Magnetic cristobalite (?): A possible new magnetic phase produced by the thermal decomposition of nontronite

Prolonged heat treatment (>1 hour) of nontronite (an iron-rich smectite clay) at 900° to 1000°C produces a phase with some unusual magnetic properties. This new phase has a Curie temperature of 200° to 220°C, extremely high remanent coercivities in excess of 800 milliteslas, and a room-temperature coercivity dependent on the magnitude of the applied field during previous thermomagnetic cycling from above 220°C. X-ray and magnetic analyses suggest that an iron-substituted cristobalite could be responsible, in part, for these observations. Formation of this magnetic cristobalite, however, may require topotactic growth from a smectite precursor.