Characterization of leached layers on olivine and pyroxenes using high-resolution XPS and density functional calculations

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Geochimica et Cosmochimica Acta 72 (2008) 69–86 www.elsevier.com/locate/gca

Characterization of leached layers on olivine and pyroxenes using high-resolution XPS and density functional calculations V.P. Zakaznova-Herzog a b

a,b,*

, H.W. Nesbitt a, G.M. Bancroft c, J.S. Tse

d

Department of Earth Sciences, University of Western Ontario, London, Ont., Canada N6A 5B7 Institute of Isotope Geochemistry and Mineral Resources, ETH Zurich, 8092 Zurich, Switzerland c Department of Chemistry, University of Western Ontario, London, Ont., Canada N6A 5B7 d Department of Physics, University of Saskatchewan, Saskatoon, Sask., Canada S7N 5E2

Received 15 June 2007; accepted in revised form 24 September 2007; available online 13 October 2007

Abstract High-resolution core level and valence band (VB) X-ray photoelectron spectra (XPS) of olivine [(Mg0.87Fe0.13)2SiO4], bronzite [(Mg0.8Fe0.2)2Si2O6] and diopside [Ca(Mg0.8Fe0.2)Si2O6] were collected before and after leaching in pH 2 solutions with the Kratos magnetic confinement charge compensation system which minimizes differential charge broadening. The leached samples yield Si 2p, Mg 2p, Ca 2p and O 1s XPS spectral linewidths and lineshapes similar to those collected from the respective pristine samples prior to leaching. As with previous XPS studies on crushed samples, our broadscan XPS spectra show evidence for initial, preferential leaching of cations (i.e., Ca2+ and Mg2+) from the near-surface of these minerals. The O 1s spectra of leached olivine and pyroxenes show an additional peak due to OH, which arises from H+ exchange with nearsurface cations (Ca2+ and Mg2+) via electrophilic attack of H+ on the M–O–Si moiety to produce the H2Mg(M1)SiO4(surf) complex at olivine surfaces, and two complexes, H2Mg(M1)Si2O6(surf) and H4Si2O6(surf) at diopside and enstatite surfaces. The olivine and pyroxene surface complexes H2Mg(M1)SiO4(surf) and H2Mg(M1)Si2O6(surf) have been proposed previously, but the second pyroxene surface complex H4Si2O6(surf) has not. Two electrophilic reactions occur in both olivine and pyroxene. For olivine, the more rapid attacks the M2–O–Si moiety producing H2Mg(M1)SiO4(surf); while the second attacks the M1–O–Si moiety ultimately producing H4SiO4 which is released to solution. For pyroxenes, the first electrophilic reaction produces H2Mg(M1)Si2O6(surf), while the second produces.H4Si2O6(surf). These two reactions are followed by a nucleophilic attack of H2O (or H3O+) on Si of H4Si2O6(surf). This reaction is responsible for rupture of the brigding oxygen bond of the Si–O–Si moiety and release of H4SiO4 to solution. The intensity of the OH peak for the leached pyroxenes is about double the OH intensity for the leached olivine, consistent with the equivalent of about a monolayer of the above surface complexes being formed in all three minerals. Valence band XPS spectra and density functional calculations demonstrate the remarkable insensitivity of the valence band to leaching of Ca2+ and Mg2+ from the surface layers. This insensitivity is due to a dearth of Ca and Mg valence electron density in the valence band: the Ca–O and Mg–O bonds are highly ionic, with metal-derived s orbital electrons taking on strong O 2p character. The valence band spectrum of leached olivine shows an additional very weak peak at about 13.5 eV, which is assigned to Si 3s valence orbitals in the surface complex H2Mg(M1)SiO4, as indicated by high quality density functional calculations on an olivine where Mg2+ in M2 is replaced by 2H+. The intensity of this new peak is consistent with formation of the equivalent of a monolayer of the surface complex.  2007 Elsevier Ltd. All rights reserved.

1. INTRODUCTION

*

Corresponding author. E-mail address: [email protected] (V.P. Zakaznova-Herzog).

0016-7037/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2007.09.031

It is important to understand the detailed kinetics and mechanism of the dissolution of common simple silicate minerals because their weathering is an important part of

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the geochemical cycle for many elements (e.g., Si, Mg, and Fe). There are a host of leaching studies of silicate minerals and glasses in literature (e.g., recent overview by Oelkers, 2001b). Early solution-based room temperature leaching studies of crushed olivine and pyroxene indicated that their dissolution proceeded incongruently with preferential loss of Mg, and formation of Si-rich layers sufficiently thick to slow dissolution by inhibiting diffusion through them (Luce et al., 1972; Grandstaff, 1977). In contrast, a large number of more recent leaching studies on these two mineral classes using both solution and surface analyses (e.g., XPS- and IR-based techniques) have shown that the overall dissolution process of olivines and pyroxenes involves essentially congruent dissolution at room temperature and acidic solutions (e.g., Schott et al., 1981; Schott and Berner, 1983; Schott and Berner, 1985; Casey et al., 1993; Seyama et al., 1996; Chen and Brantley, 2000; Pokrovsky and Schott, 2000a,b; Rosso and Rimstidt, 2000; Oelkers, 2001a; Oelkers and Schott, 2001; Duro et al., 2005; Giammar et al., 2005; Hanchen et al., 2006; Dixit and Carroll, 2007). Both Schott et al. (1981) and Eggleston et al. (1989) showed that the studies of the surface chemistry and leaching behaviour of these minerals is heavily dependent on the initial treatment of the crushed samples. High-resolution TEM studies of naturally weathered and oxidized olivines (Banfield et al., 1990, 1992) show very thin leached layers and dissolution channels. All recent XPS studies on leached olivines show a thin surface layer depleted in Mg2+, and some evidence for Si–O–Si dimers from the broadening of the O 1s spectra (Casey et al., 1993; Seyama et al., 1996; Pokrovsky and Schott, 2000a,b) and DRIFT (Diffuse Reflectance Infrared Spectroscopy) evidence for OH groups (Pokrovsky and Schott, 2000a). It was suggested that the rate controlling surface complex in leached olivines is the formation of the Si dimer. In contrast, the formation of these thin surface layers is usually thought to involve an initial exchange of Mg2+ in M2 for 2H+ from solution (Schott and Berner, 1985; Rosso and Rimstidt, 2000; Liu et al., 2006) after adsorption and reaction of an absorbed hydronium ion (Rosso and Rimstidt, 2000; Liu et al., 2006). There are only a few XPS studies of pyroxene leaching (e.g., Schott et al., 1981; Schott and Berner, 1985). These studies also show evidence for a cation-depleted layer; and the exchange of Ca2+ and Mg2+ (in M2) for 2H+ is also considered to be the initial leaching step (Schott et al., 1981; Schott and Berner, 1985; Oelkers and Schott, 2001). The interpretation of the O 1s XPS of the leached nearsurface layer is somewhat circumspect, primarily because charge broadening of XPS spectra generally is dramatic for non-conductors. The O 1s and other linewidths are always greater than 2 eV compared to linewidths of less than 1 eV on conductors and semiconductors. The increase in O 1s linewidths and the associated asymmetry observed after leaching could well be attributed to enhanced charge broadening rather than to addition of new peaks resulting from new chemical signals derived the leached zone. Consequently, the increase in O 1s linewidth and asymmetry could be assigned either to charge broadening or to generation of a new peak (Si-OH moiety) in the spectrum at slightly different binding energy from the Si–O–Si signal (Nasu et al., 1988;

