Periclase surface hydroxylation during dissolution

Geochimica et Cosmochimica Acta, Vol. 59, No. 9, pp. 1875- 1881, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 0016.7037/95 $9.50 + .oO

Pergamon

0016-7037( 95)00070-4

LETTER

Periclase surface hydroxylation ROY A. WOGELILJS,’ * KEITH

REFSON,’DONALD G.

FRASER,

during dissolution ’GEOFF

W.

GRIME,’

and

JONATHAN

of Oxford, Department of Earth Sciences, Parks Road, Oxford OX1 3PR, UK Scanning Proton Microprobe Unit, Nuclear Physics Laboratory, Keble Road, Oxford, 30xford Physics, Clarendon Laboratory, Oxford OX1 3PU, UK

P. GOFF~

‘University

*University

of Oxford,

(Received

December

15, 1994; accepted

OX1 3RH, UK

in revised form March 2, 1995)

Abstract-Periclase (001) surfaces were etched in dilute acid at pH 2 and 4. X-ray reflectivity measuqments on a reference crystal constrained the initial roughness of these surfaces to be approximately 30 A. The reference crystal and the crystal reacted at pH 2 were analyzed by Elastic Recoil Detection Analysis (ERDA) for proton penetration. After reaction the etched sample showed proton penetration to a depth of at least 5000 A while the reference crystal showed no significant proton inventory. Down to 900 A, the H/Mg ratio in the etched sample was approximately 2, consistent with near-surface protonation of the MgO to form hydroxylated brucite-like layers. Protonation is a far more likely mechanism to explain the proton profile than precipitation because this reaction was completed over 16 orders of magnitude below saturation with brucite. Formation of a hydroxylated near-surface layer on periclase during dissolution explains why the dissolution rates of periclase and brucite are identical in the pH range 2-5; the detachment rates are the same because the surface structures are the same. This suggests that even for this ionic solid in acid, the dissolution reaction involves a two-step mechanism with a rapid single protonation step of near-surface oxygen atoms and a slower, rate determining second protonation step. In general, product phases such as brucite are likely to be better developed under natural weathering conditions of near-neutral pH because the second step of protonation (and thus full hydration of the detaching cation, e.g., Mg +r) is much slower than in acid. Our proposed protonation mechanism relates field observations of the periclase weathering reaction to laboratory dissolution, hydration, and dehydration experiments. INTRODUCTION

plain theoretically. In this study we have taken a compositionally simple oxide mineral and made detailed chemical and structural measurements both before and after reaction with an aqueous fluid. These observations were then used to constrain ab initio calculations of the energetics of possible surface structures. By using well-characterized single crystals these measurements allow us to assess the crystallographic controls on the dissolution reaction. Intergrown reactant-product mineral textures occur naturally in the case of the hydration of contact metamorphismproduced periclase (Gorshkov et al., 1992). It has been noted, both in natural samples (Gorshkov et al., 1992) and in experimental studies (Feitknecht and Braun, 1967; Giovanoli et al., 1968; Gorshkov et al., 1992), that the (111) planes in the reactant periclase typically are parallel to the (0001) planes of the product brucite. The same relationship has been noted during the dehydration of brucite to periclase (Goodman, 1958; Ball and Taylor, 1962). Aquahydroxylpericlase, similar in structure to periclase but with structurally associated water, has been suggested as a possible intermediate phase (Gorshkov et al., 1992). Along with the parallel relationship between the periclase ( 111) and brucite (0001 ), previous experimental work has determined the following three key facts about the reaction of MgO with H20. ( 1) Hydration rates with steam depend strongly on the annealing temperature of the parent MgO (Feitknecht and Braun, 1967). (2) The (0001) brucite Bragg peak appears late in the hydration reaction and remains faint

