Calcite surface structure observed at microtopographic and molecular scales with atomic force microscopy (AFM)

Geochimica

et Cosmochimica

Acta, Vol.

58. No.

14, pp. 3023-3033,

1994

ElsevierScienceLtd Printedin the USA. All rightsreserved

Copyright

Pergamon

0

I994

0016-7037/94 $6.00+ .OO 0016-7037(94)00099-9

Calcite surface structure observed at microtopographic and molecular scales with atomic force microscopy (AFM)* S. L. S. STIPP,’ C. M. EGGLESTON,~and B. S. NIELSEN~ ’ DCpartement de Chimie Mintrale, Analytique et Appliquke, Universitk de Gentve, 30 Quai E.-Ansermet, CH- 121I Genkve 4, Switzerland 2 Institute for Environmental Science and Technology (EAWAG), Swiss Federal Institute of Technology (ETH), 3 Niels Bohr Institute,

CH-8600 Diibendorf, Switzerland Blegdamsvej 17, DK-2 100 Copenhagen M, Denmark

(Received August 4, 1993; accepted in revisedjwm February 8, 1994)

Abstract-Atomic force microscopy (AFM) was used to study calcite cleavage surfaces in air and under aqueous solution at the microtopographic and molecular scales. Surfaces freshly fractured and imaged in air showed wide, flat terraces with steps only a few monolayers high. Exposure to distilled water promoted development of etch pits with differential dissolution rates on inequivalent sides. The pits became broad and shallow with apparent approach to equilibrium after about fifteen minutes. Exposure of the freshly fractured surfaces to air for more than a couple of hours resulted in the appearance of narrow and deepening, irregular-sided pits. These observations indicate a highly dynamic surface capable of incorporating adsorbed material into the near-surface bulk. Molecular resolution images revealed a regular array of corrugations that were interpreted to represent the covalent carbonate groups. The unit cell dimensions of the lattice were the same as those expected for the cleavage plane of bulk calcite but the two-dimensional surface symmetry was not conserved. Apparent displacement of every second carbonate row probably results from frictional forces as the tip scans over rows of alternating orientation; likewise, it may be an expression of surface hydration species adsorbed to these alternate rows and/or the adsorbed species may enhance the effect of the frictional forces. A 2 X 1 structure is observed on surfaces of pure calcite imaged by AFM in air and under water and supports observations made previously using low energy electron diffraction (LEED) in vacuum. The 2 X 1 pattern may be caused by slight twisting of some surface carbonate groups in order to stabilize “dangling bonds” and surface charge after cleavage and surface hydration. However, exact interpretation of row-pairing and the 2 X 1 structure is difficult because of the convolution of information in the AFM images resulting from topography, elastic surface deformation, and electrostatic forces. INTRODUCTION

cations into bulk, calcite-like sites has been confirmed in recent XAS studies by PINGITOREet al. (1992) and Xu (1993). The exact role that surface structure plays in each of these studies remains unclear. Several investigations have examined the surface chemistry and structure of calcite and other carbonate minerals using wet-chemical and/or spectroscopic methods. DAVIS et al. ( 1987) observed adsorption of aqueous Cd’+ by calcite and explained the diffusion-controlled rate of uptake by postulating an amorphous hydrated surface layer. Recently, CICERONE et al. (1992) stated that “the present status of knowledge favors the usage of models which propose that a strongly hydrated and disordered calcium carbonate layer overlies the calcite crystals”. However, low-energy electron diffraction (LEED) results of STIPP and HOCHELLA (199 1) and STIPP et al. (1992) showed that even freshly precipitated calcite and otavite (CdC03) have ordered lattice structure in the top lOA, but it could be that the ultra-high vacuum (UHV) conditions required for LEED analysis may have altered the surface from that present in an aqueous environment. The surface chemistry of calcite was studied by STIPP and HOCHELLA( 199 I), who used X-ray photoelectron spectroscopy (XPS) to study calcite after exposure to air, water, and carbonate-containing solutions. They found evidence for 8 - C03H and 8 * CaOH surface species (where 8 * represents a calcite surface site); the latter species predominated on dried

SEVERALRECENTSTUDIEShave raised questions concerning the relationship between calcite surface atomic structure and chemical reactivity. Surface chemistry and structure have direct influence on the kinetic behaviour of calcite surfaces in dissolution and growth reactions, including mechanisms by which calcite crystals incorporate and sequester trace metals. PAQUETTE and REEDER (1990) and STAUD~ et al. (1994) observed sector- and intrasector-zoning of calcite precipitated from solutions containing trace amounts of divalent metals and ligands; this implies surface structural control on the incorporation mechanism. DICKSON (199 1) found a weak influence of surface structure on isotope (13C, 180) fractionation during calcite growth. STIPP et al. (1992) showed that adsorbed Cd’+ was incorporated from the surface into the near-surface bulk structure to form a solid solution even in ultra-high vacuum where precipitation was impossible. PLUMMERet al. (1992) observed a period of fast initial uptake of S?’ from solution by aragonite, followed by desorption and a period of slower uptake. Incorporation of adsorbed

* This work was originally presented as an oral contribution to the American Chemical Society Meeting, Symposium for Structure, Bonding, and Kinetics at Mineral Surfaces, San Francisco, USA, April 5-10, 1992. 3023

