Early silicate diagenesis is characterized by the precipitation of minerals with high silica activity, such as opaline silica, smectite, and clinoptilolite, and by pore-water silica activities exceeding quartz saturation. With burial, this early, high-silica activity diagenesic facies is succeeded by a low-silica activity facies distinguished by quartz precipitation and progressive development of illite as the nonexpandable component of mixed-layer clays. We suggest that the smectite-illite reaction occurs as a consequence of the reduction of silica activity at the onset of quartz precipitation. Data from oceanic and sedimentary basins confirm that the smectite-illite reaction does not control aqueous silica activities. Where the smectite-illite reaction occurs at higher temperatures, aqueous silica activities likewise reach quartz saturation at higher temperatures.