Mineral surface reactions are the key to understanding a wide variety of geochemical processes. The pH dependence and activation energies for the dissolution of many silicates, carbonates, and oxides have been measured in numerous laboratory experiments (see Brady and House, 1995, and references therein), yet there are few detailed studies of the structural and chemical changes that occur near mineral surfaces during reaction with aqueous fluids. Those studies that have been completed have had difficulty in determining both how cations exposed to solvent attack form precursor surface complexes and in determining exactly how these precursor complexes finally detach from the solid substrate and become fully solvated aqueous species (Casey et al., 1993a). Furthermore, laboratory experiments have often been in direct conflict with field observations: from field evidence the dissolution of minerals typically involves the formation of an intergrown texture of secondary minerals with the reactant primary mineral (Banfield and Eggleton, 1990; Casey et al., 1993b). Laboratory experiments, particularly those concerned with the dissolution of feldspar, have shown the development of a compositionally distinct leached layer (Casey et al., 1988; Hellman et al., 1990), but experiments have not produced an intergrown texture and that texture has been difficult to ex* Present address: University ology, Oxford Road, Manchester

of Manchester, Department Ml 3 9PL, UK.

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(Feitknecht and Braun, 1967). (3) Dissolution rates of periclase and brucite are identical in the pH region 2-5 at 25 and 75°C while above pH 5 periclase dissolves more rapidly than brucite (Vermilyea, 1969). This suggests that in the pH range 2-5 the two minerals share common surface structures and intermediate reaction complexes which strongly implies that a protonation (or hydroxylation) step is involved in the detachment of Mg from the periclase structure. (Note that we use the term hydroxylation to indicate the creation of a surface hydroxyl group either by protonation of surface oxygen or by inner sphere complexation of a solution hydroxyl group with a cation exposed at the surface.) Therefore, a study of this simple dissolution reaction with an oriented sample may shed light on the development of intergrown textures and on the epitaxial relationships noted in more complicated natural systems and will allow us to test the hypothesis that a simple oxide may undergo a structural conversion during dissolution in acid. Consideration of the previous computational work discloses a paradox. Since periclase shows perfect cleavage along the (00 1) plane, this is the dominant surface presented to reactant fluids. Although computations show that Hz0 physisorbs readily on the (001) cleavage surface, no dissociative chemisorption is predicted to occur on this surface (Scamehom et al., 1993; Lange1 and Parrinello, 1994). However, MgO powders easily hydrate in contact with water (Kuroda et al., 1988; Jones et al., 1984; Feitknecht and Braun, 1967). Thus, a fundamental reason for studying this dissolution reaction is to explain how the observed reactivity of periclase occurs despite the fact that the dominant surface apparently does not chemisorb water. EXPERIMENTAL

METHODS

Dissolution Single crystals of chemomechanically polished periclase measuring 1 cm X 1 cm X 0.1 cm were used in these experiments. Prior to reaction the pet-i&se crystals were ultrasonically cleaned for 15 min in successive baths of reagent grade acetone and reagent grade propanel, then rinsed with distilled deionized water. After cleaning, the excess water was blown off of the surface with a high pressure jet of ultrapure Nz (all Nz referred to is 99.999% pure) and the crystals then stored at 60°C and used within 6 h of cleaning. Only Teflon tools were used to touch the crystal edges and nothing came in contact with the polished surface after cleaning. For reaction the crystals were weighed and then loaded into a Teflon crystal holder which allowed free flow of reactant fluid over all crystal faces and the holder was then press-fit into a titration cup. Immediately prior to reaction the cup was filled with approximately 45 mL of distilled deionized water and the solution titrated to the pH of interest, in the case of the dissolution experiments to either pH 2 or 4, with 0.1 M HNO, using a Mettler DL-77 auto-titrator. The auto-titrator was then used in pHstat mode to keep the pH of the fluid constant (kO.05 pH units) throughout the remainder of the experiment. The large, polished 1 cm2 (001) faces were analyzed by SEM. All product fluids were analyzed by ICP-AES with expected concentrations bracketed by prepared standards. Blanks of laboratory distilled deionized water were also measured to check background levels of the pertinent cations. ERDA The unreacted periclase reference crystal and the crystal reacted at pH 2 were analyzed using a 7.5 MeV I60 beam with a surface barrier detector placed at a scattering angle of 24 degrees. A thin (6.2 pm)