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S. L. S. Stipp, C. M. Eggleston, and B. S. Nielsen

samples, no matter what the pH of the contacting solution had been. These hydration species were still observed even after dried samples had been exposed to ultra-high vacuum for many days. CHARLET et al. (1990) examined surfaces of rhodochrosite (MnC03) and siderite (FeC03) in a flowthrough apparatus at pH ranging from 4 to 8, and demonstrated that the surfaces were dominated by bicarbonate species in solutions where pH exceeded 5 or 6. In eiectrokinetic studies, CICERONE et al. ( 1992) observed that the zeta potential of calcite was not dependent on pH at constant Ca2+ concentration, and that Ca2+ and CO:- were the only potential determining ions. They found their results to be compatible with the presence of the hydration species, CaOH and C03H. A computer simulation of surface complexation for carbonate minerals in aqueous solutions by VAN CAPPELLEN et al. ( 1993) also supported the presence of these species. The surface atomic structure of calcite was examined by STIPP and HOCHELLA ( 199 1) using LEED on surfaces freshly prepared by cleavage in air; the patterns showed faint extra reflections that corresponded to a 2 X I structure (a doubling of the unit cell in the a dimension) relative to a simple termination of the bulk lattice and suggested it might be caused by slight twisting of surface CO3 groups. WENK et al. (1983) and REEDER (1992) observed microstructures in carbonates with transmission electron microscopy (TEM). Some oftheir samples showed extra reflections in electron diffraction patterns. Although not well understood, some of these features seemed to be connected with exsolution lamellae in Ca dolomites and Mg- and other divalent-ion calcites containing significant (>5%) quantities of trace metals. WENK et al. ( 1983) proposed that they resulted from periodic planar defects and were sometimes due to twisting of carbonate groups to accomodate size discrepancies of the substituting cations. G~JNDERSON and WENK (198 I ) found modulated structures in the bulk of stoichiometric calcite from oolitic (biogenic) carbonates and proposed it to be the result of periodic disorder of CO3 groups. WENK et al. (I 983) proposed that internal structural defects might induce twisting of alternate carbonate groups in opposite directions, thus doubling the unit cell over the space of several unit cells in three dimensions. Atomic force microscopy (AFM) has been used to study calcite by several groups. HILLNER et al. (1992a.b) observed the dissolution and growth of calcite in situ, and directly confirmed different growth and dissolution kinetics on structurally inequivalent steps at the microtopographic scale. GRATZ et al. ( 1993) suggested that calcite crystal growth is not limited by surface diffusion, and that steps advance by direct precipitation of Ca’+ and CO:- ions adsorbed within five lattice spacings of active step edges. DOVE and HOCZIELLA (I 993) used AFM to observe precipi~tion on calcite surfaces exposed to saturated and supersaturated solutions with and without PO:- and found that the mechanism of crystal growth changes with time, solution conditions, and history of surface exposure to air. RACHLIN et al. (1992) studied cleavage surfaces exposed to water for several hours with AFM. They interpreted the topographically higher spots on their high resolution images to represent Ca atoms and using Fourier Transform patterns of~~~icity, they dete~ined the surface unit cell parameters to be the same as those for bulk calcite. Although AFM can be sensitive to surface restructuring, they

did not observe a doubling of the a (4.99A) lattice dimension but stated that the 2 X 1 surface structure previously observed in LEED patterns was “probably a result of cationic substitution”; this seems unlikely for the chemically pure Iceland spar used in that (and this) study. OHNES~RGE and BINNIC (1993) recently observed the calcite surface at high resolution with a varying tip force. At very low forces. they interpreted negative contrast spikes to indicate Van der Waals attraction between the tip and the uppermost oxygen atoms above the (IOi4J plane of the bulk structure. With increasing force, they were able to observe repulsive interaction and the effects of deformation. Their work proved AFM to be capable of at least molecular scale imaging, and that the corrugation observed in force images is not restricted to Moir6 patterns resulting from interfering sets of osciilations. In this paper, we use in situ AFM to observe microtopography and to study the molecular-scale structure of calcite { lOi cleavage surfaces in air and under solution. Our goal is to answer the following questions: (I) Can we unde~tand the processes occurring at the surface of calcite in air and in water that could explain the uptake and incorporation of divalent metals that suggest solid-state diffusion? (2) Are calcite surfaces always ordered (c~stalline) in air and in water, as they are in ultra-high vacuum (UHV)? (3) Does a 2 X 1 surface structure exist in air and in water, as in UHV? If so, what might cause it? (4) What can we learn about chemical reactivity of the calcite surface? EXPERIMENTAL

DETAILS

Calcite samples (~10 mm on each side and 2 mm thick) were prepared from precleaned, optical-quality Iceland spar (Chihuahua, Mexico; Ward’s Scientific), Precleavage cleaning and methods for minimizing contamination are described in STIPP and HOCHELLA (1992).Cleavage was induced by repeated scoring-with gentle pressure using a fine scalpel along a line parallel to the ( 1014) plane. Chemical analyses by inductively coupled plasma (ICP) spectroscopy for a piece of the material used for this study showed the material to be chemically pure CaCOJ with less than I% divalent metal substitution for Ca. Samples were imaged with a Digital Instruments Nanoscope III AFM, equipped with a fluid cell (volume z 0.1 ml), and using 200 pm long, wedge-shaped S&N4cantilevers with a force constant of about 0.6 Newton/m. Forces between tip and sample were controlled at IO-90 nN for most scans. We used a I2 Gumpiezoelectric scanner for all images. Drift (see EGGLESTONand H~CHELLA,1992) and pressure on the scanner during use of the fluid cell caused distortion of the images, resulting in a lo-30% error in measured horizontal distances. Height of steps (within calibration error) was of integral multiples of 3,& the thickness of a calcite monolayer normal to the cleavage surface. Images were collected with a line-scan frequency of 4-8 Hz for Frn-scale images and 30-60 Hz for high resolution images. Scanning direction was from left to right, unless otherwise stated in the figure caption. The experiments were repeated using a Park Scientific Instruments (PSI) Universal Scanning Probe Microscope (SPM) with the same tip and instrument conditions. We first imaged fresh cleavage surfaces in air, at micro-, and then nanometer-scales. Next. we injected distilled, deionized water that had been exposed to the atmosphere for several hours (pH z 5.6) into the fluid cell over the sample. We collected a series of topographic scale images during a sequence in which water injection was alternated with time for an approach to equilibrium (about 10 min for each cycle). After the final injection, we waited until there was little change in surface morphology on a 5-min timescale before imaging at molecular resolution on wide, flat terraces. Following these experiments, the sample was left exposed to laboratory air and imaged again after six days. The experiments in air and in water. and at micro- and nanometer-scales were repeated to confirm and to reproduce the re-

Calcite surface structure

observed

with AFM

30’5

sults. Imaging parameters such as rotation, scale, and scan-rate were varied in order to test for artifacts. The images presented here have been low-pass filtered using the Digital software 3-point smoothing routine to remove high-frequency noise. In order to avoid possible cutoff frequency back-transformation artifacts, we used no Fourier filtering on any of the images.

