Experimental determination of the dissolution mechanism of calcium and magnesium pyroxenes/pyroxenoids in alkaline solutions

Far from equilibrium dissolution rates of diopside, enstatite and wollastonite have been studied in alkaline solutions (from nominal pH 8 to 13), concentrating on the effect of dissolved carbon dioxide in the experimental system. As argued by Brady and Walther (1989) dissolution rates of inosilicates should increase above neutral pH with increasing pH. But the critical summary of literature data on inosilicate dissolution by Brantley and Chen (1995) does not show the alkaline enhancement as observed in aluminosillicate dissolution (Brady and Walther, 1989). The objective of the present study is to determine the effect of dissolved carbon dioxide on three major Ca-, Mg-bearing pyroxenes and pyroxenoids over a certain range of precisely defined alkaline pH buffer solutions and their comparison with literature data on major Ca-, Mg-silicates to improve understanding of the dissolution rates and mechanism of weathering of these silicates.;All the dissolution experiments were carried out in batch reactors set up by pouring about one gram of finely ground mineral samples into a 500 ml. precisely defined buffer solutions. Experiments in a N2 atmosphere were undertaken inside a CO2 purged glove box. Buffer solutions were purged of CO2 by boiling them vigorously for 12-15 minutes in plastic polyethylene beakers. Concentrations of Ca, Mg and Si in samples extracted as a function of time with a glass syringe were measured with a Beckman direct current plasma emission spectrometer (DCP).;Early dissolution of these silicates was nonstoichiometric with preferential Ca and to a lesser extent Mg release from diopside and preferential Ca release from wollastonite with respect to Si. Enstatite dissolution shows a higher rate of release of Si with respect to M1 and M2 cations during the early incongruent period of dissolution. Dissolution of these silicates was stoichiometric eventually although high pH experiments (nominal pH 12 and 13) on diopside and enstatite do not show stoichiometric dissolution because of likely precipitation of Mg-hydroxide in those experiments. Stoichiometric dissolution has been observed at all pHs for wollastonite by the end of the experiments.;The diopside dissolution in alkaline solution was significantly affected by the presence of CO2, as evidenced by the increase in dissolution rate with increase in solution pH from nominal pH 8 to 13 in CO2 purged dissolution experiment, in comparison to nearly pH independent dissolution in the open to atmospheric CO2 experiment. It is proposed that formation of a stable uncharged surface carbonate complex bridging between M1 and M2 surface sites on diopside shields the bonded surface Si between them. Similar inhibition of dissolution has also been observed in the open to air enstatite dissolution experiment. Apparently, adsorption of solution carbonate to the positively charged Mg surface sites on enstatite forms a >Mg2-CO3 surface complex that stabilizes the mineral surface as well as the surface Si-O bonds from further solution attack. Because of the formation of this uncharged surface complex, Si-O bond polarization at the mineral surface by negatively charged Si surface sites is significantly inhibited causing pH independent dissolution behavior in open to atmospheric condition. Therefore, increase in dissolution rates of enstatite and diopside as a function of pH in CO2 purged condition suggests that the reactions at the silica surface sites control the pH dependency of dissolution of these silicates in alkaline solutions.;The wollastonite dissolution experiment, performed under the same experimental conditions of open to atmospheric CO2 and CO2 purged conditions as our diopside and enstatite dissolution experiments, does not show inhibition of dissolution of wollastonite, in equilibrium with atmospheric CO2. Steady-state alkaline dissolution rates of wollastonite do not show a rate minimal at or close to the pHZPC of wollastonite. I believe that a different mechanism controls the dissolution of this Ca-pyroxenoid. Exchange of solution H+ with Ca2+ ions at the surface exposes surface silica tetrahedral chains to solution. Wollastonite dissolution is completed through breaking of surface silica tetrahedral chains by further solution attack. Rapid Ca-proton exchange at the wollastonite surface during alkaline dissolution of wollastonite precludes adsorption of solution carbonate below the pHZPC of CaO.