Paparazzo et al., 1992; Uchino et al., 1992; Tsuchida and Takahashi, 1994). Interpretation of Infra-Red (IR) studies are also circumspect in that IR measurements on a leached olivine yield a very broad peak centered at 3350 cm1 which may be due to a thin microporous surface layer containing OH bonds (Pokrovsky and Schott, 2000a). In spite of concerns related to interpretation of the above spectra, there seems to be agreement that the thickness of the leached layer (reflected by Mg and Ca surface depletions in XPS broad˚ for acid leachscan spectra primarily) does not exceed 20 A ing of olivines and pyroxenes (Schott et al., 1981; Pokrovsky and Schott, 2000a,b; Giammar et al., 2005). Leached layers ˚ , however, have been calculated for pyroxof less than 10 A enes (e.g., Schott et al., 1981). Recent XPS studies of non-conductor silicate minerals, have shown that the rather new Kratos Axis Ultra XPS instrument yields much narrower core level (e.g., Si 2p, O 1s, Mg 2p) linewidths than previously collected for silicates; and that peak shapes are Gaussian–Lorentzian, as expected from theoretical considerations. Highly resolved valence band (VB) spectra of non-conductor silicates also have been obtained (Nesbitt et al., 2004; Zakaznova-Herzog et al., 2005, 2006). The Kratos instrument uses a novel magnetic lens system to effectively eliminate differential charge broadening in non-conductors; and indeed, Nesbitt et al. (2004) showed that the same linewidth can be obtained on analogous semiconductor and bulk non-conductors. In addition, XPS valence bands of these minerals reveal many features heretofore unobserved, and high-quality pseudopotential density functional calculations on quartz, olivines and pyroxenes (Zakaznova-Herzog et al., 2005, 2006) reproduce well the valence band spectra of the three classes of minerals, thus providing insight into the origins of these details. This agreement gives us considerable confidence when interpreting the spectra of pristine and leached surfaces of these minerals. The greatly improved XPS spectra obtained for nonconductors are exploited in this study to probe the changes in surface composition of fractured pristine and leached forsteritic olivine, orthopyroxene and clinopyroxene. The O 1s XPS spectra, the VB spectra, and the theoretical calculations are combined to detect and characterize the surface complexes on these leached minerals, and to estimate the surface coverage of these complexes. 2. MATERIALS AND METHODS Mg-rich olivine, clinopyroxene (diopside) and orthopyroxene (bronzite) samples are of natural origin from the mineral collections at the University of Western Ontario and the South Australian Museum. Their composition was characterized structurally and compositionally by X-Ray diffraction and a JEOL JXA-8900 Superprobe Electron Microprobe with the following results: (Mg0.87Fe0.13)2SiO4, Ca(Mg0.8Fe0.2)Si2O6 and (Mg0.80Fe0.20)2Si2O6, respectively. The XPS spectra of these pristine (vacuum-fractured) minerals have been reported and interpreted elsewhere (Zakaznova-Herzog et al., 2005, 2006). The leaching experiments were performed at 22 C, in HCl solutions (weight of mineral was less than 0.02 g for

Characterization of leached layers on olivine and pyroxenes using XPS and DFT

50 ml of acid), on freshly fractured, air-cleaved surfaces; the reactor system was open and static. Olivine has imperfect cleavage (Deer et al., 1969); hence smooth surfaces are not attainable. Pyroxenes have one good cleavage, along with a number of parting planes (Deer et al., 1969). For good comparison of olivine and pyroxene spectra, no attempt was made to cleave the pyroxenes. All of our analyzed surfaces were visibly rough; and such surfaces are excellent tests of the XPS neutralization system. Eggleston et al. (1989) have emphasized that large freshly fractured surfaces should be used to avoid problems with many types of sample pre-treatments. The Mg-rich olivine was leached for 3 h, (pH 2.4), 27 h (pH 2.2) and 38 days (pH 2.1); the acid was not replaced because no pH changes were detected. Diopside was leached for 3 and 16 days (pH 2.1), whereas bronzite was leached for 24 h and 16 days (pH 2.1). In bronzite and diopside leaching experiments for longer than a day, the acid was replaced every 24 h acid with a new solution (from the original stock solution) to maintain the same pH throughout the leaching period. Continuous monitoring of the pH showed that the pH varied by no more than 0.2 over the duration of the experiment. XPS studies of pristine surfaces were conducted as follows: samples were cleaved in situ in the vacuum of the transfer chamber (low 108 torr range) system, and immediately transferred to the analytical chamber where pressures were in the low 109 torr range. Leached samples were rinsed with deionized water in the air, lightly dried with a tissue on all sides except the fractured side to be analyzed, and immediately transferred to the introduction chamber (all procedure taking 30 s–1 min). A Kratos Axis Ultra Xray photoelectron spectrometer (with magnetic confinement charge compensation system) at Surface Science Western (SSW) was used to collect the room temperature XPS spectra using Al-Ka radiation at 1487 eV, a 300-lm spot size, and take-off angle of 90 for all analyses. The assumed anal˚ (e.g., for SiO2 Hochella and Carim, ysis depth (3k) is 78 A 1988). A smaller take-off angle improves the surface sensitivity (Hellmann et al., 1990); however, our studied surfaces are visibly rough, as compared to the smooth feldspar surfaces studied by Hellmann et al., 1990). No good spectra could be consistently obtained on our rough surfaces with much smaller take-off angles. A 10-eV pass energy and 25-meV step were used to collect all core level spectra (Si 2p, Mg 2p, Ca 2p, Na 1s, and O 1s), and 20 eV pass energy and 50-meV step were employed to collect all valence band spectra. The instrumental resolution at 10 eV pass energy is about 0.35 eV (Nesbitt et al., 2004). Pass energy (160 eV) and 700-meV step were used to collect survey spectra. The low C 1s signal indicated minimal C contamination, even for leached surfaces. The charge compensation system was tested for effectiveness over a large range of settings with only small changes in linewidths. Optimal settings yielded Si 2p3/2 linewidths of 1.0–1.1 eV (1.2–1.4 eV for O 1s) and these were highly reproducible over months of testing. Differences in linewidths of 0.1 eV for the same mineral have been detected due to the variance in fractured surfaces, the smoother and flatter surfaces giving the narrower linewidths. This is expected because all charge neutralizer

