It has a chemical formula of KFe32+AlSi3O10(OH)2.[7] Annite is the iron end member of the biotite mica group, the iron rich analogue of magnesium rich phlogopite.[9] Annite is monoclinic and contains tabular crystals and cleavage fragments with pseudohexagonal outlines.[5] There are contact twins with composition surface {001} and twin axis {310}.[10]