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.