RESULTS

Microtopographic

AND DISCUSSION

Images

Figure 1 is a low-resolution image of a fresh calcite cleavage surface taken in air within a few minutes of fracture. All fresh surfaces were remarkably flat, with a few long, straight cleavage steps. Steps between terraces were, typically, 1-3 monolayers high. We saw no evidence of microfractures. The “lightening-shape” of the cleavage steps is similar to morphology observed by BETHGE ( 1990) with scanning electron microscopy (SEM) and the gold-dot decoration technique on NaCl crystals. His work pointed out the relationship of this shape to crack velocity and dislocation mobility during cleavage. Figure 2, taken under solution, shows consecutive images from a sequence of water-injection cycles (see Experimental Details). Because the fluid cell is essentially closed to the atmosphere, solid/solution equilibration causes the pH to approach 9.9 (GARRI:LS and CHRIST, 1965). Upon initial injection of water, small, straight-sided etch pits formed within seconds; as the system approached equilibrium, these evolved into broad, flat-bottomed pits with crystallographically controlled curved and straight sides (Fig. 2a). Further injection cycles repeated the process (Fig. 2b). A morphological inequivalence of the acute and obtuse sides developed (for definition, see Fig. 3a, left), where the acute edges were straight and met at well-defined corners and the obtuse edges were curved. suggesting higher kink density. The inequivalence of reactivity on acute and obtuse sides in these images confirms

-800

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2do

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6dO

860

nm

FIG. 1. AFM image of Sample 1, a { 1074) cleavage face of calcite, taken in air within 15 min of cleavage. Step height in the middle of the image is I monolayer (3A). The lines drawn to the right of the image show the orientation of the other two cleavage planes that intersect the observed surface. Image taken in constant force mode, net force = 90 -C 15 nN, line scan rate of 4.4 Hz.

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FIG. 2.(a) Image of Sample I, taken in solution about 15min after the first injection of distilled water. when the solution had neared equilibrium with the surface (little change ofsurface topography with time). Some areas show wide. atomically flat terraces with single monolayer or bunched steps. Other areas show wide “pits” with depth of about 10 A. The angles of the pit corners were not the ideal cleavage angles expected for calcite (Fig. 3; 78.4” and 101.6”: REEDER, 1983) because of imaging artifacts; the apparent stretching is a result of thermal drift and piezo nonlinearity. The lines drawn at right show the ideal intersection of cleavage planes. and the orientation of the carbonate and Ca rows (see Fig. 3). The arrows mark spots that remain fixed during removal of surrounding material. (b) The same area of Sample I (with slight offset due to drift), imaged 4 min later, after the second injection of distilled water. The original wide, shallow etch pits have become less distinct and new. smaller etch pits have developed. Both images were taken with net force = 90 t IS nN; line scan rate of 4.7 Hz.

evidence for large differences in their kinetic behaviour during dissolution and growth (HILLNER et al.. 1992a,b). We can understand the inequivalence of sites by understanding how the termination of the rhombohedral structure reacts to the environment that is normal to its surface. On the two dimensional cleavage plane represented in Fig. 3a, there are alternating A- and B-type carbonate rows. Rotation of the plane of the paper by 180” about the diad axis, D, (centre of crystal face) shows that there is no rotational sym-

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S. L. S. Stipp, C. M. Eggleston,

-

0

underbonded

II’

empty oxygen site

and B. S. Nielsen

oxygen site coordinated

with 0

in plane of paper slightly above pIane

Calcite

(CaC03)

RZk

( 1014 } cleavage face FIG. 3. Drawing of the simple termination of the bulk atomic structure at the { tOi4) cleavage face for calcite. D represents a diad axis to the surface layer, about which there is no symmetry of rotation.(a) The outlined unit cell at left has been drawn for an ideal termination of the bulk calcite structure. Note that the centers of the carbonate groups on the A-type rows are exactly halfway between the centers of the carbonate groups in the B-type rows. The outlined unit cell at right is based on the ugtilted oxygen atoms from the COJ groups on the unrelaxed, unreacted termjnat~on ofthe butk structure. ‘F’he C-O bond distances have been exaggerated in the drawing for clarity. If we project the vector representing the uppermost 0 atom onto the plane, and assume a C-O bond length of about I .3 A, we can calculate the true maximum offset of alternating rows to be about 0.8 A. (b) A surfacecovered only with H20. The unit cell is outlined. In this case, the inequivalent row is again offset, but the location of the “center” spot with respect to the unit cell will depend on the exact location of the highest surface species (whether an 0 from CaOH or from adsorbed C07H) and the frictional components which depend on imaging force and relative scanning direction.

3027

Calcite surface structure observed with AFM metry; the orientation of Ca octahedra and the upward tilting 0 atoms of the CO3 groups changes orientation. If we could look through the plane of the paper, normal to the surface, we would see that the second (represented with smaller symbols in the diagram) and subsequent planes of CO3 groups and Ca octahedra are each offset with respect to the preceding one. Thus, cleavage results in planes that slope away from the observed surface: x-sides (Fig, 3a) tilt to produce obtuse edges; y-sides produce acute edges, so that when we rotate about D, the slope of the cleavage planes also changes. In a bulk calcite crystal where each atom is surrounded in its rhombohedral network, bonding energy is balanced. However. when part of the bulk is removed by cleavage, the new surface looses its rhombohed~l environment. Surface reactions and our AFM imaging take place on the surface, where what lies perpendicularly below has the greatest effect. The offset of second layer atomic positions means that the bonding environment on the x-side of any given CO:, row is different than the bonding environment on its y-side and the free energies are different. As dissolution proceeds (Fig. 2a), species are easily removed along the steps of the acute edges but along the obtuse sides, rounded edges result as kink sites develop. We often observed submicron “bumps” (Fig. 2 at arrows); they usually remained fixed during dissolution, even if significant material was removed from adjacent areas. They could represent small regions of lower solubility, possibly due to trace substitution at metal sites, but probably result from foreign particles attached to the surface. After the sample shown in Fig. 2 was allowed to dry and to age in air for six days, it was reimaged in air (Fig. 4). Faint traces of the wide, dissolution etch pits remain, but their edges are no longer sharp, and the surface layer now also contains irregular pits that vary from 85 to 115 A in depth. Often, the pit edges are oriented in the cleavage directions. We have observed this “surface aging” process in air on many calcite samples. Such a dynamic surface layer will play a role in the kinetics of other surface processes such as trace metal uptake and incorporation. Molecular Resolution Images

