Abstract :
The thermodynamic properties of two-dimensional Ising-type models of albite/high albite transition occurs continuously with temperature. First, a nearest-neighbor Ising model is used to calculate the thermodynamic properties accurately. The value of the excess entropy obtained is very close to values found by other approximate calculations (for the same model) at low temperatures but differs at high and especially at intermediate temperatures. Within various approximate calculations for the model, these differences are a few percent at low temperatures where the low albite is the stable phase. At high temperatures, where high albite is the stable phase, the entropy calculated by the approximate methods differs from the present result by as much as 10 percent. At intermediate temperatures (where the transition occurs) the calculated entropy of the present work differs from the previous results by 20 to 30 percent at the same temperature. Comparison of the calculated order parameter with experimental results suggests that the nearst-neighbor Ising-type model is not an appropriate model for albite. Second, we assumed that the site preference energy depends linearly on p (the fraction of T***10 sites occupied by A1 atoms), a fairly good agreement is found with experimental results. The excess thermodynamic properties are calculated and compared with those given by the Landau theory.
