Termination and hydration of forsteritic olivine (010) surface

Hongping Yan*, Changyong Park, Gun Ahn, Seungbum Hong, Denis T. Keane, Curtis Kenney-Benson, Paul Chow, Yuming Xiao, Guoyin Shen

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

14 Scopus citations

Abstract

Termination and hydration of the forsteritic (Fo90Fa10) olivine (0. 1. 0) surface have been investigated with high-resolution specular X-ray reflectivity and Atomic Force Microscopy. The surface was prepared by polishing a naturally grown {0. 1. 0} face, from which we found the polished surface in acidic (pH 3.5) alumina suspension exhibits regular steps while the basic (pH 9.5) silica polished surface is irregularly roughened, indicating there are two distinguishable mechanochemical processes for the surface dissolution. The quantitative interpretation of the regular steps from the alumina-polished surface suggests that the observed step heights correspond to multiples of crystallographic unit cell. Only this atomically terraced surface is investigated with the high-resolution X-ray reflectivity (HRXR) to determine the surface termination and hydration. The basic silica paste polished surface turned out too rough to measure with X-ray reflectivity. HRXR reveals that the alumina polished olivine (0. 1. 0) surface in pure water is terminated at a plane including half-occupied metal ion sites (M1), an oxygen vacancy site, and a silicate tetrahedral unit with one of its apices pointing outward with respect to the surface. An ideal termination with the oxygen vacancy would fulfill the stoichiometry of the formula unit; however, in the observation, the vacancy site is filled by an adsorbed water species and about a quarter of the remaining metal ions are further depleted. The terminating plane generates two distinct atomic layers in the laterally averaged electron density profile, on which two highly ordered adsorbed water layers are formed. The first layer formation is likely through the direct interaction with the M1 plane and the second layer is likely through the hydrogen bonding interaction with the first water layer. With this multilayered adsorbed water structure, the surface metal ion is partially hydrated by the vacancy-filling water species and adsorbed water molecules. The bulk water links to these distinct adsorbed water layers, with weak density oscillations that almost completely damp out after the first bulk water layer. The total thickness of the layered water structure including the two distinct adsorbed layers and the first layer of bulk water is slightly less than 1. nm, which corresponds to roughly three molecular layers of water. These results describe the steric constraints of the surface metal ion hydration and the iron redox environment during water-olivine interactions in this particular crystallographic orientation.

Original languageEnglish (US)
Pages (from-to)268-280
Number of pages13
JournalGeochimica et Cosmochimica Acta
Volume145
DOIs
StatePublished - Nov 5 2014

Funding

This work is a part of the Deep Carbon Observatory-Deep Energy project, supported by the Alfred P. Sloan Foundation in the United States. H. Yan is partially supported by the High-Pressure Collaborative Access Team (HPCAT) . HPCAT is supported by DOE-NNSA under Award No. DE-NA0001974 and DOE-BES under Award no. DE-FG02-99ER45775 , with partial instrumental funding by NSF . Portions of this work were performed at the DuPont-Northwestern-Dow Collaborative Access Team (DND-CAT) located at Sector 5 of the Advanced Photon Source (APS). DND-CAT is supported by E.I. DuPont de Nemours & Co. , The Dow Chemical Company and Northwestern University . Some preliminary tests were performed at the HPCAT 16ID-D beamline through the auspices of Carnegie / DOE Alliance Center (CDAC) for the beamtime. APS is supported by DOE-BES , under Contract No. DE-AC02-06CH11357 . AFM experiments conducted at Materials Science Division, Argonne National Laboratory by G. Ahn and S. Hong were supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division under Contract No. DE-AC02-06CH11357 .

ASJC Scopus subject areas

  • Geochemistry and Petrology

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