Figures Sa-d show AFM images taken in air of different regions on the calcite cleavage surface. Imaging conditions are given in the figure caption. Considering calibration and drift uncertainty, the measured unit cell dimensions (5.1 + 0.2 A by 7.8 _t 0.2 A) are in good agreement with the idea1 bulk calcite lattice constants (4.99 f 0.1 A by 8.10 k 0. l A; REEDER, 1983) and LEED results for calcite surfaces (STIPP and HOCHELLA,1991). Figures 6a-d are images obtained in situ under solution from terraces that were at or near equilibrium with water. They show regular, molecular-scale structure, similar to that observed in air. The unit cell dimensions (4.6-6.7 by 8,912.0 A) vary more on these images than on those taken in air, but are well within the range of error expected for the fluid cell setup. Distortion results from instrument drift and pressure on the piezoelectric scanner by the fluid cell (details in the experimental section). Immediately apparent from Figs. 5 and 6 is that the surface is ordered both in air and in water. Moreover, the images

-7.5

(

-5.0

-2.5

2;5

5.0

7.5

-0

10.0

em

FIG. 4. A different site on Sample 1, imaged again in air, after 6 days exposure to air. Note m~ulation of the original topography and the development of a chaotic pattern of pits aligned along the cleavage directions. Vertical depth is on the order of 10 nm or more than 30 monolayers. Net force = 90 k 15 nN; line scan rate = 4.4 Hz: scan direction, bottom to top (rotated 90% for comparison with Fig. 2).

have a similar appearance under both conditions. We see no evidence for an amorphous calcite layer, but it is clear from samples aged in air that there is considerable mobility of species at the surface (Fig. 4). Rather than invoking a uniform amorphous layer to describe the diffusion-controlled kinetics of trace metal uptake and incorporation (reported by DAVIS et al., 1987), we suggest that a physically more realistic model for calcite would consider surface diffusion and mixing effects in a dynamic, atomically organized surface layer that is many unit cells thick. Closer examination of Figs. 5 and 6 reveals some differences between the observed patterns and those expected for an unrelaxed termination of the bulk calcite structure. In Fig. 3a, we see coplaner Ca and carbonate ions arrayed in evenly spaced rows, but all of the AFM images show fewer “bumps” than we would expect from a surface containing Ca and CO3 groups. Moreover, every second row of atoms appears to be offset from the position it is expected to occupy in a simple termination of the bulk lattice (the central spots in the unit cell are off-center). In many images, A- and B-type rows seem to alternate in apparent elevation (Figs. 5a-d, 6a and d) but occasionally we see a change from “paired rows” to single rows on the same scan (Fig. 7). Several images (Figs. 5a,d and 6a,d) show areas where bright and not-so-bright spots alternate along the same row in a pattern which corresponds to the 2 X 1 structure observed in the ultra-high vacuum LEED studies. The cross-section (Fig. 6e) and frequency plot clearly demonstrate the doubling of the unit cell in the a direction. Following we discuss these differences and possible explanations for them. Tip and Surface Structure: Image Interpretation

The inte~retation of molecular scale AFM images requires an understanding of how the tip and sample interact. Although details of these interactions are not completely un-

3028

S. L. S. Stipp, C. M. Eggleston, and B. S.

Nielsen 4.0

-4.0

cleavage traces

3.0 \J1 7.d

-3.0

5.d I

c 2.0

2-o

1.0

-1.0

t

d

l.bO

2.bo

3.bo

nm

/ i.

4.iIO

d

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2.00

3.00

nm

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cleavage

1

-6.

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--,0.6

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2.bo

4.bo

nm

6.k

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FIG. 5. High resolution images of the fresh calcite cleavage surface, all taken in air with scanning direction from left to right; scan rate is 20.3 Hz. except for(b) at 30.3 Hz. The orientation of the cleavage plane intersections for each surface. and the measured unit cell dimensions are shown to the right of each image. The black dots drawn on some of the images outline the unit cell positions as shown on Fig. 3. We have used low-pass filtering only; no Fourier filtering. (a) Rows alternate in height and there is some alternation of bright and not-so-bright spots along the rows. Net force = 80 + 12 nN. (b) Another example of inequivalent rows. Net force = 7 I It 11 nN. (c) Strong row-pairing. Net force = 67 + 10 nN. (d) The 2 X 1 structure and the magnitude of offset of A-type rows gives a herringbone appearance to this image. Net force = 75 + I I nN.

derstood, if we remain aware of the limitations of the technique and the po~ibility of artifacts, and we consider data gained from other techniques, we can make some interpretations that extend our understanding of surface chemistry and structure. In “contact” or repulsive mode AFM, the sample is lightly “pressed” against the tip until the electron clouds of the tip and sample begin to overlap and repulse each other. Any feature of the surface that modulates the interaction between tip and sample will have an effect on the AFM image: tip properties such as composition, size, and shape play a role. Thus, an AFM image is a combined record of many forces, some of which are: (1) repulsion of overlapping electron clouds, (2) electrostatic interaction, and (3) frictional forces. The character of both tip and surface can contribute to vertical or lateral deformation of the surface by the tip.

