ATOMIC-SCALE STRUCTURE OF THE ORTHOCLASE (001)-WATER INTERFACE MEASURED WITH HIGH-RESOLUTION X-RAY REFLECTIVITY

Abstract

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°C. These are the first in situ measurements of the orthoclase-water interface structure performed to #9ngstrom-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 #92), (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 ~26 #9 into the orthoclase with a maximum relaxation of ~0.15 #9 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.