71

systems work optimally on flat surfaces. Therefore, we report the narrowest spectra of multiple spectra. Even for rough surfaces, however, linewidths were broader by no more than 0.1 eV (compared with the smoothest surfaces). Binding energies (BE) for pristine (vacuum fractured) and leached samples were referenced to the adventitious C 1s peak at 285.0 eV; values (average for pristine samples) are given in the figures. For direct comparison or subtraction in figures leached and pristine spectra were, however, additionally refined by alignment of the Si 2p3/2 peak to the Si 2p3/2 peak position for the pristine sample (refinements ranged from 0.05 to 0.22 eV). This re-alignment is not surprising because C 1s peak BE may vary by up to 0.3 eV (Zakaznova-Herzog et al., 2005, 2006). Apparently, adventitious C is not always in good electrical contact with the mineral surface. The reported values for leached samples are given without alignment. As for previous papers (Nesbitt et al., 2004; ZakaznovaHerzog et al., 2005, 2006), core level spectra were fit with a 70% Gaussian–30% Lorentzian function. Spin–orbit components were constrained to the same linewidths and fixed to atomic values where the p1/2 peak was constrained to half the intensity of the p3/2 peak. The spin–orbit splitting parameters used are 0.617 eV for the Si 2p, 3.5 eV for Ca 2p, and 0.28 eV for Mg 2p. Because of the constraints on the p1/2 position, linewidth and intensity, just the p3/2 position and linewidth were iterated to minimize root mean square deviations for a given spectrum. The Si and Mg spin–orbit splittings were unresolved in these spectra, so that peak widths (FWHM) were evaluated using the best visual fit of the low energy side of the Si 2p and Mg 2p peaks. All spectra were corrected for the background using the Shirley approach (Shirley, 1972) and CasaXPS software was used to fit all the spectra. The O 1s spectra for pristine olivine were fitted with a very small asymmetric factor to account for the asymmetry on the high binding energy side of this spectrum. The two and three peak fits for pristine pyroxenes are taken from the previous publication (Zakaznova-Herzog et al., 2006). The O 1s spectra from leached olivine samples were fit with two peaks where the main contribution included the same asymmetric factor obtained from the fit to the O 1s spectrum of the pristine sample. The O 1s spectra of the leached pyroxenes were fitted with two peaks using constraints established from fitting the spectra of the pristine samples. For each O 1s spectrum of the leached pyroxenes, the two peaks were assigned the same linewidth, and the area of the high and low binding energy peaks were constrained to the ratio obtained from the fit to the spectra of the pristine samples. In addition, the distance between these two peaks was taken from the fit to the spectra of the pristine samples. With these constraints, the linewidth of the peaks then was adjusted to fit the low binding energy side of the spectrum and finally the intensity of the stronger (low energy) peak was adjusted to obtain the best least-squares fit. 3. THEORY Electronic structural calculations within the pseudopotential density function theory (DFT) with generalized gra-

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dient (GGA) approximation (Perdew et al., 1996) were performed with the program SIESTA (Ordejon et al., 1996; Soler et al., 2002) on forsterite (Mg2SiO4) (Zakaznova-Herzog et al., 2005) and two hypothetical leached forsterites. The calculations on the H+substituted olivines were performed on a periodic cell beginning with the experimental geometry of Mg2SiO4 (Smyth and Hazen, 1973; Wyckoff, 1981), which has four Mg2+ each in the M1 and M2 sites in the unit cell. One of the four Mg2+ ions in the respective site was removed and replaced with 2H+ ions. A 16 · 8 · 16 uniform k-point set was used in the computation of the electron density of states. Full-geometry optimization (cell parameters and atomic positions) of the H+-substituted olivines were then performed. The effect of core electrons is replaced by a Troullier–Martin type pseudopotential (Troullier and Martins, 1991). For the valence orbitals, Sankeydouble zeta basis sets (Sankey and Niklewski, 1989) augmented with a set of d-polarization functions were used for Si, O and Mg. This method gives accurate density of states (Sanchez-Portal et al., 1997), as indicated by the very good agreement found in our previous papers between the theoretical and observed valence band XPS spectra for quartz, olivines and pyroxenes (Zakaznova-Herzog et al., 2005, 2006). Following the Gelius model (Gelius, 1972), the valence level X-ray photoelectron spectra were calculated from the theoretical, projected density of states of individual atoms weighted by the corresponding theoretical atomic cross-sections (Yeh and Lindau, 1985). All peaks were broadened with a Gaussian experimental width of 0.37 eV, a value similar to the measured instrumental resolution of 0.35 eV. It should be noted that at finite temperature the H+ ions are highly fluxional around the substitution sites (Dawn and Tse, 2007). Since the present calculations are on the static structures only, any photoelectron peak assigned to the H  O bond is expected to somewhat broader than the theoretical prediction.