Tip efects The tips that we used, composed of Si3N0, are known to oxidize after exposure to air for a few days and to behave much like SiOz in aqueous solution (PAPARAZZO et al., 1992; BUTT, I99 1). At the high pH of solutions at or near equilibrium with calcite (closed system, pH > 9) the tip carries a negative surface charge (e.g., DUCKER et al., 1991, 1992). Thus, over an ionic surface such as calcite, the tip could contribute an electrostatic component to the total image corrugation. This will be discussed later. Clearly, the smaller and “sharper” a tip, the better will be the resolution of the image. Unfortunately, we can never be sure of the character of a tip because its mo~hology changes constantly, even during scans. It can break, wear down, or accumulate debris from the surface, resulting in a larger,

Calcite surface structure

observed

3029

with AFM

r6.0

-8.0 cleavage

-6.0 12.oA

d

2.bo

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nm

6.b

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nm

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FIG.6. High resolution images of the calcite cleavage surface after approach to equilibrium with distilled water, imaged under solution; the orientation of the cleavage intersections and measured unit cell dimensions are shown. The black dots represent unit cell positions as on Fig. 3. Scanning direction was from left to right with scan rate 30.5 Hz, except for (a) at 6 1.O Hz. (a) Row-pairing with alternating bright and not-so-bright spots corresponding to a 2 X 1 structure. Total force = 22 k 3 nN. (b) Slight row-pairing visible. Total force = 22 f 3 nN. (c) Alternating rows have clearly distinct width differences, probably an example of lateral deformation during scanning (see text); Deflection mode image, average force about 50 nN. (d) One set of carbonate rows are difficult to see in this image and the 2 X I structure dominates. Total force < 1 nN. (e) Expanded view of image (a), with cross-section along line marked by arrows. Alternation of height along row and Fourier Transform frequency pattern demonstrate 2 X I structure with unit cell doubling in the a direction.

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S. L. S. Stipp, C. M. Eggleston, and B. S. Nielsen

blunter probe that is less able to resolve details, or resulting in double or multiple points. This latter effect can sometimes be recognized by a doubling or multiplication of features on both the image and the Fourier transform. We have not included such an image here, but Fig. 7 shows an example of a change in the resolving ability of the tip during a scan. Although the lattice dimensions are unchanged in this image, details of molecular spacing are not apparent in the lower portion of the image. This figure demonstrates the need for caution in attributing image details only to the surface structural features. Swface

structure

The purely topographic component of the AFM image is the corrugation caused by the “shape” of the surface-most atoms, molecules, or adsorbed species; it results from the repulsive force between overlapping electron clouds of tip and sample. An ideal AFM image of an ideal surface composed of identical, uncharged, “hard-sphere”, coplanar atoms would show all atoms as “spots” of equal brightness and spacing. For an ionic solid such as calcite, one would expect the difference in size, charge, shape, and bonding character of the cationic and anionic components to result in an uneven, but regular pattern of bright spots. In calcite (Fig. 3), the covalent CO3 group has short C-O bond distances because of electron-sharing. We have assumed it to be about 1.3 A for subsequent calculations @TREKWEISER and HEATHCOCK, 1981). On the { 1074) plane of bulk calcite, coplanar Ca and C are separated by 3.2 A. On the images presented here, we cannot resolve each surface Ca, C, and 0 atom, probably because the AFM tip is simply not sharp enough or, as suggested by OHNESORGE and BINNIG (1993),the imaging forces are too high for true atomic resolution. Nevertheless, in inte~reting the images, it is reasonable to expect that high points correspond to the highest atoms in the lattice or more precisely, the highest and most electron-dense regions of the mineral surface. If we assume the maximum density is directly centered over the CO3 group, we would expect the pattern of bright spots with the lattice spacing as shown in Fig. 3a, left, where A-type carbonates are in line with those of B-type rows; this pattern does not match any of the images. In a simple termination of the bulk structure on the ( lOi4) plane, the trigonal planar CO3 groups are tipped at about 44” such that an underbonded 0 atom from each group protrudes about 0.8 .& normal to the plane. If we assume that the maximum electron density sensed by the tip is over the underbonded, protruding 0 atoms, then we would expect a lattice spacing as shown in Fig. 3a, right. This pattern matches the images a little better. However, the maximum offset (calculated geometrically) of each of the A-type and B-type uptipped 0 atoms is only about 0.4 A for each row, or about 0.8 A in total, which is less than half of the offset we see in some of the images. It is doubtful if friction, even at the highest forces that we used, could be responsible alone for this large shift. A more serious critisism of this interpretation concerns the validity of assuming that the calcite surface after fracture conserves the lattice positions of the original bulk structure. Although Ca2+ and CO:- fit the calcite surface most effectively

in terms of size and charge, any reactive species from the environment in contact with a newly created surface will be taken up in order to decrease excess surface energy. Previous work has shown that the dangling bonds created during cleavage react immediately with Hz0 and CO2 in air or in water (STIPPand HOCHELLA, I99 1). Though still vibrationally, ro~tionaliy, and translationally mobile, we would expect these adsorbed species to orient themselves according to local charge imbalance and to space available on the surface, with local concentration varying according to the composition of the adjacent solution or atmosphere. For example, if Hz0 were the only species available for reaction, we would expect that on a time-average, H20 might sit with its 0 attached to underbonded surface Ca atoms and with one H directed toward underbonded 0 from CO3 groups. This assumption is supported by XPS binding energy data (STIPP and HOCHELLA, 199 1) for the surface hydration species 8 - C03H and 8 - CaOH. The exact location ofthe site of maximum atomic density over these adsorbed species would affect the offset of the spots on the AFM image, but the lattice pattern for this arrangement, centered over the 0 atom of the adsorbed HZ0 (Fig. 3b), better matches many of the AFM images, and conforms with previous undemanding of the behaviour of the calcite surface. Previous work also showed enrichment of 8. C03H species on hydrated calcite surfaces (STIPP and HOCHELLA, 1991). A trigonal planar carbonate group is wider than a water molecule by at least a factor of two. Certainly the orientation of adsorbed carbonate species (as opposed to structural carbonate groups) would be sterically controlled. We know that AFM is sensitive to height differences in adsorbed commensurate and incommensurate layers (CHEN and GEWIRTH, 1992; LAGRAFF and GEWIRTH, 1993), and it seems reasonable that an ordered overlayer of adsorbed HZ0 or CO3 or a mixture of the two could give rise to the 2 X 1 structure that we have often observed both in air and under solution. In many images, rows are paired and alternate rows are topographically higher (Figs. 5, 6). These height differences are preserved after rotation of the sample by 180” with respect to the scanning direction; that is, a low row on the left in the original image appeared on the right after rotation, indicating that height difference of rows is a real feature of the surface relating to the near-surface atomic structure, but its interpretation depends on an understanding of tip interaction with the surface. Previously, we discussed the effect of the differing bond energies on the x- and y-sides of each carbonate row. This asymmetry combined with the effects of friction (to be discussed later) could contribute to row-pairing and alternating higher and lower rows.

