Aluminite

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 1–2 and a specific gravity of 1.66–1.82.

An outcrop of aluminite at the top of the white chalk of the cliff at Newhaven, East Sussex, England.

Aluminite
Aluminite from Newhaven, Sussex, England
General
Category	Sulfate mineral
Formula (repeating unit)	Al2SO4(OH)4·7H2O
IMA symbol	A[1]
Strunz classification	7.DC.05
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) (same H-M symbol)
Space group	P21/c
Unit cell	a = 7.44, b = 15.583 c = 11.7 [Å]; β = 110.18°; Z = 4
Identification
Color	White to grayish white
Crystal habit	Needles and fibrous masses
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	[2][3][4]

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.[2]

It was first described in 1807 from Halle, Saxony-Anhalt, Germany and named for its aluminium content.[3] It is also known as alley stone, halite and websterite (named after Orcadian geologist Thomas Webster).

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