4. RESULTS AND DISCUSSION 4.1. Atomic ratios from broadscans of leached surfaces Atomic percentages (and ratios) of the major elements in pristine and leached samples are obtained from XPS broadscans (Table 1 and Fig. 1). Small C 1s signals were present in spectra of pristine and leached samples. However, the surface compositions of vacuum fractured minerals revealed no obvious compositional differences with small changes in the C 1s intensities from one sample to another. The reproducibility of the results is quite good (Fig. 1), and the errors in atomic percentages are generally less than 10%. The errors, however, are large compared with the differences in concentrations reported for pristine and leached samples (Table 1). The compositions (Fig. 1 and Table 1) of the vacuumfractured samples are close to those obtained by electron microprobe, with XPS atomic ratios generally within 10–20% of the microprobe results. For olivine, the microprobe analyses give Mg/Si, Fe/Si and Fe/Mg ratios of 1.74, 0.26, and 0.15, respectively, compared to the XPS values of 1.57 (±0.07), 0.28 (±0.06), and 0.18 (±0.04), respectively (Table 1). Atomic percentages for pristine and leached olivine are similar (Fig. 1 and Table 1), and although there is a suggestion that the Mg concentration decreased and the O concentration increased with leaching, these changes are not statistically significant. There is, for diopside, an appreciable (and statistically significant) decrease in the Ca concentration with leaching (Fig. 1), as indicated by decreases in Ca%, and Ca/Si and ratios (Table 1). There is also a suggestion (but not statistically significant) decrease in Mg% in diopside. Also, the Ca depletion is very similar after 3 and 16 days leaching, indicating that the thickness of the leached layer is constant over this period. This result is consistent with previous suggestions from solution analysis and

Table 1 Atomic ratios obtained from XPS survey spectra of vacuum-fractured and acid-leached Mg-rich olivine (Ol), diopside (Di) and bronzite (Opx) Mineral

na

Fe

O

Si

Mg

Mg/Si

O/Si

Fe/Si

Fe/Mg

Ol vac. fract. ±2r

6

4.1 ±0.6

58.4 ±4.4

14.6 ±1.8

22.9 ±2.8

1.57 ±0.07

4.00 ±0.81

0.28 ±0.06

0.18 ±0.04

Ol leach. 3 h Ol leach. 27 h Ol leach. 38 days

1 1 3

4.0 4.1 4.0

61.8 61.3 62.1

14 14 13.7

20.2 20.6 20.2

1.44 1.47 1.47

4.41 4.38 4.53

0.29 0.29 0.29

0.20 0.20 0.20

Di vac. fract. ±2r

4

2.8 ±1.22

58.2 ±1.05

20.1 ±1.30

5.8 ±0.77

0.5 ±0.36

12.6 ±0.81

0.29 ±0.03

2.90 ±0.23

0.14 ±0.06

3.4 2.3

62.1 59.6

20.3 21.8

5.0 6.1

n/d n/d

9.1 10.2

0.25 0.28

3.05 2.73

Di leach. 3 days Di leach. 16 days

2

Na

Ca

Ca/Si

Ca/Mg

0.49 ±0.29

0.63 ±0.07

2.17 ±0.29

0.17 0.11

0.68 0.38

0.45 0.47

1.83 1.67

Opx vac. fract. ±2r

7

4.5 ±1.3

61.7 ±6.7

19.7 ±4.8

14.1 ±2.9

0.71 ±0.04

3.12 ±1.07

0.23 ±0.12

0.33 ±0.16

Opx leach. 24 h Opx leach. 16 days

2 2

3.9 2.8

65.1 63.3

19.1 20.9

11.9 12.9

0.62 0.62

3.41 3.03

0.20 0.13

0.33 0.22

15.90 15.80

Time of leaching is indicated. Carbon contribution of the surface varied from 7% to 20%, however, no systematic effect on the compositions was observed in the reproducing runs. These atomic ratios were not affected by carbon. a n is the number of measurements, from which the average was calculated.

Characterization of leached layers on olivine and pyroxenes using XPS and DFT

Diopside

atomic %

Olivine

Bronzite

70

70

70

60

60

60

50

50

40

40

30

30

20

20

20

10

10

10

50

Fe O Si Mg Na Ca

40

30

0

0

0 1

1000

1000000

73

0

time (log(min))

10

time (days)

20

0

5

10 15 20

time (days)

Fig. 1. Atomic compositions of olivine, diopside and bronzite plotted versus leaching time. Vacuum-fractured composition is plotted at zero time. Please note that for olivine, a logarithmic scale is used for plotting clarity and marked with leaching time labels.

XPS studies on powdered samples where, after an initial preferential leaching of divalent cations from the surface, congruent dissolution occurs (Schott et al., 1981). There is also an indication in Table 1 that the Si and O atomic % increased with leaching of diopside, but these increases are not statistically significant. For bronzite, there are no statistically significant changes in the atomic compositions with leaching, although there are indications that the leached surface is weakly depleted in both Mg and Fe. Previous XPS studies (all on powder samples) are consistent with our results. However, they showed more significant changes in cation concentrations, but no errors were quoted. For example, the Mg/Si values decreased from 1.80 on a similar pristine olivine (Seyama et al., 1996; Pokrovsky and Schott, 2000a) to 1.47 (Pokrovsky and Schott, 2000a) and 1.2 (Seyama et al., 1996), indicating an Mg-deficient leached layer (or layers). Similarly in diopside, the Ca/ Si value decreased by about 30% after leaching for 2 or 22 days at pH 1 (Schott et al., 1981) compared to our decrease of about 20% (Table 1). These larger decreases are probably due to the much greater surface areas and surface irregularities of the powdered samples compared to our bulk fractured samples. 4.2. Core level Si 2p, Mg 2p and Ca 2p spectra High-resolution conventional XPS spectra of Si 2p, Mg 2p, Ca 2p and Na 1s lines of vacuum-fractured surfaces are shown on the left side of Figs. 2–4; from Zakaznova-Herzog et al., 2006); and the same spectra of some of the leached samples are shown on the right side of Figs. 2–4. The fits to the spectra of both pristine and leached samples are reasonable, with the exception of the poor fit at the high binding energy tail on the Ca 2p spectrum (2p3/2:2p1/2