We have estimated the relative contribution of the “hardsphere” repulsive and the electrostatic components to the total force. Several empirical relationships have been proposed to describe the repulsive force with respect to the radii of the atoms involved and various constants representing the properties of the interacting materials, such as “sphere hardness” and the compressibility of atomic lattices (LENNARD-JONES and DENT, 1928; BORN and G~PPERT-MAYER, 1933; LASAGA, 1990: SARID, 1991; ISRAELACHVII.~, 1992). For the

Calcite surface structure

observed

3031

with AFM

17.5

-5.0

0.8 nm

-2.5

Ii

2:5

5:o

0.4

7:s

FIG.7. Image taken in air immediately after Fig. 5d; the orientation ofthe cleavage traces and the unit cell ofthe two images are identical. We see a change from double rows to single rows resulting from a change in resolving ability of the tip during & I1 nN; scan rate = 30.5 Hz.

imaging.

Net force = 70

case of calcite, determining these constants is very difficult. CaC03 is rhombohedral; the atomic spacing perpendicularly below the { 1074) face is complex; the COa group is trigonal planar, not spherical; it is covalently bonded, therefore not “hard”; and its divalent negative charge is not centered but shared. Therefore, we have not attempted to determine the repulsive forces for a surface of CaCOj. Instead, we have assumed the minima1 possible distance of closest approach to be approximately two oxygen radii, or about 3A. Below this distance, hard sphere repulsion is assumed to increase to infinity. An upper limit for the electrostatic forces that exist between the tip and the surface has been obtained by calculation of the force at the minima1 distance defined above. We have represented the tip as a single negative charge and the surface as a series of alternating double positive charges over the Ca ions and double negative charges over the C of the CO3 groups. Assuming all atoms are point sources of charge, we use Coulomb’s Law,

to calculate the total electrostatic force, F, as a function of the separation distance, d, between the tip and the surface. We sum, vectorially, all of the forces between the tip and surface charges, where r,, is the distance between the tip with charge, zI, and each surface charge, zi. to represents the permittivity of vacuum. Figure 8 shows F plotted against d, along with a line indicating the assumed minimal distance of closest approach. From Fig. 8, we can see that for large imaging forces (greater than 10 nN), the electrostatic component is negligible, while for smaller forces, the electrostatic component plays a role in AFM images. Most of our images were taken with imaging force greater than 10 nN; therefore, electrostatic interactions provide only a small component of image corrugation.

0

12

3

4

5

6

I

0

FIG. 8. Electrostatic force, F, as a function of separation, d, between the tip and a calcite surface for various surface geometries. The dotted curve represents the force from only one double (point) charge directly below the tip. The dashed curve is calculated from a single plane of alternating doubly-charged sites at positions corresponding to the rhombohedral calcite lattice. Finally, the solid curve shows the force when the full, three-dimensional calcite lattice is considered. The vertical line at d = 3A indicates the assumed minimum distance of closest approach of the tip to the surface based on a hard-sphere approximation.

Elastic surface deformation and rebound In contact AFM, the force applied by the scanning tip has both vertical (normal) and lateral (frictional) components. Because atoms and adsorbed species at a surface are not rigidly bound, these forces may be strong enough to modify their position (OHNESORGE and BINNIG, 1993), and the shape and location of the “bumps” in the image becomes a record of the vertical and/or lateral deformation that took place during scanning. It has been shown that frictional forces affect apparent molecular topography in AFM images (OVERNEY et al., 1992; MARTI, 1993). When the tip traverses the calcite surface, it rides over the surface atoms or adsorbed species that are oriented differently with respect to the scan direction in each A or B-type row (Fig. 3). As the tip interacts with the species of each of the rows, their position is recorded during deformation. For example, H20 molecules adsorbed to balance underbonded surface Ca and 0 atoms (Fig. 3b) alternate in orientation. Scanning the tip over the surface with a high force such that the scan line runs across one row and nearly along the other, results in broad “bumps” for the first and elongated streaks for the second, and gives alternate rows a very different appearance. Such interactions could explain images that look like Fig. 6c. Rather than simple elastic deformation, pressure exerted in the region of tip/sample interaction could result in a temporary and local phase transition. At pressures exceeding 3 kbar at room temperature, calcite structure is no longer stable; a higher pressure polymorph would be expected

S. L. S. Stipp, C. M. Eggleston, and B. S. Nielsen

3032

(CARLSON, 1983), in which Ca ions are alternately displaced up or down perpendicular to the { 1014) plane and alternate CQ groups are rotated. Therefore, it is possible that the 2 x 1 structure results from rotational and vertical displacements of surface species as a result of the localized pressure ofthe AFM tip. However, the 2 X I structure is also present in LEED patterns where there was no local tip pressure. Thus, we can say that the surface doubling of the bulk calcite unit cell in the a direction is a result of surface restructuring and/ or hydration. Although we cannot state the mechanism exactly, it may simply be that surface reaction with CO2 and Hz0 forms an ordered adsorbed layer, as is seen for adsorbed layers on metals and semi-conductors (KOLB et al., 1990; CHEN and GEWIRTH, 1992: MAGNUSSEN, 1993; CARNAL, 1993). SUMMARY

