By thermal decomposition of CaFe5O7 at 1125 degrees C in a sealed silica tube during about ten days, we obtained the oxide Fe0,832Ca0,028 O-1 referred to as "Fe6O7". Like wustite, it is iron deficient, very rich in iron (III) cations (Fe2+/Fe3+ = 1.97), has the rock-salt structure (a = 429.4 pm), but the x-ray diffraction pattern does not show satellite reflections around the Bragg peaks. Like wustite, "Fe6O7" is antiferromagnetic (T-N = 207 K), but the room temperature Mossbauer spectrum is different. It seems that this phase is stabilized by traces of calcium and that there is no tetrahedral interstitial iron (III).