EFFECTS OF ORGANIC-ACIDS ON THE DISSOLUTION OF ORTHOCLASE AT 80-DEGREES-C AND PH 6

The dissolution of K-rich feldspar (orthoclase), quartz and Al(OH)(3) was investigated at 80 degrees C and pH 6 in buffered solutions of org anic acids. Previous studies of the effects of organic acids (OA) on f eldspar dissolution have typically been conducted in acidic, low-ionic strength solutions often under conditions which preclude isolation of effects of pH from those due to organic acids. Our experiments were c onducted at constant pH, temperature, ionic strength, and buffer compo sition to allow direct comparison of experiments with and without OA. The dissolution experiments were conducted under closed-system conditi ons to: (1) determine the magnitude of mineral solubility enhancement by OA; (2) examine changes in reaction stoichiometry as equilibrium is approached; and (3) investigate the effects of OA on secondary minera l precipitation. The carboxylic acid species, oxalate and citrate, sig nificantly enhanced the dissolution of orthoclase at pH 6. The concent rations of Si and Al in 10 mM oxalate and citrate solutions were nearl y 3 times that in solutions of acetate buffer without oxalate or citra te. Aluminum was below the limit of detection (< 0.007 mM) in the acet ate buffer alone. Citrate increased the release of Si and Al from orth oclase more than did oxalate at the same concentration. Equilibrium mo deling indicates that solutions with oxalate and citrate attained supe rsaturation with respect to gibbsite, kaolinite, and smectite, and sat uration with respect to quartz. Nevertheless, orthoclase dissolution r emained congruent with respect to Si and Al release. Separate experime nts using pure quartz and Al(OH)(3) suggest that a mechanism other tha n formation of Si-organic complexes may be involved in the OA-enhanced release of Si from orthoclase. The increase in dissolved silica was m odest in 10 mM oxalate and 10 mM citrate solutions reacted with quartz , and did not increase with increasing OA concentration. The solubilit y of quartz was similar in solutions of oxalate and citrate. In contra st, Al(OH)(3) dissolution was 50% greater in citrate than in oxalate, similar to the behavior of orthoclase. Citrate may be more effective i n dissolving orthoclase than oxalate due to a stronger interaction bet ween citrate and Al, rather than due to a synergistic effect of Al-cit rate and Si-citrate complexes. These results have implications for the mass transport of Si and Al in diagenetic environments.