Monte Carlo simulation of Al, Si ordering in albite

The substitutional disorder of tetrahedral atoms in the feldspar albite has been studied by a Monte Carlo method, which takes into account atom 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. The interdependence between atom ordering and lattice relaxation has been analyzed by means uf radial distribution functions for framework atoms and Na cations, as well as by mean-square displacements of the atoms as a function of temperature. Our results indicate a strong coupling between Al, Si atom distribution and lattice relaxation. The Monte Carlo method is shown to be very helpful to analyze this order-disorder problem at a microscopic level.