Al window was inserted between sample and detector to filter out forward scattered oxygen and the sample was tilted 15 degrees. Spectra were simulated using normal differential Rutherford cross-sections (Kotai, 1993) and the Bohr straggling coefficient was adjusted in order to fit the ERDA peak shape. H:Mg ratios were calculated assuming a uniform distribution of Mg in the crystal surface. X-Ray Reflectivity Commercially prepared crystals were purchased with (001) surfaces that were nominally 15 A rough. A reference crystal of the starting material was analyzed by X-ray reflectivity. To ensure as clean a surface as possible, this crystal was subjected to a rigorous preparation regime. 24 h before X-ray analysis, the crystal was placed in a furnace at approximately 500°C for 12 h to drive off water vapour and other volatile components. When this heating period was completed, the furnace was purged with Ar and the crystal removed from the furnace and quickly placed into an Ar-purged sealed desiccator to cool. The entire Ar-purged desiccator was then placed inside a N?purged glove bag, and the crystal removed from the desiccator and mounted inside a custom-built Al sample chamber. X-ray transparent mylar windows were used to seal the chamber which was kept at a positive pressure of Nz using an attached portable tank during transfer from the glove bag to the X-ray source and during the analysis. Specular X-ray reflectivity measurements were made using Cu-K, radiation from a rotating anode. The reflectivity profile was measured by scanning the incident angle at each detector angle to enable the integrated intensity to be determined and the background to be subtracted. Data mentioned here are the integrated and background subtracted results of 111individual scans completed over a 24 h period. For more details of mineral surface analysis by X-ray reflectivity see Chiarello et al. ( 1993). In addition, diffraction peaks were located in order to assess the degree of misalignment (miscut) between the actual surface and the (001) crystallographic planes. Surface miscut was 0.372” (100) and 0.010” (010). Calculations The ab initio quantum mechanical calculations were based on density functional theory within the local density approximation and used pseudopotentials and plane-wave basis sets (Payne et al., 1992; Car and Parrinello, 1985). Comparison of the total energies from separate calculations on the reactants and products yields the AE of the reaction in question. The initial states were a clean (001) surface of periclase in a periodic supercell and the energy of an isolated water molecule plus an experimental value for the binding energy of water into ice at 0 K (Eisenberg and Kauzman, 1969). The final states consisted of a slab with the pertinent hydroxyl surface in a similar supercell. Further details will be published elsewhere (Refson et al., 1995). Solution activities were calculated using EQ3 (Wolery, 1983) with the extended Debye-Htickel approximation used for activity coefficient computation. Brucite solubility data were taken from the associated EQ3 database, version data 0.3245R54 (Wolery, 1983).

RESULTS X-ray reflectivity analysis of the reference crystal determined the r.m.s. surface roughness to be 3 1 A (+3 A). The measured critical angle of the reference crystal was 0.0392 A -‘, within 1% of the theoretical value for MgO (0.0389 A -’ ). Note that the critical angle of Mg( OH)* is 0.0424 A ’, so that a thick hydroxide layer would be plainly visible in the reflectivity data. Furthermore, no oscillations are present in the reflectivity data. Such oscillations would be expected if the starting material had a hydroxide layer as thin as several unit cells. Therefore, the reflectivity data constrain any hydroxylated layer on the original crystals to be no thicker than approximately 30 A.

