Order-disorder phase transitions in silicates and oxides: recent observations of strain coupling

Coupling between spontaneous strains and cation exchange dominates high-temperature mineral behaviour. Cation ordering processes, such as Al-Si ordering on the tetrahedral sites of aluminosilicates, are frequently associated with the development of local strains. In framework structure, comprising a network of Q⁴tetrahedra, these local strains usually result in macroscopic phenomena and the development of microstructures. Generally, their long-range nature ensures that ordering interactions are essentially mean-field. Archetypal examples include Al-Si order-disorder processes in the feldspar structures, where convergent ordering and ferroelastic instabilities occur hand-in-hand as a function of temperature. In these cases Landau models work well to describe the high-temperature behaviour, and the long-range order parameter couples directly with the ferroelastic or coelastic spontaneous strain. Recent results from ilmenite and related oxides reveal, however, that such models break down when interactions are dominantly short range, but strains may still reflect ordering behaviour over a range of length scales.