Application of Landau theory to cation ordering in omphacite; 2, Kinetic behaviour

The kinematics of cation disordering have been followed in single- crystals of omphacite which were annealed in evacuated silica tubes at 900, 950 and 1000 degrees C. The X-ray intensities I (sub k) , of superlattice reflections were used to monitor changes in the macroscopic order parameter, Q, with annealing time (I (sub k) alpha Q (super 2) ). A purely empirical analysis of the kinetic data gives a rate law of the form ln(Q (super 2) ) alpha t (super 1/2) and an enthalpy of activation of 615 +- 65 kJ/mole. The data are also approximately described by Q (super 2) alpha ln t for a range of t. Time-dependent order parameter theory provides a more rigorous basis for analysing the disordering transition and the data have been compared with a specific solution of the Ginzburg-Landau equation. This analysis gives an example of how Landau theory for equilibrium behaviour can be coupled with the Ginzburg-Landau equation for kinetics to describe structural phase transitions in minerals under non-equilibrium conditions.