Double sulfates with the general formula are also known, where "A" is a monovalent cation such as sodium, potassium, rubidium, caesium, or thallium(I), or a compound cation such as ammonium (), methylammonium (), hydroxylammonium () or hydrazinium (), "B" is a trivalent metal ion, such as aluminium, chromium, titanium, manganese, vanadium, iron(III), cobalt(III), gallium, molybdenum, indium, ruthenium, rhodium, or iridium. Analogous selenates also occur. The possible combinations of univalent cation, trivalent cation, and anion depends on the sizes of the ions.