Abstract: SiO2 generated in clay mineral transformation cannot develop into authigenic quartz because of narrow space resulting from high clay minerals in pure shale of Ordos basin, and appear among clay minerals in the occurrence of amorphous state. Authigenic quartz with perfect crystal morphology grows in micron-sized pores of silty mudstone interlayers accompanying with chlorite and ferrocalcite. Amorphous silica gathers among detrital minerals in silty mudstone interlayers and arises along quartz particles in the form of granular and sheet silica by secondary enlargement. Clay mineral transformation and quartz dissolution are the sources of silica.