In the process disclosed herein, siliceous plant matter is steeped in water, soaked in an aqueous solution containing a solute which solubilizes inorganic oxides, soaked in an aqueous solution containing an oxidizing solute, rinsed, dried and thermally pyrolyzed to produce amorphous silica with of low carbon content, low water content, low inorganic impurity content and is of high porosity. Practice of the invention yields usable energy, does not produce carbonization of the atmosphere and is of lower nitrogen oxide and sulphur emission than currently used processes. Lignin, hemicellulose and cellulose derived sugars may be recovered from the steep or soak water. By varying steps of the process herein disclosed, the carbon content, inorganic impurities and porosity of the resulting amorphous silica may be selectively controlled.