THE EFFECT OF ORGANIC-ACIDS ON PLAGIOCLASE DISSOLUTION RATES AND STOICHIOMETRY

The rates of plagioclase dissolution in solutions containing organic acids are up to ten times greater than the rates determined in solutions containing inorganic acids at the same acidity. Initial rates of dissolution are poorly reproduced in replicate experiments. After a day, however, the rates of plagioclase dissolution calculated from the rates of silicon release are reproducible and constant for up to nineteen days. Steady-state rates of dissolution are highest (up to 1.3 X 10(-8) mol/m2/sec) in acidic solutions (pH almost-equal-to 3) and decrease (to 1 X 10(-11) mol/m2/sec) as acidity decreases toward neutral pH. The polyfunctional acids, oxalate, citrate, succinate, pyruvate, and 2-ketoglutarate, are the most effective at promoting dissolution. Acetate and propionate are not as effective as the other organic acids but are nonetheless more effective than solutions containing only inorganic acids. The degree of ligand-promoted enhancement of dissolution rate (rate in organic-containing solution/rate in inorganic solution at the same pH) decreases as acidity increases, indicating that the ligand-promoted dissolution mechanism becomes relatively more important as the rate of proton-promoted dissolution decreases. The stoichiometry of release to solution indicates that dissolution is selective even after the rates of dissolution become constant. As in previously published studies, Na and Ca are rapidly released from the plagioclase feldspar, leaving a surface enriched in Si and/or Al. The ratio of Al/Si released to solution indicates that the stoichiometry of the residual plagioclase surface is a function of pH and the nature of the organic ligand. The ligands which remove Al in preference to Si from the dissolving mineral surface are also those which enhance overall plagioclase dissolution rates.