
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English:
- Alunite
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Russian:
- Алунит
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- Alunit
German:
- Alunit
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Mandarin Chinese:
- 明矾石
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