Aluminite
Aluminite - Newhaven, Sussex, England.jpg
Aluminite from Newhaven, Sussex, England
General
CategorySulfate mineral
Formula
(repeating unit)
Al2SO4(OH)4·7H2O
IMA symbolA[1]
Strunz classification7.DC.05
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP21/c
Unit cella = 7.44, b = 15.583
c = 11.7 [Å]; β = 110.18°; Z = 4
Identification
ColorWhite to grayish white
Crystal habitNeedles and fibrous masses
CleavageNone
FractureIrregular/uneven
TenacityFriable
Mohs scale hardness1 - 2
LusterDull to earthy
StreakWhite
DiaphaneityTranslucent, opaque if massive
Specific gravity1.66–1.82
Optical propertiesBiaxial (+)
Refractive indexnα = 1.459 nβ = 1.464 nγ = 1.470
Birefringenceδ = 0.011
2V angleMeasured: 90°, calculated: 86°
References[2][3][4]

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

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.[citation needed]

References

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b Handbook of Mineralogy
  3. ^ a b Mindat w/ locations
  4. ^ Webmineral

Drone Video of the Outcrop at Newhaven