Aluminium oxide is generally the basis for the production of aluminium sulphate, it is industrially produced from bauxite, a reddish soil of sedimentary origin rich in iron and alumino oxides. The same oxide is the main source for the final production of metallic aluminium.
Magmatic and volcanic activity also leads in certain cases to the formation of aluminium sulphate, in magmas containing sulphur and various metals, including aluminium. Thus, hot groundwater formed in later magmatic phases (hydrothermal phases) have in certain world areas impregnated layers of clay, giving rise to stratified deposits. From here you can extract aluminum sulfate. It was in fact easily removable downstream of the metalliferous mountains between Germany and Bohemia.
In small quantities but chemically interesting, aluminum sulfate is often formed at the outlet of the sulfur “fumaroles”, containing hydrogen sulfide, sulfur dioxide and water vapor. What above described can be found on the crest of the main crater of the island of Vulcano: along with sulphur, which marks the exit points of the fumaroles in yellow. In the sun it is possible to notice “with the naked eye” small, bright, needle-like crystals of aluminium sulphate.
Aluminium sulphate is a valuable ally in clarifying and treating waste and turbid water. In fact, these waters often contain colloids, solid particles (clay or silt) of microbic size, as well as bacteria and viruses that tend to have a very slow process of sedimentation. Sulphate accelerates the process heavily, making these operations much cheaper.
Aluminium sulphate in solution, added in small concentrations to the water, brings to a slightly alkaline pH, and develops a flaky layer, which settles easily incorporating all microbial parts. This part is then easily eliminated thanks to the shape of the tanks for water treatment.