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 to determine the structure of the orthoclase (001) cleavage surface in contact with deionized water at 25 degreesC. These are the first in situ measurements of the orthoclase-water interface structure performed to Angstrom-scale resolution. The orthoclase (001) cleavage surface has minimal roughness, and only one of two possible surface terminations is exposed. The X-ray data 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 Angstrom (2)), (2) the outermost layer of K+ ions have been removed with a derived 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 relaxation of similar to0.15 Angstrom near the surface. These data are inconsistent with K+ ion depletion below the topmost K+ layer. These results provide a new baseline for understanding the initial steps of the feldspar dissolution process, demonstrate the power of combining X-ray scattering techniques with scanning probe microscopies for understanding the intrinsic characteristics of complex mineral-water interface systems, and suggest a new approach for understanding feldspar dissolution mechanisms. Copyright (C) 2000 Elsevier Science Ltd.