The widespread occurrence of a novel, high coercivity magnetic phase in well-heated archeological material is reported. Its properties are defined when it represents the dominant magnetic phase, although it is nearly always found as part of a mixture of magnetic phases. They are as follows: very high coercivity (remanence coercivity >600 mT), low unblocking temperatures (≤200°C) and high degree of thermal stability-this last property istinguishing it from goethite. The phase shows striking similarities to magnetic phases produced by thermal decomposition of nontronite (an Fe-rich clay), where decomposition occurs after prolonged heating in air to high temperatures - conditions suffered by well-heated archeomagnetic material. Preliminary results of Mössbauer and X-Ray diffraction spectroscopy suggest that the phase is more likely to be a substituted hematite, rather than Fe-cristobalite or a variant of ε-Fe2O3. Copyright 2007 by the American Geophysical Union.