Abstract :
In order to use a local unit larger than that suggested by Andersen and Mazo (1979), a new method-the modified sequential construction method or MSCM -has been developed for the calculation of order parameters and configurational entropies in albite. This method has several advantages over the older methods such as being readily usable for any size of local structural unit. The number of distributions of atoms among sites is first reduced to noninteracting units, and the resulting combinatorial factor is then corected by an approximated correction factor.In comparison with Monthe Carlo results this new approximation gives a better order parameter for the 2-D model of albite of Andersen and Mazo than the quasi-chemical and the sequential construction methods. The maximum numerical deviation of the calculated order parameter (for Andersen and Mazo,s 2-D model of albite) from the Monte Carlo results is about 6,for the quasi-chemical method,2,for the sequential construction method, and only about 1,for the modified sequential construction method. It is also shown that if the size of the local unit becomes larger, the entropy curve becomes steeper at temperatures in which the high albite-low albite transition occurs.
