STATISTICAL-MECHANICS OF ATOM ORDERING IN ULTRAMARINES

The distribution of Si and Al atoms over the framework of the aluminosilicate ultramarine with a ratio Si/Al = 1 is studied by a Monte Carlo method, in the temperature range from 100 to 1800 K. For low temperatures (T < 700 K), the atom arrangement shows sublattice ordering with Si and Al atoms alternating in the network. For T > 750 K, the long-range order of the atom distribution disappears, but there remains a certain degree of short-range ordering, with partial avoidance of Al-O-Al and Si-O-Si groups. The heat capacity derived from the MC simulations shows a peak at about 750 K, suggesting an order-disorder phase transition at this temperature. Configurational entropy and free energy results confirm that at temperatures of pyrolytic synthesis this atom distribution is disordered and violates Loewenstein's rule, in agreement with results obtained earlier by Si-29 NMR spectroscopy.