A SI-29 MAGIC ANGLE SPINNING NMR AND DTA STUDY OF THERMAL CRYSTALLIZATION OF SPHENE AND ZIRCON GELS

29Si MAS NMR spectra of gels of sphene and zircon stoichiometry show wide variations in local silicon environments at intermediate stages of crystallization, depending on the pH of the starting mixtures. The broad NMR peaks of the calcined gels give way on heating to spectra of varying complexity, including a cristobalite peak which then diminishes in intensity again as the sphene or zircon peak becomes predominant on further heating. Differential thermal analysis as a function of heating rate for selected zircon and sphene gels yields activation energies of crystallization of about 300 kJ mol-1. © 1990 Chapman and Hall Ltd.