Aluminite

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For the igneous rock type "websterite", see Websterite.
Aluminite
Aluminite - Newhaven, Sussex, England.jpg
Aluminite from Newhaven, Sussex, England
General
Category Sulfate mineral
Formula
(repeating unit)
Al2SO4(OH)4·7H2O
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 [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 1–2 and a specific gravity of 1.66–1.82.

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]

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

References[edit]

External links[edit]

  • Media related to Aluminite at Wikimedia Commons

Drone Video of the Outcrop at Newhaven