Calcium ferrite, Ca(FeO₂)₂

Calcium ferrite, Ca(FeO₂)₂, is formed as deep red or nearly black crystals by heating together ferric oxide and calcium oxide.

It also results on adding limewater to a neutral solution of ferric chloride and igniting the resulting brown precipitate. It appears to dissociate at its melting-point. A basic ferrite, Ca(FeO₂)₂.CaO, is produced as black crystals on heating finely powdered ferric oxide and calcium carbonate at temperatures considerably below 1385° C. Three other basic ferrites have been described, namely Ca(FeO₂)₂.2CaO, 2Ca(FeO₂)₂.CaO, and 3Ca(FeO₂)₂.2CaO, but the existence of these has been disputed.