ratio = 2:1, Fig. 4a and c); the broad Mg KLL Auger peak overlaps the Ca 2p1/2 peak at this photon energy (Zakaznova-Herzog et al., 2006). Zakaznova-Herzog et al. (2006) demonstrated that olivine and pyroxenes display the same BE for all spectral lines except for Si 2p, with the olivine Si 2p BE being located at about 1 eV lower BE than that for the pyroxenes. Each spectrum for leached samples are similar to the respective spectrum of the associated pristine sample: the peak binding energies are within 0.1 eV, lineshapes are the same and the spectra of the leached samples are consistently slightly broader than those of the associated pristine samples (less than 0.15 eV broad, with errors of less than 0.05 eV). The results demonstrate that the charge neutralizer works extremely well for both pristine-fractured surfaces and surfaces exposed to acidic solutions for many days. The leached samples, exposed to air for about 1 min while transferring the sample from water to the vacuum system, have a significantly higher C contaminant signal. The slightly larger linewidths are probably due either to the higher C 1s signal on the leached surfaces, or the different surface roughness after leaching with more distinct surface atomic environments. The only indication of any change in these spectra from the pristine spectra is given by the Si 2p spectrum of olivine (Fig. 2d). The fit to the Si 2p spectrum of Fig. 2d is not entirely satisfactory at high binding energy, perhaps indicating another surface Si species (e.g., Si-OH) with a BE of about 103 eV (see the arrow in Fig. 2d). Alternatively, the tail might be due to a slight charge broadening on this surface. Paparazzo et al. (1992) demonstrated that the Si 2p peak from Si-OH species on aged silica was located at 105.2 eV. However, there is no indication of such a peak in our Si 2p spectra. The Si 2p, Mg 2p, and Ca 2p XPS

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Ol vac. frac. FWHM3/2,1/2=0.96eV 101.7 eV FWHMtotal=1.26eV

Ol leached 27 h FWHM3/2,1/2=1.08eV FWHMtotal=1.31eV

101.8 eV

3/2 3/2

1/2

1/2

Di leached 3d FWHM3/2,1/2=1.19eV FWHMtotal=1.39eV

Di vac. frac. FWHM3/2,1/2=1.04eV FWHMtotal=1.27eV

102.8 eV

Intensity

102.7 eV

3/2

3/2

1/2

1/2

Opx leached 16 d FWHM3/2,1/2=1.25eV FWHMtotal=1.44eV

Opx vac. frac. FWHM3/2,1/2=1.17eV FWHMtotal=1.38eV 102.7 eV

102.6 eV 3/2

3/2

1/2

1/2 108

106

104

102

100

98

108

106

104

102

100

98

Binding energy (eV) Fig. 2. The Si 2p XPS spectra of pristine (dots and fit) and leached (empty dots), (a) Mg-rich olivine, (b) diopside and (c) bronzite. The Si 2p spectra of leached samples fitted on (d) Mg-reach olivine (27 h leached), (e) diopside (3 days leached), (f) bronzite (Opx) (16 days leached). Underlined numbers are binding energy positions of Si 2p peak fitted without spin–orbit splitting.

spectra of olivine and pyroxenes apparently yield little information related to formation of new surface species on these leached surfaces (Figs. 2–4). 4.3. The O 1s spectra 4.3.1. Olivine The O1s spectrum of the vacuum-fractured, pristine olivine is illustrated in Fig. 5a (black dots). An intense peak at 531 eV has been fitted to the data, and this peak arises

from the the non-bridging oxygen (NBO), or the Si–O–M moiety in bulk olivine (Fig. 5a, peak 1). There is also a small tail on the high binding energy side of the spectrum which cannot be accounted for with just peak 1; and as a result, a second peak has been added to fit this tail (Fig. 5a, peak 2). The sum of the two peaks (the envelope) includes the entire O 1s signal from pristine olivine, and this asymmetric ‘‘envelope’’ represents the O 1s signal of pristine olivine in Fig. 6a–c. The origin of the second peak is not obvious.

Characterization of leached layers on olivine and pyroxenes using XPS and DFT

Ol leached 27 h FWHM3/2,1/2=1.03eV FWHMtotal=1.08eV

Ol vac. frac. FWHM3/2,1/2=0.97eV FWHMtotal=1.05eV

50.3 eV

50.2 eV

3/2

3/2

1/2

1/2

Di vac. frac. FWHM3/2,1/2=1.15eV FWHMtot=1.19eV

Intensity

75

Di leached 3 d FWHM3/2,1/2=1.15eV FWHMtot=1.20eV

50.3 eV

50.4 eV

3/2

3/2

e

1/2

1/2

Opx vac. frac. FWHM3/2,1/2=1.44eV FWHMtot=1.48eV

Opx leached 16 d FWHM3/2,1/2=1.41eV FWHMtot=1.45eV

50.5 eV

f

50.5 eV

3/2

3/2

1/2

1/2 54 53 52 51 50 49 48 47 46

54 53 52 51 50 49 48 47 46

Binding energy (eV) Fig. 3. The Mg 2p XPS spectra of pristine (dots and fit) and leached (empty dots) (a) olivine, (b) diopside and (c) bronzite. The Mg 2p spectra of leached samples fitted for (d) olivine (27 h leached), (e) diopside (3 days leached), (f) bronzite (Opx) (16 days leached).

