Some consequences of aluminum and magnesium impurities in naturally occurring titanomagnetites

Abstract

The distribution of impurity cations between the octahedral and tetrahedral sublattices of the spinel structure is discussed. Six cation models are considered, ranging from a random distribution of impurities in the Akimoto model of titanomagnetite to an ordered distribution of impurities in the Neel-Chevallier model of titanomagnetite. Of 39 natural impure titanomagnetites analyzed by electron microprobe, a change from B- to A-site dominance of spontaneous magnetization will occur as a result of migration of the impurity cations from an initially random distribution to their preferred sites in 5 cases for ‘Akimoto’ titanomagnetite and in 27 cases for ‘Neel-Chevallier’ titanomagnetite. The dependence with temperature of activation energy associated with cation mobility makes extrapolation of results of laboratory experiments performed at several hundreds of degrees above the ambient of limited value in assessing the extent to which self-reversal may have occurred naturally in impure titanomagnetites. The variation of Curie point with cation distribution has been calculated and a self-reversal mechanism in impure titanomagnetites similar to that described by Ishikawa and Syono (1963) in hemo-ilmenites follows if the cation distribution was nonuniform during the aquisition of thermoremanent magnetization.