Cluster ordering in synthetic ultramarine pigments

Blue ultramarine pigments with composition Na7.7[(Al,Si)12O24] · (SO4)0.9(S3)0.8 and Na6.5K0.5[(Al,Si)12O24] · (SO4)0.9(S3)0.7 were synthesized in order to get a deep knowledge of its structure and microstructure. These pigments were prepared following the current industrial method synthesis, which consists of the calcination of the metakaolin, sulphur, sodium carbonate and charcoal under controlled atmosphere. The structural characterization of the obtained pigments was performed in detail by the complementary use of different techniques such as X-ray powder diffraction and transmission electron microscopy (SAED and HRTEM) together with 29Si MAS-NMR and IR spectroscopies. The obtained results have allowed to propose a structural model with the lowest symmetry for ultramarine pigments. The suitable space group to describe this structure is P23 (No.195), which takes account the observed β-cage cluster ordering at the unit cell level and the framework adaptation to the different size of the β-cage clusters.