It may be caused by: an Si–O–Si moiety (surface-polymerized SiO4 tetrahedra) which would give a BE close to the quartz value of 532.8 eV; an Si–O–H surface moiety derived from H2O from the residual gases in the vacuum; or peak 1, which represents the Si–O–M moiety, may be asymmetric due to a large vibrational manifold derived from ion state vibrational splitting, which by its nature is asymmetric (Nesbitt et al., 2004, Nesbitt and Dalby, 2007). To obtain the intensity of the new O 1s peak, the O 1s spectra of leached olivine samples (Fig. 6b and c) were scaled to match the maximum intensity of the pristine spectrum (Fig. 6a) and the scaled results are plotted on Fig. 6b

and c. To facilitate direct comparison of leached and pristine samples, the asymmetric envelope representing the pristine spectrum is also plotted on Fig. 6a–c, and differences (in counts) obtained by subtracting the asymmetric envelope from the data shown in each diagram (difference spectra shown at bottom). As apparent from the difference spectra, there is additional intensity on the high binding side of the peak of each leached sample. The additional contribution is located at about 532.2–532.7 eV with a linewidth of 1.2–1.6 eV. The peak in the difference spectra for 27 h leaching is well-defined at 532.2 eV. This peak at 532.2 eV is assigned to an Si-OH at the surface, and the

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Di vac. frac. 347.9 eV Ca 2p FWHM3/2,1/2=1.13eV

Di leached 3d Ca 2p FWHM3/2,1/2=1.20eV

3/2

3/2

Mg KLL

Mg KLL

1/2

Intensity

1/2 356

354

347.9 eV

352

350

348

346

344

356

354

352

350

348

346

344

Di leached 3d Na 1s

Di vac. frac. Na 1s FWHM=1.66eV 1072.4 eV

1078 1076 1074 1072 1070 1068 1066

Binding energy (eV)

1078 1076 1074 1072 1070 1068 1066

Binding energy (eV)

Fig. 4. The Ca 2p (a) and Na 1s (b) XPS spectra of pristine (dots and fit) and leached (empty dots) diopside. The fitted Ca 2p (c) and Na 1s (d) spectra of 3 days leached diopside.

position is very similar to the OH-position of OH on SiO2 (Duval et al., 2002), and also OH on other oxides such as MgO (Lazarov et al., 2005). The area of the new OH peak for the 27-h leached sample is about 7% of the total O 1s intensity of Fig. 6c. A surface monolayer of S in FeS2 yields about 10% of the total S 2p intensity, (at about 1320 eV kinetic energy, Nesbitt and Muir, 1994), and this is very similar to that expected from the first monolayer of SiO2 from the escape depth quoted earlier (Hochella and Carim, 1988). Thus our results indicate no less than a monolayer of OH on these olivine surfaces. These O 1s spectra, along with the Si 2p spectra, show no evidence for formation of Si–O–Si moieties, as suggested in previous papers from much broader O 1s spectra (e.g., Pokrovsky and Schott, 2000a). The O 1s BE for peak 2 in the leached samples of 532.2 eV is substantially lower than that for Si–O–Si groups in quartz or pyroxenes (Zakaznova-Herzog et al., 2005, 2006), and we would expect a Si– O–Si layer to increase in thickness with increase in leaching time until a steady state thickness is obtained. 4.3.2. Diopside The O1s spectrum of vacuum-fractured, pristine diopside shows two distinct peaks (Fig. 5b), one representing bridging oxygen (BO) and the other non-bridging oxygen

(NBO). There are, however, three equally abundant but energetically distinct oxygen sites in the mineral, a Si–O– Si, Ca–O–Si and an Mg–O–Si moiety. Zakaznova-Herzog et al. (2006)) consequently, fit three equally intense peaks (same areas) to the O 1s spectrum (Fig. 5b), the Si–O–Si bridging oxygen contribution at about 532.8 eV (peak 3, Fig. 5b), the Mg–O–Si (NBO) peak (Mg in the M1 site) at about 531.5 eV (peak 2), and the Ca–O–Si (Ca in the M2 site) NBO peak at 531.1 eV (peak 1). A simpler, but arbitrary, two-peak fit to the diopside O 1s spectrum also reproduces the O 1s pristine diopside spectrum well (Fig. 7a), where the two fitted peaks are constrained to the same linewidth. This empirical fit yields an NBO:BO intensity ratio of 1.6:1.0, which is not realistic because the structural ratio should be 2:1. Nevertheless, this simple fit faithfully reproduces the pristine diopside spectral data of Fig. 7a. The pristine diopside O 1s spectrum has been simulated and plotted in Fig. 7b and c to facilitate comparison with the leached diopside spectra This simulated spectrum is constrained as follows: the linewidths of the two peaks are set equal; the BE separating the two peaks is fixed and derived from the fit in Fig. 7a; and the peak ratio is set to 1.6:1.0. Although the FWHM of the two peaks are constrained to the same value, this value was determined so as to fit accurately the low binding

Characterization of leached layers on olivine and pyroxenes using XPS and DFT

Mg-rich olivine FWHM1 =1.21eV FWHM2 =2.02eV

Diopside FWHM1 =1.11eV FWHM2 =1.41eV FWHM3 =1.14eV

531.0 eV

16 days

77

Bronzite (Opx) FWHM1 =1.16eV FWHM2 =1.51eV FWHM3 =1.40eV

24 hours

3 days

16 days

Intensity

pristine 1

532.8 eV 2

3

pristine

531.1 eV

1

532.5 eV

531.5 eV

531.1 eV

3 2 531.6 eV

1

27 hours 3 hours vac.fract.

536

2

534

532

530

528

536

534

532

530

528 536

534

532

530

528

Binding energy (eV) Fig. 5. The O 1s XPS spectra of pristine (dots and fits) and leached (empty dots) of (a) olivine, (b) diopside and (c) bronzite. Note the assignments: for diopside, peak 1 to NBO in M2, peak 2 to NBO in M1 and peak 3 to BO; for bronzite, peak 1 to NBO in M1, peak 2 to NBO in M2, and peak 3 to BO.