AND CONCLUSIONS

On a microtopographic scale, freshly cleaved surfaces of Iceland spar show atomically smooth, flat terraces of 100’s of nm width, with no evidence of microfractures. After exposure to water, numerous etch pits develop, which grow during approach to equilibrium to broad, shallow pits (order of 10’s nm width) showing differential dissolution on inequivalent sides. The atomic structure controls dissolution and the difference in bond energy on the two sides of the row would likewise lead to asymmetry during precipitation. After exposure to air, deep (210 nm), irregularly shaped pits develop chaotically over the surface, with microscopic edges trending along the cleavage directions. The highly dynamic nature of calcite surfaces exposed only to the humidity in air is consistent with evidence (STIPP et al., 1992) showing incorporation of adsorbed trace metals into the bulk. The molecular resolution images demonstrate unequivocally that the calcite surface is ordered and crystalline both in air and in solution, as in vacuum. The similarity of images taken in air with those taken under solution indicates a similarity of surface behaviour and suggests the presence of hydration species on all surfaces, corroborating evidence gained using other techniques (XPS, LEED; STIPP and HOCHELLA, 1991). It also suggests that the contact force of the tip is insufficient to displace the adsorbed hydration layer just as exposure to ultra-high vacuum for periods of days or weeks was insufficient. The molecular resolution images taken in air show lattice dimensions consistent with accepted values, i.e., 5.1 k 0.2& 7.8 + 0.2& but the unit cells do not reflect the spacing expected for a simple termination of the bulk structure. Differences in appearance of images taken with different scan directions and instrument conditions indicate that the relative weight of the various components of tip/sample interaction, viz. topography, elastic surface deformation, and electrostatic effects, determines the character of the image. Row-pairing and alternate row-offset probably result from adsorbed surface species that are affected by friction and elastic deformation during scanning over inequivalent rows. We have observed a doubling of the unit cell in the a direction (and by crystal symmetry, also in the b direction) on the surface with respect to the bulk lattice. In these chemically pure calcites, this feature cannot be explained by structural adjustments to divalent metal substitution. The 2 x 1 surface

structure is apparent on images taken in air and under solution as well as in ultra-high vacuum. It probably represents an ordered adsorbed layer, containing the hydration species Se C03H and S - CaOH. Acknowkedgments-We express our gratitude to Werner Stumm for his encouragement and support of this collaboration; Jacques Defeme of the Museum of Natural History in Geneva for the donation of some calcite samples; Pierre Glynn, Mike Hochella, Fred Wicks, and an anonymous reviewer for their suggestions; and the National Research Foundation of Switzerland and the R. E. Scott Memorial Fund for financial support. Editorial handling: M. F. Hochella Jr.

REFERENCES

H. (1990)Molecular Kinetics on Steps. In Kinetics of Ordering and Growth at Swfaces (ed. M. G. LAGALLY).pp. 125144. Plenum. BORNM. and COPPERT-MAYER M. (1933) Dynamische Gittertheorie der Kristalle. In Handbuch der Physik ZweiteAuflage, A@au der Zusammenhdngenden Materie (ed. A. SMEKAL),pp. 623-794. Springer-Verlag. BUTT H. J. (1991) Electrostatic Interaction in Atomic Force Microscopy when Imaging in Water. STM ‘9 I, Intl. Conf. Scanning Tunnelling Microscopy (abstr.). CARLSONW. D. (1983) The Polymorphs of CaCO, and the Aragonite-Calcite Transformation. In Carbonates., Minerakogy and Chemistry (ed. R. J. REEDER):Rev. Mineral. Vol. 11, pp. 191225. CARNALD. (1993) STM-investigation of underpotential deposition of Tl and Pb at chemically polished Ag( 11 I) electrodes in perchlorate electrolyte. Ph.D. dissertation, University of Berne. CHARLETL., WERSINP., and STUMMW. (1990) Surface charge of MnCO, and FeCO-,. Geochim. Cosmochim. Acta 54,2329-2336. CHEN C.-H. and GEWIRTHA. A. (1992) AFM study of the structure of underpotentially deposited Ag and Hg on Au( I II). Uitramicroscopy 42-44, 437-444. CICERONED. S.. RECAZZONIA. E., and BLESAM. A. (1992) Electrokinetic Properties ofthe Calcite/Water Interface in the Presence of Magnesium and Organic Matter. J Co//. Interface Sci. 154, 423-433. DAVISJ. A., FULLERC. C., and COOK A. D. (1987) A model for trace metal sorption processes at the calcite surface: Adsorption of Cd’+ and subsequent solid solution formation. Geochim. Cosmochim. Acta 51, 1477-1490. DICKSONJ. A. D. (1991) Disequilibrium carbon and oxygen isotope variations in natural calcite. Nature 353, 842-844. DOVE P. M. and HOCHELLAM. F., JR. (1993) Calcite precipitation mechanisms and inhibition by orthophosphate: In situ observations by Scanning Force Microscopy. Geochim. Cosmochim. Acta 57, 705-714. DUCKERW. A., SENDEN T. J., and PASHLEYR. M. (1991) Direct measurement of colloidal forces using an atomic force microscope. Nature 353, 239-24 1. DUCKER W. A., SENDENT. J., and PASHLEYR. M. (1992) Measurement of Forces in Liquids Using a Force Microscope. Langmuir 8, 1831-1836. EGGLES~ONC. M. and HOCHELLAM. F., JR. (1992) The structure of hematite {00 1) surfaces by scanning tunneling microscopy: Image interpretation, surface relaxation, and step structure. Amer. Mineral. 77, 9 I l-922. CARRELSR. M. and CHRISTC. L. (1965) Solutions, Minerals, and Equilibria. Freeman, Cooper, and Co. GRATZA. J., HILLNERP. E., and HANSMAP. K. (1993) Step dynamics and spiral growth on calcite. Geochim. Cosmochim. Acta 57,49 I 495. GUNDERSONS. H. and WENK H. R. (198 I) Heterogeneous microstructures in oolitic carbonates. Amer. Mineral. 66, 789-800. HILLNERP. E., MANNES., GRATZA. J., and HANSMAP. K. (1992a) BETHGE

3033

Calcite surface structure observed with AFM AFM images of dissolution and growth on a calcite crystal. U/tramicroscopy 42-44, 1387- 1393. HILLNERP. E., GRATZ A. J., MANNES., and HANSMAP. K. (1992b) Atomic-scale imaging of calcite growth and dissolution in real time. Geology 20, 359-362.

