FELDSPAR DISSOLUTION AT 25-DEGREES-C AND PH 3 - REACTION STOICHIOMETRY AND THE EFFECT OF CATIONS

Feldspar powders, An(0)-An(76), were dissolved in flow-through reactor s at 25 degrees C, pH 3, to investigate the effect of feldspar composi tion, electrolyte concentration, and cation identity upon dissolution rates. BET surface area increased 1.5-7 times over the approximately 2 000 hour reaction times; we, therefore, calculated dissolution rates w ith the final, rather than the initial surface area. This correction r esulted in calculated rates which were, correspondingly, 1.5-7 times l ower than several previously published rate estimates. Dissolution rat es increase linearly with increasing anorthite content over the compos ition range studied. Rates decreased with increasing NaCl, and to a le sser extent, increasing (CH3)(4)NCl concentrations. We interpret our r ate data with a surface-controlled rate model: rate = k . =SOHex(n), where =SOHex is the concentration of H+ which reacts with the felds par surface through proton-cation exchange reactions. Previous workers have used =SOH to represent protons adsorbed to surface hydroxyl si tes. We express =SOHex with a Langmuir competitive adsorption isothe rm, and fit our rates to the model: GRAPHICS where k = the rate cons tant, N-s = the surface site density, K-H = the H+ constant for adsorp tion at the exchange site, K-Na = the Na+ constant for adsorption at t he exchange site, and {i} denotes the activity of species i. Aluminum and the network-modifiers, Na, K, and Ca, were preferentially released compared to Si during the initial phase of dissolution. After 500-100 0 hours in H2O-HCl, dissolution became stoichiometric for the microcli ne, albite, and bytownite compositions. Oligoclase and labradorite con tinued to exhibit preferential Ca and Al release even after 3000 hours of dissolution. Exsolution texture, observed in labradorite, may prov ide a structural control for preferential Ca and Al release. Apparent nonstoichiometric dissolution in oligoclase is due to the presence of Ca- and Al-rich accessory phases, present in the original feldspar sam ples. This work suggests that in the absence of accessory phases and m ineral defects, steady-state feldspar dissolution is stoichiometric fo r all compositions.