Aluminite

(repeating unit)Al₂SO₄(OH)₄·7H₂OIMA symbolA^[1]Strunz classification7.DC.05Crystal systemMonoclinicCrystal classPrismatic (2/m)
(same H-M symbol)Space groupP2₁/cUnit cella = 7.44, b = 15.583
c = 11.7 [Å]; β = 110.18°; Z = 4IdentificationColorWhite to grayish whiteCrystal habitNeedles and fibrous massesCleavageNoneFractureIrregular/unevenTenacityFriableMohs scale hardness1 - 2LusterDull to earthyStreakWhiteDiaphaneityTranslucent, opaque if massiveSpecific gravity1.66–1.82Optical propertiesBiaxial (+)Refractive indexn_α = 1.459 n_β = 1.464 n_γ = 1.470Birefringenceδ = 0.0112V 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.

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

^ 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.
^ ^a ^b "Handbook of Mineralogy" (PDF). Archived (PDF) from the original on 2012-04-26. Retrieved 2012-06-10.
^ ^a ^b "Mindat w/ locations". Archived from the original on 2009-11-09. Retrieved 2005-09-05.
^ "Webmineral". Archived from the original on 2013-01-17. Retrieved 2005-09-05.