High Quality Content by WIKIPEDIA articles! Rhomboclase is an acidic iron sulfate mineral with a formula reported as H5Fe3+O2(SO4)2·2(H2O) or HFe(SO4)2·4(H2O). It crystallizes in the orthorhombic system and typically occurs as tabular crystals with a rhombic outline. It occurs as transparent colorless, blue, green, yellow or grey crystals with a vitreous to pearly luster.