5000

Ol vac. fract. FWHM1=1.21eV

5000

Ol leached for 3 hours FWHM1=1.21eV

531.1 eV

531.0 eV

Counts

4000

Ol leached for 27 hours FWHM1=1.21eV

531.0 eV

4000 4000 3000

3000

3000

2000 2000

1

1000

Diff., counts

538

1000

1000 536

534

532

530

528

500

538

536

534

532

530

528

500

538

0

0

0

-500 538

-500 538

534

532

530

528

536

534

532

530

528

536

534

532

530

528

500

-500 538

536

1

2000

1

536

534

532

530

528

Binding energy (eV) Fig. 6. The O 1s spectra of pristine (a) and leached (b and c) olivine. The asymmetric ‘‘envelope’’ obtained by fitting the data for pristine olivine (Figs. 5a and 6b) has been subtracted from each spectrum of the two leached samples to obtain difference spectra in counts (plotted below the spectra). Scaling of the leached spectra to the peak maximum of the pristine spectrum was required to obtain the difference spectra.

energy side of the leached spectra (Fig. 7b and c), leading to a FWHM approx. 0.1 eV broader than that obtained by the two-peak fit to the pristine sample (Fig. 7a). The O 1s spectra of leached diopside show additional intensity on the high binding energy side of the major (NBO) peak (Fig. 7b and c). Broadening of the O 1s signal

for leached diopside consequently is minor and consistent with the small amount of broadening seen on all other peaks in these leached samples (Figs. 2–4). Most importantly, broadening does not explain the additional intensity in the 532–533 eV BE range observed for the leached samples. The most reasonable explanation for the additional

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V.P. Zakaznova-Herzog et al. / Geochimica et Cosmochimica Acta 72 (2008) 69–86 11000

5500

O 1s, Diopside 10000 FWHM1,2=1.26 eV ΔBE1,2=1.52 eV

Di leached for 3 days 5000 FWHM1,2=1.36eV

10000

Di leached for 16 days 9000 FWHM1,2=1.38 eV

531.2 eV

9000

4500

8000

4000

7000

3500

531.2 eV

8000

531.3 eV

Counts

7000

6000

532.8 eV

2

3000

1500

2000

2000

1000

1000 536

534

532

530

528

500

538

536

534

532

530

528

1000

538

1000

1000

2000

0

500

1000

536

534

532

530

528

2

4000

2000

3000

Diff., counts

1

2

2500

1

4000

-1000 538

532.8 eV

1

5000

5000

538

6000

532.8 eV

3000

0 538

536

534

532

530

528

0 538

536

534

532

530

528

536

534

532

530

528

Binding energy (eV) 3500

4000

O 1s, Opx FWHM1,2=1.38 eV 3000 ΔBE 1,2=1.23 eV

Opx leached for 24 hours FWHM1,2=1.39eV 531.3 eV

3000

532.5 eV

6000 532.5 eV

2000

1

531.3 eV

7000

2500 2000

FWHM1,2=1.39 eV

531.2 eV 8000

2500

Counts

3500

9000 Opx leached for 16 days

1

532.6 eV

5000

1

1500 2

2

1500

2

4000

1000

3000 1000 2000

500 500

Diff., counts

538

536

534

532

530

528

538

536

534

532

530

528

538

536

534

532

530

528

536

534

532

530

528

500 500

1000

0 -500 538

0 536

534

532

530

528

538

536

534

532

530

528

0 538

Binding energy (eV) Fig. 7. The O 1s spectra of pristine (a) and leached (b and c) diopside and pristine (d) and leached (e and f) bronzite. Two peak fits (equal widths) are shown. For leached samples, the distances and ratio between peaks were additionally constrained to be equal to the fit to the pristine sample [(a) and (d) for diopside and bronzite, respectively]. Differences in counts are given below. Binding energy positions for peaks are given as underlined.

intensity of the leached samples is generation of a new O 1s peak to the spectrum after leaching. The spectral characteristics of the new contribution are evaluated by taking difference spectra. The pristine diopside

spectrum (represented by the sum of peaks 1 and 2 of Fig. 7b and c) was subtracted from spectra of leached samples. Before subtraction, however, the O 1s spectra of the two leached samples were scaled to the maximum intensity

Characterization of leached layers on olivine and pyroxenes using XPS and DFT

of the simulated O 1s spectrum, broadened to fit low binding energy edge as described above (shown in Fig. 7b and c), and the difference then taken (differences in counts are plotted below each spectrum). The difference spectra reveal a substantial contribution with maximum intensity at about 532.2 eV, which coincides with the BE of the OH peak in the olivine difference spectrum, although considerably more intense (12% of the total O 1s intensity in Fig. 7b, compared to 7 % in olivine, Fig. 6c). This peak likely is due to the formation of OH in the near-surface of diopside, just as observed on the olivine surface. There is, in addition, a minor contribution near 534 eV (Fig. 7b and c), and this may be due to water adsorbed to the surface (Knipe et al., 1995; Liu et al., 1998). The leached olivine spectrum shows no indication of a water species on the surface. It is also noticeable from Fig. 7b and c that the intensity of the extra OH peak in diopside spectra is relatively greater after 3 days leaching (12%) than after 16 days leaching. (9%). A much more complete kinetic study now needs to be performed to characterize the intensity of the OH peak as a function of time. 4.3.3. Bronzite Zakaznova-Herzog et al. (2006) based on the structure of orthopyroxene, fitted the vacuum-fractured, pristine bronzite O1s spectrum with three peaks, a BO and two NBO peaks. The fit is reproduced in Fig. 5c. As for leached-diopside, leached-bronzite displays additional spectral intensity on the high binding energy side of the most intense (NBO) peak (compare Fig. 7d–f). The pristine bronzite spectrum was simulated with a two-peak fit (Fig. 7d), and the simulated spectrum was constrained in precisely the same manner as the simulated diopside peak. The pristine bronzite O 1s simulated spectrum is shown in (Fig. 7e and f). Difference spectra of the leached and pristine spectra then were obtained using the same method outlined for the diopside difference spectra, and they are plotted below the associated spectrum of Fig. 7d–f. The maxima in the bronzite difference spectra are located at 532.3 eV, just as observed in the leached diopside difference spectra (Fig. 7b and c). Again, as for diopside, the peak is assigned to OH at the mineral surface; and as for diopside, the intensity of the new peak is about 12% of the total O 1s intensity in both spectra Fig. 7e and f. As for diopside, there is a contribution near 534 eV in the difference spectra, which might be due to water adhered to the surface of bronzite.

