The structure and Si,Al distribution of the ultramarines

Abstract

The structure and Si,A1 distributions of various ultramarine pigments have been investigated using X-ray and neutron powder diffraction with Rietveld refinement, and magic-angle-spinning NMR (MAS NMR). The basic structures of the ultramarines studied are found to be very similar: refinement proceeds far better in space group I713m than in PT~3n which indicates that Si and A1 in the framework of ultramarines are disordered. This conclusion is supported by 29Si MAS NMR spectra which contain five signals, rather than a single signal which would be present if AI-O-AI linkages were forbidden. The intensities of the five peaks are consistent with the presence of Si-O-Si, Si-O-A1 and A1-O-AI linkages in the structure. Such disordered Si,A1 distributions in pyrolytically formed ultramarines are in striking contrast to the ordered distributions found in both the naturally formed counterpart lazurite and in hydrothermally synthesized zeolites.