Commonly found as small octahedra it is white, but impurities of realgar or orpiment may give it a pink or yellow hue.

It can be associated with its dimorph claudetite (a monoclinic form of As2O3) as well as realgar (As4S4), orpiment (As2S3) and erythrite, Co3(AsO4)2·8H2O.

[4] It was first described in 1854 for an occurrence in the St Andreasberg District, Harz Mountains, Lower Saxony, Germany.

[4] It occurs by the oxidation of arsenic-bearing sulfides in hydrothermal veins.

It also occurs as a result of mine or coal seam fires.