J. N. (1992) Intermolecular and Surface Forces. 2nd ed. Academic. KOLBD. M., LEHMPFUHLG., and ZEI M. S. (1990) Surface structural investigations by electron diffraction techniques. In Spectroscopic and Dzfraction Techniques in Interfacial Electrochemistry (ed. C. GUTI~RREZand C. MELENDRES).Kluwer. LAGRAFF3. R. and GEWIRTHA. A. (1993) In-situ Observation of Oxide Monolayer Formation on Cu Electrode Surfaces. NATO ASI, Nanoscale Probes of the Solid/Liquid Interface. Sophia Antipolis, France, July 1O-2 1, 1993. LASAGAA. C. (1990) Atomic Treatment of Mineral-Water Surface Reactions. In Mineral- Water Interface Geochemistry (ed. M. F. HOCHELLAJR. and A. F. WHITE):Rev. Mineral. Vol. 23, pp. 1785. LENNARD-JONES J. E. and DENT B. M. (1928) Cohesion at a Crystal Surface. Trans. Farczday Sot. 24, 92- 108. MAGNUSSEN0. M. (1993) STM Investigation ofAnion Adsorption. NATO ASI, Nanoscale Probes of the Solid/Liquid Interface. Sophia Antipolis, France, July 10-21, 1993. MARTI 0. (1993) Friction Microscopy of the Solid/Liquid Interface. NATO ASI. Nanoscale Probes of the Solid/Liquid Interface. Sophia Antipolis, France, July 10-21, 1993. OHNESORGEF. and BINN~GG. (1993) True Atomic Resolution by Atomic Force Microscopy Through Repulsive and Attractive Forces. Science 260, 145 1- 1456. OVERNEYR. M. et al. (1992) Friction measurements on phase-separated thin films with a modified atomic force microscope. Nature

ISRAELACHVILI

359,133-135.

PAPARAZZOE., FANF(~NIM., SEVERINIE., and PRIORI S. (1992) Evidence of Si-OH species at the surface of aged silica. J. Vat. Sci. Tech&. AIO, 2892-2896. PAQUETTEJ. and REEDERR. J. (1990) New type of compositional zoning in calcite: Insights into crystal-growth mechanisms. Geology 18, 1244-1247. PINGITOREN. E., JR., LYTLE F. W., DAVIES B. M., EASTMAN M. P., ELLER P. G., and LARSONE. M. (1992) Mode of incor-

poration of S?’ in calcite: Determination of X-ray absorption spectroscopy. Geochim Cosmochim. Acta 56, 153 I-l 538. PLUMMERL. N., BUSENBERGE., GLYNN P. D., and BLUM A. E. ( 1992) Dissolution of aragonite-strontianite solid solutions in nonstiochiometric Sr(HCOs)2-Ca(HC0,)Z-COz-Hj0 solutions. Geochim. Cosmochim.

Acta 56. 3045-3072.

A. L., HENDERSON G. S., and GOH M. C. (1992) An atomic force microscope (AFM) study of the calcite cleavage plane: Image averaging in Fourier space. Amer. Mineral. 77, 904-9 10. REEDERR. J. (1983) Carbonates: Mineralogy and Chemistr_v; Rev.

RACHLIN

Mineral.

Vol. I I.

REEDER R. J. (1992) Carbonates: Growth and Alteration Microstructures. In Minerals and Reactions at the Atomic Scale: Transmission Electron Microscopy (ed. P. R. BUSECK):Rev. Mineral. Vol. 27, pp. 38 l-424. SARID D. (199 1) Scanning Force Microscopv with applications to electric, magnetic, and atomic,forces. Oxford University Press. STAUDTW. J., REEDERR. J., and SCHOONEN M. A. A. (1994) Surface structural controls on compositional zoning of SO:- and SeO:- in synthetic calcite single crystals. Geochim. Cosmochim. Acta 58, 2087-2098.

STIPPS. L. and HOCHELLAM. F., JR. (1991) Structure and bonding environments at the calcite surface as observed with X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). Geochim. Cosmochzm Acta 55, 1723- 1736. STIPPS. L., HOCHELLAM. F., JR., PARKSG. A., and LECKIEJ. 0. ( 1992) Cd*’ uptake by calcite, solid-state diffusion and the formation of solid-solution: Interface processes observed with nearsurface sensitive techniques (XPS, LEED, and AES). Geochim. Cosmochim. Acta 56, 1941-1954. STREITWIESER A., JR. and HEATHCOCKC. H. (1981) Introduction to Organic Chemistry. 2nd ed. Macmillan. VANCAPPELLENP., CHARLETL.. STUMMW., and WERSINP. (1993) A surface complexation model of the carbonate mineral-aqueous solution interface. Grochim. Cosmochim. .4cta 57, 3505-35 18. WENKH.-R., BARBERD. J., and REEDERR. J. (1983) Microstructures in Carbonates. In Carbonates: Mineralogy and Clzemistry (ed. R. J. REEDER):Rev. Mineral. Vol. Il. pp. 301-367. Xv N. (1993) Spectroscopic and Solution Chemistry Studies of Cobalt (II) Sorption Mechanisms at the Calcite-Water Interface. PhD Thesis. Stanford University.