Green crystals of the rare mineral meta-alunogen from South Dakota, USA (Runge Mine, Edgemont, Fall River County, South Dakota, United States of America). Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY-SA 4.0
Alunogen
Alunogen (from French alun, "alum"), also called feather alum and hair salt is a colourless to white (although often coloured by impurities, such as iron substituting for aluminium) fibrous to needle-like aluminium sulfate mineral. It has the chemical formula Al2(SO4)3·17H2O. It is often found on the walls of mines and quarries as a secondary mineral. It can be found in the oxidation zones of some ore deposits as well as on burning coal dumps (i.e., as the product of millosevichite hydration). It also forms as a low temperature deposit in fumaroles.
Intro adapted from Wikipedia (CC BY-SA).
Alunogen is an IMA-recognized mineral species in the sulfate class, with the chemical formula Al₂(SO₄)₃(H₂O)₁₂·5H₂O.
It crystallizes in the triclinic crystal system (space group P-1).
Key properties: Mohs hardness 1.5–2; specific gravity 1.72–1.77; Vitreous to silky luster; refractive index 1.476; optically biaxial.
IMA symbol Alg, grandfathered status — known before the IMA’s founding, named in 1832.
Forms by reaction of sulfates from decomposing sulfides with aluminous minerals in shales and slates; in gossan or altered wall rock of pyritic deposits in arid regions; in coal seams; in relatively low-temperature fumaroles.
It is found associated with Pyrite, marcasite, halotrichite, pickeringite, epsomite, alum, melanterite, gypsum.
The name comes from the Latin for alum and the Greek for to make.