Summary: Two compounds, one an apatite and the other a perovskite, have been synthesized by sintering mixtures containing CaO and Cr₂O₃, and CaO, Cr₂O₃, and Fe₂O₃ respectively. The apatite formed by partial oxidation of the Cr₂O₃ and subsequent pick-up of moisture from the atmosphere, to give Ca₅(CrO₄)₃OH. The perov-skite formed as a non-stoicheiometric compound Ca₄(Fe,Cr)₄O₁₁ by partial oxidation of the Cr₂O₃ during firing. Lattice parameters and interplanar spacings are given for the two structures.