SI-29 AND AL-27 NMR-STUDY IN THE MULLITE SOLID-SOLUTION AL-2(AL2-2X)O-10-X(2XSI2)

The Si-29 MAS NMR study has shown the existence of four different site s for Si whatever the composition of mullite. Their population evolve linearly with the composition in the solid solution. The major site, n ot reported in precedent studies due to its broad distribution of chem ical shifts, increases as 2x, x being the number of oxygen vacancies. It is assigned to all the tetrahedra near an oxygen vacancy. By extrap olation it is not present in sillimanite (x = 0). Two other sites are modified silimanite-type sites and decrease as 1x. The (27)AlMAS NMR s tudy has been carried out at two different magnetic fields (7 and 11.7 T). The spectra have been simulated with quadrupolar bands and show t hat the Al(IV)/Al(VI) ratio decreases abnormally for alumina-rich mull ites. The Al-27 NMR is not quantitative. Particularly the most distort ed tetrahedra, more present in alunina-richer mullites, are only parti ally seen by NMR.