Zinc ferrite, Zn(FeO₂)₂

Zinc ferrite, Zn(FeO₂)₂ or ZnO.Fe₂O₃, occurs in nature as franklinite and may be artificially prepared in the laboratory in a variety of ways, such as by fusing an intimate mixture of the sulphates of sodium, zinc, and ferric iron at cherry-red heat, and extracting the melt with boiling water. Any basic zinc sulphate is removed with dilute acetic acid. Crystalline zinc ferrite also results when zinc chloride is heated in moist air with ferric chloride or haematite. The zinc chloride may be replaced by the corresponding fluoride.

As obtained by these methods crystalline zinc ferrite closely resembles the natural franklinite. It crystallises in regular octahedra, small crystals being reddish brown and translucent, whilst the larger ones are opaque. Their hardness is 6-5 and density 5-33, the corresponding values for the mineral being H = 5.5 to 6.5 and D = 5 to 5.2 respectively. The crystals are not magnetic, are resistant to heat, and are but slowly attacked by acids. The natural mineral is magnetic, but this is attributed to impurities.