The 18O/16O fractionation accompanying the hydrothermal crystallization of quartz from silicic acid at 265°-465°C has been studied in order to assess the influences of (a) rate of quartz formation, (b) the nature of the reaction mechanism, and (c) temperature. At 360° and 465°C there are no indications of rate effects influencing isotope partitioning, but at 265°C (where fractionation factors show an unusually large scatter) the evidence is inconclusive in this respect. All reaction appears to occur through solution-precipitation processes, with the intermediate phases, cristobalite and silica K, giving identical fractionation factors (within experimental errors) to quartz samples formed at the same temperature. The temperature-dependence of fractionation in the range 265°-465°C is given by the equation
The data are in good agreement with other experimental calibrations of the quartz-water fractionation.