Hydroxylation of periclase surfaces Dissolution of periclase as determined by acid consumption was apparently linear throughout the reactions while pH-statted at both pH 2 and 4. During the first stage of reaction most of the acid added to the reaction vessel was used in changing the pH of the solution and therefore if a short initial period of nonlinear release occurred it cannot be resolved. Because these were reactions with low-surface area solids, only small quantities of acid were consumed during reaction at fixed pH and therefore rates calculated from acid consumption would be prone to large errors. To avoid such errors, the quantity of periclase dissolved was determined by Mg+’ concentration change and by crystal mass change. Table 1 presents the dissolution data for these experiments and Table 2 compares our results with the powder results of Vermilyea ( 1969) at pH 2 and 4. Note that for our experiments, because some Mg+* is added to solution as a result of cation-proton exchange these are maximum rates; however, since Vermilyea ( 1969) computed rates based on H’ consumption, his results incorporate the same exchange process and thus remain comparable to ours. Furthermore, because of the known surface area normalization problem involved with comparing powder data to single crystal experiments we expect the absolute rate values to disagree slightly as indeed they do. However, the difference is constant between the two datasets at pH 2 and pH 4, and hence we are confident that our experiments have reproduced the same relative rate vs. pH dependency (A log R/A pH = -0.35) and that these are therefore comparable dissolution results. We also show the saturation state of the product solutions with respect to brucite in Table 2. The etching solutions were 16.2 and 11.9 orders of magnitude below saturation for the experiments at pH 2 and 4, respectively. These solutions were vigorously stirred by propeller stirrer throughout the length of the experiments, and so this extreme degree of undersaturation makes the possibility of any precipitation reactions highly unlikely. We likewise assume that at these low Mg+* concentrations self-adsorption and other interference effects are minimized. ERDA results from the pH 2 etched and unetched (reference) samples are shown in Fig. 1. The clear peak between 900 and 1300 keV in the spectrum from the etched sample (open squares) shows the presence of protons in the nearsurface, while the spectrum obtained from the unetched sample (thin line) shows almost no recoiled protons. The ERDA result from the unetched sample agrees with the reflectivity analysis. Figure 1 also compares the simulated spectrum (heavy line) with the data. This simulation of the spectrum from the etched sample was used to unfold the ERDA spectrum into a proton depth profile. First, a proton distribution Table 1 Moles single crystal periclase dissolved and saturation state of experiments

~“2

-491

-4 97

02618

I6 1807

oH4

-4 74

-4 78

4 5368

II 9057

’Log moles dissolved calculated from concentration change of Mg in solution b,, II II II * crystal mass change ’Log of ratio of Mg+’activity to square of proton activity ’ Log of bnxite equilibrium constant (log I
1877 Table 2. Kinetic data (rates, R, in mol cm” s.‘) single crystal’

powde? log %,

log %, log Rsc

At (s)

logRs, pH2

-9 I4

-7 8

I3

706x10’

~“4

-9 82

-8 6

I?.

500x10’

A log R/A0

-035

-0 4

‘Calculated assuming B surface area of 2 4 cd bFrom Vermilyea (1969)

profile was assumed, and the resultant ERDA spectrum for the given experimental conditions calculated. After the simulation was completed, the assumed profile was changed and a new spectrum calculated to provide a better fit to the experimental data. When all parts of the simulated spectrum agreed with the actual data, the iterative procedure was stopped. The inset on Fig. 1 shows the H-depth profile in the etched periclase that results from unfolding the ERDA spectrum. Clearly, the periclase near-surface region becomes extensively protonated during reaction with acidic fluids. An H:Mg stoichiometry of 2:l as deep as 900 A is consistent with the data. Protons penetrate even deeper at much lower concentrations: from 900 to 5000 A depth the etched sample is calculated to have a H:Mg ratio of 8:lOO. This suggests that the detachment of Mg from the mineral-solution interface is not the only step involved in periclase dissolution. An examination of the energetics of chemisorption of HZ0 onto MgO surfaces exposed to solvent attack can allow us to determine exactly how the observed hydroxylation reaction may proceed and also allows us to make predictions about the crystallographic relationship between the MgO single crystal substrate and the hydroxylated surface thin-film which developed in acid. Extensive protonation of the periclase near-surface could proceed by a number of pathways. The simplest is by dissociating H20 on a (001) surface and bonding the resulting proton onto a substrate oxygen and the hydroxyl to an exposed magnesium atom. This surface reaction will have a specific energy change associated with it. By calculating the energy changes of several different chemisorption reactions we can determine which surface atomic configurations are the most stable and hence the most likely to form. Ab initio calculations of the energy changes for several periclase hydroxylation reactions at 0 K were completed. Thermodynamic corrections were applied to extrapolate the resulting values up to 298 K. Results of these calculations and an atomic model comparing two final states are presented in Fig. 2. Note that a bare ( 111) surface of periclase is unstable in ultra-high vacuum and will reconstruct into microfacets of (001) steps. But our calculations show that when H20 is present the surface behaviour of periclase is markedly different. Chemisorption of water will create and stabilize a hydroxylated ( 111 )-type surface because this reaction is favourable with a large negative energy. Figure 2a shows this energetically favourable surface atomic arrangement (AEOK = -90.2 kJ mol-’ = - 1.69 J mm*, AC,,,, = -87.24 kJ mol-’ = -1.63 .I mm*), where H20 has chemisorbed to form a periclase ( 111 )-hydroxyl surface. In this orientation, the Mg and 0 atoms are arranged in

