Aluminite

Aluminite is a hydrous aluminium sulfate mineral with formula: Al₂SO₄(OH)₄·7(H₂O). 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 is very soft Mohs hardness of 1 with a specific gravity of 1.66—1.7. It forms as an oxidation product of pyrite and marcasite along with aluminous silicates and clays.

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It was first described in 1807 from Halle, Saxony-Anhalt, Germany. It is also known as alley stone and websterite (named after English geologist Thomas Webster).

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

References

• Mindat w/ locations
• Webmineral