The Magnetic Properties and Crystal Chemistry of Oxide Spinel Solid Solutions

The palaeomagnetic signal of rocks arises mainly due to the presence of Fe-bearing oxide solid solutions with the spinel crystal structure (such as the titanomagnetites). The ability of these minerals to acquire a strong and stable remanent magnetization in the presence of the Earth’s magnetic field is determined to a large extent by their Curie temperature (Tc), saturation magnetization (Ms), coercivity (Hc), and remanence (Mrs). Tc and Ms are determined mainly by the fundamental crystal chemical state of a mineral, which is effected by the processes of cation ordering, magnetic ordering, and subsolvus exsolution. Hc and Mrs are determined mainly by the microstructure of the mineral, which is also a function of the cation ordering and subsolvus exsolution processes. This paper reviews recent developments in the theoretical description and experimental observation of these processes in Fe-bearing spinel solid solutions and describes the magnetic and palaeomagnetic consequences of the interaction between them. A general thermodynamic model for coupled ordering processes is developed and used to describe quantitatively the temperature and composition dependence of the cation distribution and saturation magnetization, the interaction between cation and magnetic ordering, the relationship between cation distribution and Curie temperature, and the kinetics of cation ordering. Each new concept is illustrated using the spinel solid solution between magnetite (Fe3O4), magnesioferrite (MgFe2O4), hercynite (FeAl2O4), and spinel (MgAl2O4) as an example. This solid solution serves as a synthetic analogue to the natural titanomagnetite solid solution. The phenomenon of subsolvus exsolution is discussed and the different stages of microstructural development are illustrated using transmission electron microscopy. The magnetic consequences of subsolvus exsolution are investigated and the implications for natural exsolved material are discussed.