Ammonium iron(III) sulfate
Ammonium iron ( III ) sulfate , NH4Fe ( SO4 ) 2·12 H2O , or NH4 [ Fe ( H2O ) 6 ] ( SO4 ) 2·6 H2O , also known as ferric ammonium sulfate ( FAS ) or iron alum , is a double salt in the class of alums , which consists of compounds with the general formula AB ( SO4 ) 2 · 12 H2O . 
<triplet> Ammonium iron(III) sulfate <subj> alum <obj> instance of <triplet> alum <subj> double salt <obj> subclass of