What is Ammonium iron(III) sulfate?

Ammonium iron sulfate, NH4Fe2·12 H2O, or NH46]2·6 H2O, also known as ferric ammonium sulfate or iron alum, is a double salt in the class of alums, which consists of compounds with the general formula AB2 · 12 H2O. It has the appearance of weakly violet, octahedrical crystals. There has been some discussion regarding the origin of the crystals' colour, with some ascribing it to impurities in the compound, and others claiming it to be a property of the crystal itself.

FAS is paramagnetic, acidic and toxic towards microorganisms. It is a weak oxidizing agent, capable of being reduced to Mohr's salt, ferrous ammonium sulfate.