Usually yellow, it is a rhombic crystalline salt and soluble in water at room temperature. It is used in dyeing as a mordant, and as a coagulant for industrial wastes. It is also used in pigments, and in pickling baths for aluminum and steel. Medically it is used as an astringent and styptic.
In sewage plants it is used to assist in settling minute particles of untreated sewage in tank water.
Mikasaite is the name of mineralogical form of iron(III) sulfate. This anhydrous form occurs very rarely and is connected with coal fires. The hydrates are more common, with coquimbite (nonahydrate) as probably the most often met among them. Paracoquimbite is the other, rarely met natural nonahydrate. Kornelite (heptahydrate) and quenstedtite (decahydrate) are rarely found. Lausenite (hexa- or pentahydrate) is a doubtful species. All the mentioned natural hydrates are unstable compounds connected with Fe-bearing primary minerals (mainly pyrite and marcasite) oxidation in ore beds. In the solutions of the ore beds oxidation zones the iron(III) sulfate is also an important oxidative agent.