alunite

{{infobox mineral | name = Alunite | category = Sulfate minerals | boxwidth = | boxbgcolor =#c8803f | image = Alunite_-_USGS_Mineral_Specimens_015.jpg | imagesize = | alt = | caption = Alunite from Utah - USGS | formula = KAl3(SO4)2(OH)6 |IMAsymbol=Alu | molweight = | strunz = 7.BC.10 | dana = | system = Trigonal | class = Hexagonal scalenohedral (m) Space group: ( 2/m) | symmetry = Rm | unit cell = a = 6.98, c = 17.32 [Å]; Z = 3 | color = Yellow, red, to reddish brown, colorless if pure; may be white, pale shades of gray | habit = fibrous to columnar, porcelaneous, commonly granular to dense massive | twinning = | cleavage = On {0001}, perfect | fracture = Conchoidal | tenacity = Brittle | mohs = 3.54 | luster = Vitreous, somewhat pearly on {0001} for crystals, earthy if massive | streak = White | diaphaneity = Transparent to translucent | gravity = 2.62.9 | density = | polish = | opticalprop = Uniaxial (+) | refractive = nω = 1.572 nε = 1.592 | birefringence = δ = 0.020 | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence= | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = Strongly pyroelectric 25px Radioactive 9.44% (K) | alteration = | references = }} Alunite is a hydroxylated aluminium potassium sulfate mineral, formula KAl3(SO4)2(OH)6. It was first observed in the 15th century at Tolfa, near 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 three decades later to alunite. left|thumb|Alunite from Slovakia Alunite crystals morphologically are rhombohedra with interfacial angles of 90° 50', causing them to resemble cubes. Crystal symmetry is trigonal. Minute glistening crystals have also been found loose in cavities in altered rhyolite. Alunite varies in color from white to yellow gray. The hardness on the Mohs scale is 4 and the specific gravity is between 2.6 and 2.8. It is insoluble in water or weak acids, but soluble in sulfuric acid.

left|thumb|Alunite from Marysvale, Utah Sodium can substitute for potassium in the mineral, and when the sodium content is high, is called natroalunite.