Geochimica et Cosmochimica Acta. 1976, Vol. 40. pp. 41 to 49. Pergamon Press. Printed in Great Britain The dissolution kinetics of feldspars at 25°C and 1 atm CO, partial pressure EURYBMDES BUSENBERG and CHARLES V. CLEMENCY Department of Geological Sciences, State University of New York at Buffalo, 4240 Ridge Lea Road. Amherst, N.Y. 14226,U.S.A. zyxwvutsrqponmlkjihgfedcbaZYXW (Received 11 December 1974; accepted in revised form 2 June 1975) Abstract-The dissolution kinetics of 5% by weight suspensions of two potassium feldspars and six plagioclases were studied for 400~1200 hr at 1 atm Pco,. The less than 37 pm particles had surface areas ranging from 0.83 to 1.84m2/g. At frequent intervals, the pH of the stirred suspensions were measured in the reaction cell as the solutions were sampled. The filtered aqueous samples were acidified and then were analyzed for Si, Al, K. Na. Ca, Mg, and Fe. The following sequence of events were recognized in the artificial weathering of all the feldspars: (1) an initial ion exchange stage lasting approximately 1 min during which the surface catIons were replaced by hydrogen ions from the bulk solution, (2) an up to 4 day non-parabolic stage characterized by the very rapid release of cations and silicic acid into the bulk solution, (3) a diffusion-controlled parabolic stage lasting approximately 19 days, and (4) a steady-state stage characterized by the very slow release of cations and silicic acid. All the rate constants for all the feldspars were evaluated at the pH of about 5. The calculated parabolic rate constants range from 3.4 x lO-‘4 to 9.3 x IO-l3 mole/cm2/se&, the linear rate constants range from 3.2 x 10-l’ to 2.5 x 10eL5 mole/cm2/sec. and the apparent diffusion coefficients range from 1 x lo-” to 5 x IO-*’ cm2/sec. The steady-state composition of the surface layer of all the plagioclases appears to have a I:1 Si:Al molecular ratio, whereas in contrast, the potassium feldspars appear to have a 2:l ratio. INTRODUCTION THE ARTIFICIAL weathering of feldspars has been ex- tensively investigated; however, most of the earlier studies have been of relatively short duration, con- ducted in chemically complex pH buffers (K~rro and PATTERSON, 1942; WOLLAST, 1967) or at elevated tem- peratures and pressures (MOREY and CHEN, 1955; LAGACHE et al., 1961a, 1961b; LAGACHE,1965). In most cases, the chemical composition of the reactants was uncertain and/or the chemical analysis of the solution incomplete. In this study, an attempt was made to eliminate some of these shortcomings. The experimental conditions were designed to resemble natural environments closely. CARRELS and HOWARD (1957) showed that the in- itial feldspar-water interaction is a reversible ion- exchange reaction between the surface cations of the solid and hydrogen ions from the bulk solution. This reaction is followed by the incongruent dissolution of the feldspar surface (CORRENS and VON ENGEL- HARDT, 1938) leaving behind a slightly soluble prod- uct layer. The release or transport of silica and/or cations through the thickening product layers have been modelled by WOLLAST (1967), HELGE~N (1971, 1972), LUCE et aI. (1972) and PACES (1973). The purpose of this paper is (1) to describe the dissolution of all the feldspars over extended periods of time, (2) to evaluate all the rate constants, and (3) to determine the steady-state composition of the residual surface layers. EXPERIMENTAL. PROCEDURES AND MATERIALS All experiments were done in a 1 liter Nalgene reaction cell with a tight-fitting Plexiglas cover. Carbon dioxide was bubbled through glass dispersion tubes in two consecutive Erlenmeyer flasks to raise the temperature of the gas to that of the constant temperature bath and to saturate the gas with water vapor. Subsequently, the gas was bubbled gently through a polyethylene tube into the reaction vessel. After bubbling CO2 for about 20 min, approximately 50 g of -400 mesh (~37 pm) purified feldspar was added to 1 liter of distilled, deionized water in the cell. The solutions were continuously stirred to keep the solid in suspension and to maintain the liquid phase homogenized. The stir- ring rates and the 45.3 ml/min CO2 flow rate were held constant in all the experiments. The entire apparatus was maintained at constant temperature in a thermostated water bath held at 25.5 k 0.2”C. Thirty ml aliquots of homogenized suspension were withdrawn periodically from the reaction vessel with a plastic syringe. The samples were immediately filtered through 0.1 pm Millipore@’ filters. The filtrates were then acidified to pH 2 with one drop of concentrated hydro- chloric acid per 1Oml of sample. The solid was not returned to the reaction vessel and therefore a constant ratio of solid to liquid was maintained throughout the ex- periments. The solid phases The feldspar samples were crushed to small pieces in a jawcrusher. and when necessary, foreign materials were removed by hand-picking. All magnetic material was removed with a Franz Isodynamic separator. The feldspars were then ground to a particle size of less than 37 pm. The surface area of each sample was determined by the 41