Room temperature X-ray powder diffraction patterns of various SiO2 matrices under various firing condition:
(a) the unfired SiO2 matrix, (b) the SiO2 matrix fired for an hour at 823K in air, (c) the SiO2 matrix fired for an hour at 1573K in air and (d) the SiO2 matrix fired for 6h at 1573K in air.