XRD and IR analyses were applied to study the thermal synthesis of ultramarine by structural transformation of zeolites reacting with sulphur under alkaline conditions. Zeolites A, X and cancrinite are transformed to sodalite at high temperature (800°C) under highly alkaline conditions when sodium is the only alkali cation present. The original structure of the parent zeolites can be retained (particularly at low temperature) when the alkalinity is low. Other structures such as nepheline hydrate II, carnegieite, kaliophilite are detected in the coloured products obtained either at intermediate alkalinity or through the introduction of cations other than sodium into the reaction mixture.

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