R. A. Wogelius

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100

et al.

T

90

Unfolded H Profile 25

80 70 60 50

depth

(angstroms)

40

pH 2 etched

0

30

unetched

20

simulated

10 0

200

400

600

800

1000

1200

1400

1t

Energy (keV) FIG. 1. ERDA spectra for H+ in periclase. Clear peak in data from the pH 2 etched sample (open squares) results from proton penetration during dissolution. Thin line represents data from the unetched sample-no significant proton presence. Heavy line is fit to the etched data computed to unfold the ERDA spectrum into a depth profile. Inset: Unfolded proton depth profile for the pH 2 etched periclase crystal. The H:Mg ratio in the near-surface is plotted as a functiyn of depth. Down to 900 A, the 2: I ratio suggests conversion to Mg(OH)2. Protons penetrate to approximately 5000 A, with the region 900-5000 modeled here as having an 8:lOO atomic ratio. Below 5000 A the data imply no proton presence.

alternating planes, similar to the arrangement of Mg and 0 atoms normal to the brucite [OOOl] direction. In fact, the periclase (111) d-spacing is within 2% of d/2 of the brucite (0001) d-spacing. Moreover, the hydroxylated ( 111) surface is also stable relative to bulk periclase plus Hz0 (A& = -0.59 J mm*), and therefore the conversion to hydroxylated planes will not stop at the surface layer but will penetrate beneath the surface and over time may convert a significant portion of the crystal to a brucite-like structure. In contrast, the simple MgO (001) surface with chemisorbed HZ0 is at a higher energy than a clean MgO (001) surface plus bulk H20 (A& = +41.6 kJ mol-’ = +0.78 J m-‘, AG298K = + 44.6 kJ mol-’ = + 0.84 J mm2). This result is in accord with previous calculations (Scamehom et al., 1993; Lange1 and Parrinello, 1994). Figure 2b shows the lowest energy atomic configuration for a hypothetical hydrated (001) MgO surface. The calculations show this is energetically unfavourable and thus indicate that Hz0 should not ad-

sorb reactively onto the dominant MgO (001) cleavage surface. Protonation of the periclase near-surface therefore must proceed by the reaction of H20 with low coordinated defect sites. Even though our starting material was polished to be as atomically smooth as possible, it still was measured by X-ray reflectivity to have an r.m.s. surface roughness on the order of 30 A. This represents surface steps ten unit cells deep. Steps such as these present a high number of (0 11) and ( 111) type sites for reaction with the fluid. Both Jones et al. (1984) and Feitknecht and Braun (1967) observed a link between increasing number of surface defects and increasing MgO hydration rates. SEM photomicrographs of the surfaces reacted at pH 2 and 4 show the development of surface mottling. At pH 7.5, surfaces reacted in related experiments (space constraints will not allow us to describe these experiments in detail here) are not only mottled but show evidence of surface layers cracking

Hydroxylation

a) Hydroxylated MgO (111) surface = -1.69 J m-* (OOl)* iz::fr = -1.63 J me2(001)

