American Mineralogist, Volume 84, pages 884–894, 1999 0003-004X/99/0506–0884$05.00 884 Aspects of goethite surface microtopography, structure, chemistry, and reactivity JOHN RAKOVAN,* UDO BECKER,† AND MICHAEL F. HOCHELLA JR. Mineral Surface Sciences Laboratory, Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, U.S.A. ABSTRACT Goethite (010), (110), and (111) growth faces and (010) cleavage surfaces of large, natural single crystals, as well as a high surface area synthetic sample were characterized using various surface sensi- tive microscopies and spectroscopies. Differential interference contrast and atomic force microscopy characterization of the natural single crystal faces showed microtopography indicative of growth, disso- lution, and cleavage. Low energy electron diffraction patterns of the goethite (010) surface exhibit sharp, intense diffraction spots, indicating long-range order on this important surface. These patterns have two-dimensional point group symmetry 2mm, consistent with an undistorted surface structure and unit- cell parameters a = 4.62 ± 0.14 Å and c = 2.99 ± 0.08 Å. These parameters equal the equivalent bulk cell dimensions given the uncertainties. Ultra-high vacuum scanning tunneling microscopy was performed on (010) cleavage faces, although the tunneling properties of the surface were very heterogeneous. Atomic resolution was not obtained; however, microtopographic images are identical to those collected with AFM. XPS spectra from the (010) faces of two natural samples as well as the synthetic powder all have peak maxima for Fe (2p 3/2 ) at 711.5 ± 0.1 eV. The O(1s) line originating from the goethite can be fit with two peaks with a chemical shift of 1.3 eV. The peak at higher binding energy (531.3 eV ± 0.1 eV) represents the protonated oxygen in the structure, and the peak at lower binding energy (530.0 eV ± 0.1 eV) represents the proton-free oxygen in the structure. Ab initio and semi-empirical models of the (010) surface suggest that cleavage occurs through the hydroxide plane at 1/4 b in the structure. This is con- trary to cleavage through the oxide plane at 1/2 b, which has been assumed in several previous studies. INTRODUCTION The surface structure and composition of a mineral play a pivotal role in the chemical interactions that take place between that mineral and its environment. Sorption and desorption be- havior, catalytic reactivity, and growth and dissolution mecha- nisms are all determined by surface structure and composition. Unfortunately, the definitions of these surface characteristics are not as straight forward as one might initially assume. The term “surface structure” has been used to denote everything from surface topography, described in scales from centimeters down to nanometers, to the local atomic structure around one particular site on a crystal surface. The thickness of the upper- most layer defined as the “surface” varies and is often deter- mined by the resolution of the particular analytical technique used. For example, scanning probe microscopes (SPM) gener- ally probe the top-most layer of atoms only, whereas X-ray photoelectron spectroscopy (XPS) typically has an analytical depth of a few tens of angstroms. Yet, all of the structural and compositional attributes of a mineral’s surface are probably important in understanding its functionality in nature, from macroscopic to microscopic scales. A large literature dealing with goethite surface chemistry already exists (see Cornell and Schwertmann 1996 and refer- ences therein), yet intriguing questions remain, especially in- volving surface structure and reaction heterogeneity. These aspects of the surface chemistry of goethite are particularly important to soil science because goethite is the most abun- dant iron oxide. Although its absolute abundance in a soil may be only between 1 and 5%, goethite surfaces may account for 50 to 70% of the total surface area of the soil as a whole (Schwertmann and Taylor 1990). This is due both to the small size of soil goethite crystals [commonly acicular crystals 10 and 100 nm in length with surface areas of 60–200 m 2 /g (Schwertmann and Taylor 1990)], and to goethite forming coat- ings on larger soil grains. Furthermore, because goethite has a high affinity for sorption of many cations and anions (Cornell and Schwertmann 1996), it is thought to be a major player in the cycling and retention of geochemically and environmen- tally important elements such as heavy metals. In this paper, various aspects of both the structural and compositional na- ture of growth and cleavage surfaces of goethite are described, with observations encompassing a wide range of scales from optical resolution down to the atomic level. A wide variety of analytical techniques are combined to characterize surface microtopography, composition, and atomic structure for goethite. X-ray photoelectron spectroscopy (XPS) was used for surface compositional analysis. Low energy elec- tron diffraction (LEED) provides information on the long-range atomic order of a surface (e.g., Hochella 1990), and quantum mechanical modeling (Crystal95 and a procrystal model) pro- vide short-range information on local atomic surface configu- rations (e.g., Becker and Hochella 1996; Becker et al. 1996). *Present address: Department of Geology, Miami University, Ox- ford, OH 45056, U.S.A. E-mail: Rakovajf@muohio.edu †Present address: Institut für Mineralogie, Universität Münster, Corrensstrasse 24, D-48149, Münster, Germany.