Resolving orthoclase dissolution processes with atomic force microscopy and X-ray reflectivity

Direct measurements of orthoclase (001) were pet-formed using in situ atomi c force microscopy (AFM) and synchrotron X-ray reflectivity to reveal the A ngstrom -scale dissolution process as a function of pH and temperature. Dis tinct processes were observed, involving mainly terrace roughening at pH = 1.1 and step motion at pH = 12.9. A gel-like surface coating was observed t o form at acidic pH under slow fluid flow-rate conditions. No coating was o bserved either at alkaline pH or at acidic pH under high fluid flow-rate co nditions. The corresponding dissolution rates were measured directly at pH = 1.1 and 12.9 at similar to 50 degreesC using real-time X-ray reflectivity measurements, and reacted interface structures were derived from crystal t runcation rod measurements after reaction at both acidic and alkaline pH. O ur observations reveal, under these experimental conditions, that 1) orthoc lase dissolution is controlled by at least two separate surface reactions h aving distinct reactive sites; 2) dissolution is stoichiometric at alkaline pH and only minimally nonstoichiometric (limited to one unit-cell depth) a t acidic pH: previously identified nonstoichiometric layer thicknesses deri ved from macroscopic measurements are associated with the formation of the gel-like coatings; 3) dissolution rates measured at freshly cleaved (001) s urfaces are comparable to those derived from steady-state powder dissolutio n rates for both alkaline and acidic pH; and 4) elevated transient dissolut ion rates are not observed for freshly cleaved surfaces but are obtained un der alkaline conditions after reacting the orthoclase (001) surface at acid ic pH, These observations clarify differences in orthoclase dissolution mec hanisms as a function of pH, demonstrate the utility of AFM and X-ray scatt ering methods for measuring Angstrom -scale structures and face-specific di ssolution rates on single crystals and place new constraints on the underst anding of alkali feldspar weathering processes. Copyright (C) 2001 Elsevier Science Ltd.