Application of Landau theory to cation ordering in omphacite I: Equilibrium behaviour
Carpenter, Michael A.; Domeneghetti, Maria-Chiara; Tazzoli, Vittorio
published: Mar 8, 1990
manuscript accepted: Sep 4, 1989
manuscript received: Jan 18, 1989
ArtNo. ESP147050201002, Price: 29.00 €
Abstract Crystallographic data for a cogenetic suite of sodic pyroxene crystals from Nybo, Norway, are used to show that the C2/c ⇄ P2/n transition in omphacite may be close to tricritical in character. Mean cationoxygen bond lengths at the Ml-type sites of the ordered structure give a macroscopic order parameter, Q, and this is found to vary as Q4 α composition. Ordering on M2-type sites appears to be linearly dependent on Ml ordering. An estimate of the equilibrium transition temperature and published calorimetric data provide a means of calibrating the coefficients in a Landau expansion. This expansion is then used to describe the excess thermodynamic properties associated with the ordering transition. For crystals with compositions close to JdAug =1:1, the resulting expression for excess free energy has the form (in J.mole-1): G = 11.4(T-1138)Q2+4317Q6.