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 th e 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 (T-c), saturati on magnetization (M-s), coercivity (H-c), and remanence (M-rs). T-c and M-s are determined mainly by the fundamental crystal chemical state of a miner al, which is effected by the processes of cation ordering, magnetic orderin g, and subsolvus exsolution. H-c and M-rs are determined mainly by the micr ostructure 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 proces ses in Fe-bearing spinel solid solutions and describes the magnetic and pal aeomagnetic consequences of the interaction between them. A general thermodynamic model for coupled ordering processes is developed a nd used to describe quantitatively the temperature and composition dependen ce of the cation distribution and saturation magnetization, the interaction between cation and magnetic ordering, the relationship between cation dist ribution and Curie temperature, and the kinetics of cation ordering. Each n ew concept is illustrated using the spinel solid solution between magnetite (Fe3O4), magnesioferrite (MgFe2O4), hercynite (FeAl2O4), and spinel (MgAl2 O4) as an example. This solid solution serves as a synthetic analogue to th e natural titanomagnetite solid solution. The phenomenon of subsolvus exsol ution is discussed and the different stages of microstructural development are illustrated using transmission electron microscopy. The magnetic conseq uences of subsolvus exsolution are investigated and the implications for na tural exsolved material are discussed.