STATISTICAL-MECHANICS OF ATOM ORDERING IN ULTRAMARINES

The distribution of Si and Al atoms over the framework of the aluminos ilicate 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 temperat ures (T < 700 K), the atom arrangement shows sublattice ordering with Si and Al atoms alternating in the network. For T > 750 K, the long-ra nge order of the atom distribution disappears, but there remains a cer tain 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 trans ition at this temperature. Configurational entropy and free energy res ults confirm that at temperatures of pyrolytic synthesis this atom dis tribution is disordered and violates Loewenstein's rule, in agreement with results obtained earlier by Si-29 NMR spectroscopy.