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b) Fictive hydroxylated MgO (001) surface A’=reac.c~ = +0.78 J me2(001) z +0.84 J m-* (001) AG,,,

FIG. 2. Atomic configurations of final surface states calculated by ab initio DlT/LDA method. (a) Creation of a (111) hydroxylated surface as shown is favorable when water is present. (b) Periclase (001) surface is the cleavage face, but water chemisorption as shown here is energetically unstable. Protons indicated as small light grey atoms, Mg as intermediate-size and intermediate-color grey atoms, and oxygen as large, dark grey atoms. Note the zig-zag arrangement of oxygen atoms in the (001) case as compared to the clear Mg and 0 layering in the (111) case. APor ‘I is for the surface reaction: MgO@“) + Hz0 = Mg(OH):” Orb). *Energies expressed in J m-* of equivalent (001) surface assuming a (OOl)/( 111) surface area ratio of 1:Js.

and separating away from the periclase substrate. Some of the platelets separating from the surface show psuedohexagonal symmetry. These platelets may be crystalline brucite (hexagonal crystal structure) or mixed brucite/periclase intergrowths that have peeled off of the substrate due to strain caused by mismatch between the brucite-periclase crystal structures. Preliminary surface sensitive X-ray diffraction analyses of these samples suggests that the periclase reacted at pH 2 and 4 is structurally deformed in the [OOl] direction. This distortion is probably caused by the penetration of protons and hydronium into the near-surface region as indicated by the ERDA measurement. However. brucite Bragg peaks have not

a) Reacted MgO 2d(111)=4.661A

b) Brucite d (0001) = 4.766 A

FIG. 3. Proposed hydroxylation pathway of periclase. (a) Reaction begins at low-coordinated surface site, with the Mg atoms exposed at the left-hand corner. Reaction continues down the (111) plane with a total of six Mg ions exchanged for twelve protons. Note the development of the stable hydroxylated structure. (b) For comparison, the brucite structure is shown in a similar orientation. Atomic symbols same as previous.

been clearly identified

from the near-surface region in the pH 2 and 4 samples, which suggests that even after extensive hydroxylation the periclase structure maintains its integrity. Diffuse scattering from the reacted crystals does increase, implying a disturbance in short-range order in the near-surface. ERDA analysis of the pH 4 sample as well as X-ray reflectivity analysis of the reaction products are currently underway. DISCUSSION Hydroxylation of the near-surface into a brucite or brucitelike layer during dissolution explains why the dissolution rates of brucite and periclase are identical in the acidic pH region. Dissolution involves a rapid first step of conversion from the oxide to a hydroxylated surface layer and then a slower, rate-limiting step of detachment of Mg from this brucite-like structure. Our ERDA results and ab initio calculations are consistent with this proposed two-step model. This model provides a specific mechanism for the observed chemical reactivity of periclase with water vapour and aqueous fluids. Conversion of the near-surface region into a hydroxidetype structure leaves each Mg atom in six-coordination with hydroxyl groups. Under acidic conditions further addition of a proton leaves the Mg atoms in six-coordination with H20, which, of course, is an Mg atom in solution. Hydroxylation creates and stabilizes a ( 111) hydroxyl surface, and this surface is nearly identical to the brucite basal plane. Once Mg atoms have been replaced by protons in the surface layer the reaction proceeds by the diffusion of Mg+’ out and H+ inwards parallel to ( 111) planes. Replacement of Mg +* by protons creates penetrative brucite-like layers of hydroxyl ions (see Fig. 3). Consideration of the electrostatics suggests that protons and Mg +’ ions can diffuse much more rapidly along these layers which therefore function as channels for Mg +’ ion transport from the bulk to the surface. The end result of this process will be a layer penetrated by numerous hydroxylated channels. Because of the structural sim-

