TEMPERATURE-DEPENDENCE OF THE SI, AL DISTRIBUTION IN ULTRAMARINES

The Si, Al ordering pattern for blue ultramarine (aluminosilicate with a ratio Si/Al almost-equal-to 1) is analyzed by a Monte Carlo method. The influence of the synthesis temperature and the large extraframework anions on the Si, Al distribution is studied. It is found that a combination of both factors (especially the first) can explain the reported violation of Loewenstein's rule in this compound. A long-range disordered Si, Al distribution was observed for temperatures above 800 K, which correspond to the pyrolytic synthesis of ultramarine. For lower temperatures, corresponding to the regime of hydrothermal synthesis, strict alternation of Si and Al was found, in agreement with experimental results for lazurite, the natural counterpart of ultramarine.