Atomic-scale structure of the orthoclase (001)-water interface measured with high-resolution X-ray reflectivity

In situ X-ray specular reflectivity and atomic force microscopy were used t o determine the structure of the orthoclase (001) cleavage surface in conta ct with deionized water at 25 degreesC. These are the first in situ measure ments of the orthoclase-water interface structure performed to Angstrom-sca le resolution. The orthoclase (001) cleavage surface has minimal roughness, and only one of two possible surface terminations is exposed. The X-ray da ta show that (1) the silica network at the orthoclase surface is terminated by an oxygen-containing species (e.g., O or OH) having a coverage of 1.9 /- 0.25 ML (the expected coverage is 2.0 ML, where 1 ML = 1 atom/55.76 Angs trom (2)), (2) the outermost layer of K+ ions have been removed with a deri ved coverage of 0.0 +/- 0.08 ML (the bulk truncated K+ coverage is 1.0 ML), and (3) a complex relaxation profile affecting the near-surface structure propagates similar to 26 Angstrom into the orthoclase with a maximum relaxa tion of similar to0.15 Angstrom near the surface. These data are inconsiste nt with K+ ion depletion below the topmost K+ layer. These results provide a new baseline for understanding the initial steps of the feldspar dissolut ion process, demonstrate the power of combining X-ray scattering techniques with scanning probe microscopies for understanding the intrinsic character istics of complex mineral-water interface systems, and suggest a new approa ch for understanding feldspar dissolution mechanisms. Copyright (C) 2000 El sevier Science Ltd.