MECHANISM OF PLAGIOCLASE DISSOLUTION IN ACID-SOLUTION AT 25-DEGREES-C

The dissolution kinetics of three plagioclase feldspars (An(13), An(46 ), and An(76)) were studied in flow-through reactors over the pH range 3-7. In accordance with the surface complexation model, dissolution r ate was described by the equation Rate = x(a)A exp (E(a)/kT)(C-H(s))(n ), where (C-H(s)) is the concentration of protonated surface sites, x( 2) is the mole fraction of these that are activated, A is the Arrheniu s pre-exponential factor, and E(a) is the activation energy for the re action. The reaction order with respect to surface charge (n) at pH < 5 is a function of mineral composition, and increases with increasing anorthite content. Orders of 0.46, 1.2, and 2.0 are obtained for oligo clase, andesine, and bytownite, respectively. This indicates that the reaction intermediate is more highly protonated in feldspars of higher aluminum content. The nonintegral reaction orders obtained demonstrat e the importance of several surface species in the dissolution reactio n. Reaction rates increase with increasing anorthite content. In acid solution this reflects the linear dependence of E(a) on the number of Al-O-Si bonds involved in the rate-determining step. Under conditions of low surface protonation (in near-neutral solutions), dissolution is independent of surface charge. As reaction occurs at neutral alumina sites, the rate is approximately proportional to the anorthite (Al) co ntent of the feldspar.