Ferric rubidium selenium alum, Rb₂SeO₄.Fe₂(SeO₄)₃

Ferric rubidium selenium alum, Rb₂SeO₄.Fe₂(SeO₄)₃.24H₂O, is obtained when the requisite amount of rubidium carbonate is added to a solution of freshly precipitated ferric hydroxide in excess of aqueous selenic acid.

The alum crystallises out in pale violet crystals, belonging to the cubic system. Its density at 15° C. is 2.1308. At 40° to 45° C. the crystals melt in their combined water, yielding a red liquid.