Alunite, or alumstone, is a
mineral that was first observed in the
15th century in Monti della Tolfa, north to
Rome, where it was mined for the manufacture of
alum. First called aluminilite by J.C. Delametherie in 1707, 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.
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General Information:
Chemical Formula:
KAl3(SO4)2(OH)6
Composition:
(Molecular Weight = 414.21 gm)
Potassium 9.44 %
K Aluminum 19.54 %
AlHydrogen 1.46 %
H Sulfur 15.48 %
S Oxygen 54.08 %
O Empirical Formula:
KAl3(SO4)2(OH)6
Environment:
Rock forming mineral where acid, often ore-bearing, solutions have altered orthoclase feldspar-rich rocks.
IMA Status:
Approved IMA 1963
Locality:
Marysvale, Utah.
Name Origin:
From the Latin alunit, meaning "alum."
Physical Properties:
Cleavage:
[0001] Good
Color:
white, gray, yellowish gray, reddish gray, or yellowish white.
Density:
2.59 - 2.9, Average = 2.74
Diaphaniety:
Transparent to translucent
Habits:
Massive - Granular - Common texture observed in granite and other igneous rock., Earthy - Dull, clay-like texture with no visible crystalline affinities, (e.g. howlite)., Fibrous - Cr ystals made up of fibers.
Hardness:
3.5-4 - Copper Penny-Flourite
Luminescence:
None.
Luster:
Vitreous - Pearly
Streak:
white
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