Computational study of tetrahedral Al-Si ordering in muscovite

The nature of Al-Si ordering across the tetrahedral sites in muscovite, K2A l4(Si6Al2O20)(OH)(4), was investigated using various computational techniqu es. Values of the atomic exchange interaction parameters J(l) were obtained . From these parameters, a two-dimensional Al-Si ordering scheme was deduce d. The transition temperature T-c for this two-dimensional ordering is 1900 K. There are several possible ordering schemes in three dimensions. based on different stacking sequences of ordered sheets of tetrahedral sites. Mon te Carlo simulations of both two-dimensional and three-dimensional ordering were performed. but in the three-dimensional simulation only the two-dimen sional ordering is seen, implying that three-dimensional ordering is too sl ow to be attained during the timescale of the simulation. The effect of the three-dimensional interactions is to raise the two-dimensional ordering te mperature to 2140 K. From the three-dimensional Monte Carlo simulation. the frequency of occurrence of 4Si0Al. 3Si1Al, 2Si2Al and 1Si3Al clusters was determined, which match those inferred by Si-29 MAS-NMR measurements reason ably well. In fact, the match suggests that the cation ordering seen in exp eriments corresponds to a configuration with considerable short-range order but no long-range order, similar to a state that is at a temperature just above an ordering phase transition.