An Experimental Study of Quartz Precipitation and Dissolution Rates at 200°C

Abstract

The understanding and quantification of quartz-water interactions is essential for modelling mass transfer in crustal fluids. Quartz dissolution kinetics at hydrothermal temperatures have been the subject of many investigations. Recently, Berger et al. (1994) used a mixed flow reactor to produce data over a wide range of solution compositions, which enabled an improved understanding of the quartz dissolution mechanism. Much less work has been performed on quartz precipitation, and in particular the effect on the rates of changing solution composition. Rimstidt and Barnes (1980) and Bird and Boon (1985) used a closed system technique to generate rate constants from their data, but to date no direct quartz precipitation rate measurements have been performed, In this study direct measurements of quartz dissolution/precipitation rates were made as a function of solution saturation state at the near equilibrium conditions typical of natural processes. Experiments were performed at 200~ using a titanium mixed flow reactor system described by Berger et al. (1994). These reactor systems are ideally suited to investigate the rates of water/mineral interactions at both near and far from equilibrium conditions (Gautier et al., 1994). Mixed flow reactors afford numerous advantages over closed system reactors for kinetic studies including 1) allowing direct measurement of steady-state rates, and 2) permitting rate measurements at specific fluid compositions by either changing the inlet solution composition or the fluid flow rate. The same natural quartz crystals were used in both dissolution and precipitation experiments. The crystals were ground and sieved to the 50-125 micron size fraction. Fine particles were removed by ultrasonic cleaning in deionized water. To remove remaining fine particles and the disturbed surface layer produced by grinding, the quartz grains were cleaned in a mixed flow reactor for five days at 250~ with deionized water. The B.E.T. specific surface area measured after the cleaning procedure was 0.098 _+ 0.005 m2/g. The aqueous silica-rich inlet solutions used for precipitation experiments were prepared by concentrating deionized water originally equilibrated with silicic acid at 90~ When starting a precipitation run, the reactor was filled with deionized water and heated to 200~ Silica-rich solution was then pumped through the reactor. This procedure avoided any potential precipitation of non-quartz silica phases on the quartz grains that could occur from supersaturated solutions at temperatures less than 200~ To assess the possible effect of silica precipitation on the reactor walls and tubing, a blank test was made by pumping a silica-rich solution through the reactor containing no quartz sample. Only a slight decrease of the outlet silica concentration, less than one percent of the inlet concentration, was observed. Dissolution rates were measured at 200~ in atmosphere equilibrated deionized water (pH of 5,65 at 25~ Steady-state outlet silica concentrations ranged frolI1 4.13 x 10 -5 to 2.70 x 10 -3 m. Each of the dissolution data points shown in Fig. 1 corresponds to an individual run conducted at a specific flow rate. Quartz solubility was measured at 200~ in a 5 x 10 -4 m NaOH solution using the same solid as used for the kinetic study but of a smaller size fraction (40-60 microns). The measured solubility of 4.55 x 10 3 m of Si corresponds to a calculated quartz dissociation constant at 200~ of 4.27 x 10 -3. Precipitation experiments were performed using inlet fluids containing from 145 to 253 mg/kg of Si and at flow rates ranging from 0.01 to 0.18 ml/min. Steady-state outlet concentrations ranged from 141 to 198 mg/kg of Si. The B.E.T. specific surface area measured after the last experiment was 0.0745 _+ 0.005 m2/g, which is slightly lower than the original measurement. Dissolution rates exhibit a linear dependance on the saturation state consistent with rate laws derived from the Transition State Theory and the principle of detailed balancing (Lasaga, 1981) given by: