The direct depolymerization of biogenic and other high surface area silica sources uses both simple and hindered diols to produce alkoxysilanes in one or two steps that can be separated and purified directly from the reaction mixture by distillation, extraction or filtration followed by solution modification and distillation or extraction. The alkoxysilanes can take the form of spirosiloxanes or simple alkoxysilanes or oligomers thereof. Thereafter they can be treated with acid to produce colloidal or precipitated silica or aerosolized and combusted to provide fumed silica without the intervention of SiCl4.