ABSTRACT
The precursor powders formed after drying monophasic gels and on heating diphasic gel at 400 C are amorphous in nature and exist up to the temperature of their crystallizations to mullite or Al-Si spinel. The characterization of the amorphous precursor powders is the subject of many discussions. In the case of pure silica gel, where Q2, Q3, and Q4 silicon sites were well resolved, the authors showed very large linewidths and contended that these spectra are indicative of a broad distribution of environments in the second coordination sphere of silicon. It can be interpreted that aluminosilicate formation takes place by gradual heat treatment of the diphasic gel system and this verifies the present solid-state reaction study of the author that a heated diphasic precursor is also aluminosilicate, as in an SH precursor. An alkali leaching study by the author showed the absence of any free silica in the heated precursor, which conclusively predicted the formation of an aluminosilicate phase.