1880

R. A. Wogelius

ilarity between periclase and brucite the reacted surface nevertheless may retain much of the periclase structure. Figure 3 illustrates the creation of a ( 111) channel in periclase and demonstrates the similarity with the brucite structure. In Fig. 3a, the alternating planes of Mg and 0 atoms are ( 111) planes rotated slightly to show perspective. Here, the detachment of Mg +* started with removal of the two lowcoordinated atoms exposed in the upper left-hand comer of the crystal. Further replacement of Mg +* by protons continued along the ( 111) plane until finally a total of six Mg +* ions have been replaced by twelve protons. Brucite layering in the [OOOl] direction is shown in Fig. 3b to underline the similarity between the hydroxide structure and the structure resulting from periclase hydroxylation. Replacing every Mg atom in a periclase ( 111) plane with two protons results in a stable, charge-balanced structure having a d-spacing within 2% of the brucite (0001) d-spacing. After this first rapid step of protonation of the periclase surface, the coordination environments of Mg atoms exposed in either the brucite or the periclase surface are nearly identical. Therefore, the observed dissolution rates of periclase and brucite are identical. Development of brucite-like layering during dissolution thus must be fastest perpendicular to the incipient brucite (0001) axis and therefore the layers stay extremely thin parallel to c. Such layers would give poor Bragg diffraction results, just as is seen in bulk MgO hydration with steam (Feitknecht and Braun, 1967). Reaction under such extreme conditions of pH as used in our experiments may be too fast to allow a well-ordered crystalline thin-film to develop. However, the critical step of forming a hydroxide precursor is still faster than the detachment of Mg from the interface and hence the macroscopic dissolution kinetics of brucite and periclase remain the same. Under conditions of neutral pH, which more closely approximate natural systems, the development of the hydroxylated layer may be slow enough to allow a well-ordered crystalline layer to develop topotactically on the periclase substrate. Our proposed channeling of protons down ( 111) planes to create a deeply hydroxylated structure may also account for the observed different chemical behaviour of “active” and “inactive” forms of periclase. Presence or absence of such hydroxylated channels is probably highly dependent on how the periclase was prepared, e.g., different annealing temperatures. Crystals which have such channels would be expected to have a much higher number of sites available for reaction with gases or fluids and hence be highly chemically reactive. Periclase formed without such channels, and thus consisting of a surface of (001) cleavage planes, would be much less reactive. Our ab initio calculations predict that the AC for Hz0 penetration into the bulk should change sign from negative to positive at about 675”C, in agreement with experimental observations (Feitknecht and Braun, 1967). Active periclase is produced by annealing below 675”C, inactive by annealing above 675°C. This is further evidence that our proposed mechanism is correct. This proposed dissolution mechanism for periclase allows us to make several predictions for testing by further experimentation. First of all, other structurally similar (fee) metal oxides should also dissolve by this two-step process in acidic solutions. If so, we should also be able to see a proton profile.

et al.

Second, a clear, reproducible epitaxial relationship should exist between the hydrated over-layer and the single crystal oxide substrate. Finally, the dissolution kinetics of such simple oxides should be similar to the dissolution kinetics of the simple hydroxide. More detailed ERDA and reflectivity studies in this system are planned in order to separate the cationproton exchange rate from the surface complex destruction rate. Acknowledgments-This work was supported by NERC under grants GR3/8970 and GR3/8310. We would like to thank E. Salje and J. Chrosch for diffraction analyses, D. Preston, C. Cranstone, and D. Robson for expert machining, S. James for help with the ICP analyses, R. A. Cowley for access to the rotating anode, and W. Casey for a helpful discussion of the work. Thanks also go to two careful reviewers. Editorial

