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For the igneous rock type "websterite", see pyroxenite.

Aluminite - Newhaven, Sussex, England.jpg
Aluminite from Newhaven, Sussex, England
Category Sulfate mineral
(repeating unit)
Strunz classification 07.DC.05
Crystal symmetry Monoclinic prismatic
H-M symbol: (2/m)
Space group: P 21/c
Unit cell a = 7.44 Å, b = 15.583 Å, c = 11.7 Å; β = 110.18°; Z=4
Color White to grayish white
Crystal habit Needles and fibrous masses
Crystal system Monoclinic
Cleavage None
Fracture Irregular/uneven
Tenacity Friable
Mohs scale hardness 1 - 2
Luster Dull to earthy
Streak White
Diaphaneity Translucent, opaque if massive
Specific gravity 1.66–1.82
Optical properties Biaxial (+)
Refractive index nα = 1.459 nβ = 1.464 nγ = 1.470
Birefringence δ = 0.011
2V angle Measured: 90°, calculated: 86°
References [1][2][3]

Aluminite is a hydrous aluminium sulfate mineral with formula: Al2SO4(OH)4·7H2O. It is an earthy white to gray-white monoclinic mineral which almost never exhibits crystal form. It forms botryoidal to mammillary clay-like masses. It has a very soft Mohs hardness of 1 and a specific gravity of 1.66—1.7.

It forms in clay and lignite deposits as an oxidation product of pyrite and marcasite along with aluminium silicates. It also occurs in volcanic sublimates, in native sulfur deposits and rarely in caves. It occurs in association with basaluminite, gibbsite, epsomite, gypsum, celestine, dolomite and goethite.[1]

It was first described in 1807 from Halle, Saxony-Anhalt, Germany and named for its aluminium content.[2] It is also known as alley stone, hallite and websterite (named after Orcadian geologist Thomas Webster).

Aluminite is used by tile and masonry workers to reduce the setting time of mortars.[citation needed]