Amorphous fumed silica produced by vapor phase hydrolysis of silicon tetrachloride
in flame of hydrogen and oxygen, the resulting product is pure x-ray amorphous
silicon dioxide (SiO2).
The chemical reaction for this process is:
2H2 + O2 + SiCl4 → SiO2 + 4HCl
Silica particle morphology can be modified by variations in flame conditions.

By varying the concentration of the reagents, flame temperature and dwell time of
the silica in the combustion chamber, it is possible to influence the particle size,
particle size distribution, surface area and surface properties of silica within
wide limits.