handling: J. D. Macdougall

REFERENCES Ball M. C. and Taylor H. F. W. ( 1962) The dehydration of brucite. Miner& Mug. 32, 754-766. Banfield J. F. and Eggleton R. A. ( 1990) Analytical transmission electron microscope studies of plagioclase, muscovite, and K-feldspar weathering. Clays Clay Mineral. 38, 77-89. Brady P. V. and House W. A. ( 1995) Su$ace Controlled Dissolution and Growth of Minerals. CRC Press. Car R. and Parrinello M. ( 1985) Unified approach for moleculardynamics and density functional theory. Phys. Rev. Lett. 55,24712474. Casey W. H., Westrich H. R., and Arnold G. W. (1988) Surfacechemistry of labradorite feldspar reacted with aqueous-solutions at pH = 2, 3, and 12. Geochim. Cosmochim. Acta 53,2795-2807. Casey W. H., Banfield J. F., Westrich H. R., and McLaughlin L. (1993a) What do dissolution experiments tell us about natural weathering? Chem. Geol. 105, I- 15. Casey W. H., Westrich H. R., Banfield J. F., Ferruzzi G., and Arnold G. W. ( 1993b) Leaching and reconstruction at the surfaces of dissolving chain-silicate minerals. Nature 366, 253-255. Chiarello R. P., Wogelius R. A., and Sturchio N. C. ( 1993) In Situ synchrotron X-ray reflectivity measurements at the calcite-water interface. Geochim. Cosmochim. Acta .57,4103-4110. Eisenberg D. S. and Kauzman W. ( 1969) The Structure and Properties of Water. Oxford Univ. Press. Feitknecht W. and Braun H. ( 1967) Der Mechanismus der Hydratation von Magnesiumoxid mit Wasserdampf. Helv. Chim. Actu 50,2040-2053. Giovanoli R., Feitknecht W., and Fahrer W. ( 1968) iiber das Epitaktische Aufwachsen von Magnesiumhydroxid auf Magnesiumoxid. J. Microscopic 7, 177 - 194. Goodman J. F. (1958) The decomposition of magnesium hydroxide in an electron microscope. Proc. Royal Sot. A 247, 346-352. Gorshkov A. I., Kolodyazhnaya Y. V., Mohkov A. V., and Smolin P. P. ( 1992) Natural topotaxic intergrowths of brucite with periclase and the Mg( OH)* - >MgO equilibrium. Dok. Ross. Akad. Nuuk. 322,584-588. Hellman R., Eggleston C. M., Hochella M. F., and Crerar D. A. ( 1990) The formation of leached layers on albite surfaces during dissolution under hydrothermal conditions. Geochim. Cosmochim. Acta 54, 1267-1281. Jones C. F., Reeve R. A., Rigg R., Segall R. L., Smart R. S., and Turner P. S. ( 1984) Surface area and the mechanism of hydroxyldtion of ionic oxide surfaces. J. Chem. Sot. Faraday Trans. I 80,2609-2617. Kotai E. ( 1993 ) RBX, The Universal RBS and ERDA Spectrum Handler-user’s Manual. Sky-Soft Ltd., Budapest. Kuroda Y., Yasugi E., Aoi H., Miura K., and Morimoto T. (1988) Interaction of water with the surface of magnesium-oxide. J. Chem. Sot. Faraday Trans. I 84, 242 l-2430.

Hydroxylation Lange1 W. and Paninello M. (1994) Hydrolysis at stepped MgO surfaces. Phys. Rev. L&t. 73,504-507. Payne M. C., Teter M. P., Allen D. C., Arias T. A., and Joannopoulos J. D. ( 1992) Iterative minimization techniques for ab initio totalenergy calculations: molecular dynamics and conjugate gradients. Rev. Mod. Phys. 64, 1045-1097. Refson K., Wogelius R. A., and Fraser D. G. (1995) Water chemisorption and reconstruction of the MgO surface. Phys. Rev. Letr. (submitted).

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Scamehom C. A., Hess A. C., and McCarthy M. C. (1993) Correlation corrected periodic Hartree-Fock study of the interactions between water and the (001) magnesium-oxide surface. J. Chem. Phys. 99,2786-2795. Vermilyea D. A. ( 1969) The dissolution of MgO and Mg(OH), in aqueous solutions. J. Electrochem. Sot. 116, 1179- 1183. Wolery T. J. ( 1983) EQ3NR, A Geochemical Aqueous SpeciationSolubiliry Code. UCRL-53414, Lawrence Livermore National Laboratory.