A COMPUTATIONAL STUDY OF AL SI ORDERING IN CORDIERITE/

The ordering of Al and Si in Mg cordierite Mg2Al4Si5O18 is considered using computer simulation. First the enthalpy of interaction J(ij) bet ween sites is derived by computer modelling 101 different Al/Si config urations and analysing their energies. They are compared with similar results for three other minerals and with ab initio calculations to as sess the whole approach. Secondly the ordering process is studied usin g Monte Carlo simulation applied to the J(ij). The ordering phase tran sition temperature T-c is found as 1800 degrees C in reasonable agreem ent with the experimental estimate of 1450 degrees C. These are much l ower than the estimate T-c (ABW) approximate to 7600 degrees C obtaine d from Bragg-Williams theory. Strong short-range order sets in below T -c (ABW), and the reasons for much lower temperature T-c of long-range ordering are discussed. Strong short-range also sets in very rapidly in a simulated anneal, in agreement with experiment. Thirdly an attemp t is made to compare our calculated enthalpies directly with the resul ts of NMR and calorimetry experiments, not completely successfully. A free energy Delta G approximate to 4.6 eV for the activation barrier f or ordering is suggested.