is a hydrous aluminium sulfate
mineral with formula: Al2
O). 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
along with aluminous silicates and clays.
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.