SILICA ACTIVITY AND THE SMECTITE-ILLITE REACTION

Early silicate diagenesis is characterized by the precipitation of min erals 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 diagenetic f acies is succeeded by a low-silica activity facies distinguished by qu artz precipitation and progressive development of illite as the nonexp andable component of mixed-layer clays. We suggest that the smectite-i llite reaction occurs as a consequence of the reduction of silica acti vity at the onset of quartz precipitation. Data from oceanic and sedim entary basins confirm that the smectite-illite reaction does not contr ol aqueous silica activities. Where the smectite-illite reaction occur s at higher temperatures, aqueous silica activities likewise reach qua rtz saturation at higher temperatures.