MONTE-CARLO SIMULATION OF AL, SI ORDERING IN ALBITE

The substitutional disorder of tetrahedral atoms in the feldspar albit e has been studied by a Monte Carlo method, which takes into account a tom displacements along with interchanges between Al and Si atoms. The atom ordering is characterized by a long-range order parameter, which suffers a sharp decrease around 1000 K, followed by a slower decrease up to 1250 K, the temperature at which the long-range order is lost. These results are in agreement with earlier X-ray diffraction data. Th e interdependence between atom ordering and lattice relaxation has bee n analyzed by means uf radial distribution functions for framework ato ms and Na cations, as well as by mean-square displacements of the atom s as a function of temperature. Our results indicate a strong coupling between Al, Si atom distribution and lattice relaxation. The Monte Ca rlo method is shown to be very helpful to analyze this order-disorder problem at a microscopic level.