79

overlaps strongly with the O 2s peak decreases on leaching as observed for the Ca 2p intensities (Section 4.1). A fit to the Ca 3p/O 2s peaks is shown in the insert, with peaks 1 and 2 being due to O 2s. Second, there is no hint of the quartz peak 5 at 15 eV in the leached olivine spectrum (see vertical dashed lines), showing immediately that there is not a significant SiO2 layer on the olivine surface. There are other more subtle changes in the leached spectra. For example, there is a small shoulder on peak 5 of bronzite at 14 eV, and a hint of a peak in olivine at about 13 eV, marked with an ovals at around 13–15 eV. There are also subtle differences around peak 3 in the bronzite and olivine VBs which may be due to minor decrease in Ca 4p and Mg 3p valence electron density on the leached samples (note that Ca 4p and Mg 3p were labeled incorrectly in previous papers by Zakaznova-Herzog et al., 2005, 2006). The O 2s peaks are all broad and asymmetric due to a breakdown of the one electron approximation as noted in our previous paper (Zakaznova-Herzog et al., 2006). There is also a significant broadening in the wings of the O 2s peak in olivine after leaching, and we do not understand this broadening. To characterize the relatively simple leached olivine spectra more closely, Fig. 9 shows a detailed view of the leached olivine VBs. There is indeed a weak peak at about 13.7 eV (note the ovals) in all the spectra of leached samples, and this peak is absent from the pristine spectrum (Fig. 9, bottom). This small peak, and the small increase in the shoulder on the bronzite spectrum at about 14 eV, is probably also associated with the surface OH species identified in the O 1s spectra; and this peak is investigated in Section 4.6 with theoretical calculations. 4.5. Mechanism for the formation of the OH layer The above observations and interpretation of the spectral changes associated with acid leaching are consistent with exchange of Ca2+and/or Mg2+ for 2H+ proposed in many previous studies, via electrophilic attack of H+or H3O+ on Non-Bridging Oxygen atoms (NBO) of the M– O–Si moieties to cleave the O–M bond (Budd, 1961; Blum and Lasaga, 1988; Rosso and Rimstidt, 2000; Oelkers and Schott, 2001; Liu et al., 2006): 2Hþ + Si–O–M ! M2þ + Si–O–H2

ð1Þ

With regard to olivine, the above general reaction may be represented by the stoichiometric reaction: 2Hþ + Mg2 SiO4 ! Mg2þ + H2 Mg(M1)SiO4ðsurfÞ

4.4. Experimental valence bands The valence band (VB) background corrected spectra (including the O 2s region) of leached olivine, pyroxenes, and (previously published) spectra of pristine pyroxenes, quartz and olivine (Zakaznova-Herzog et al., 2005, 2006), are shown in Fig. 8. Valence bands were referenced to the C 1s peak at 285 eV and the intensity of the valence bands of leached samples were normalized to the intensity of peak 5 in the respective pristine sample. There are two important observations from these spectra. First, the intense Ca 3p peak of diopside (the most intense peak in that spectrum at about 26 eV BE), which

+

ð2Þ 2+

where 2H has preferentially exchanged with Mg of M2 sites (Rosso and Rimstidt, 2000; Liu et al., 2006) because the M2 site is more distorted and less stable than the M1 site. Preferential leaching of the M2 site leads initially to incongruent dissolution with preferential release of Mg2+ relative to SiO2(aq). This electrophilic exchange reaction is followed by a second electrophilic reaction, where the H2Mg(M1)SiO4(surf) in Eq. (2) is attacked by two additional H+ ions (where the H+ ions attack the NBO associated with Mg2+ in the M1 site): H2 Mg(M1)SiO4 + 2Hþ ! Mg2þ + H4 SiO4ðsolÞ

ð3Þ

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V.P. Zakaznova-Herzog et al. / Geochimica et Cosmochimica Acta 72 (2008) 69–86

O 2s

Quartz

5 4

3

1

2

Diopside

Intensity

Ca 3p 3d

1

2

16d 30

26

22

18

3 4

5

16d

2? 1

Bronzite 3

4 5

2?

1

5

Mg-rich olivine

4

3

2

1

O 2s 35

30

25

20

15

10

5

0

Binding energy (eV) Fig. 8. The O 2s and valence band XPS spectra (VBXPS) of pristine (black dots and black line) and leached (coloured dots and solid colored lines): leached Mg-rich olivine for 27 h (red), leached bronzite for 16 days and 24 h (red and blue, respectively), leached diopside for 16 days and 3 days (red and blue, respectively). In addition, the spectrum of vacuum fractured quartz is given for comparison at the top. The insert indicated by an arrow near diopside’s valence band is the Ca 3p peak at about 26 eV BE overlapping with O 2s peak of diopside represented by peaks 1 and 2. Changes in valence bands with leaching are indicated with ovals. Please note that all spectra were normalized by peak 5. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Characterization of leached layers on olivine and pyroxenes using XPS and DFT

81

Olivine

Intensity

38 days leached

27 hours leached

3 hours leached

Vac. fract

14

.

12

10

8

6

4

2

0

Binding energy (eV) Fig. 9. Detailed valence band spectra of vacuum fractured and leached Mg-rich olivine (3, 27 h and 38 days). Note the small peak at about 13.5 eV BE for the leached samples.

As this second electrophilic reaction proceeds, all Mg2+ of both the M1 and M2 sites are exchanged, all Mg–O–Si bonds around the surface SiO4 tetrahedra are ruptured and these (surface) tetrahedra are then released to solution. Once steady state is achieved (both reactions proceed at the same rate) congruent dissolution ensues as observed in most leaching studies (eg. Pokrovsky and Schott, 2000b; Hanchen et al., 2006). By this mechanism, the electrophilic reaction alone leads initially to preferential release of Mg2+ to solution and to congruent dissolution once steady state is achieved. The initial reaction for pyroxene dissolution has been suggested to be analogous to Eq. (2) for olivine, from XPS and kinetic studies (Schott et al., 1981; Oelkers and

Schott, 2001). Considering bronzite (Oelkers and Schott, 2001), 2H+exchange via an electrophilic reaction with the Mg2+ in M1 and M2 sites, with the reaction at M2 being the more rapid: Mg2 Si2 O6 + 2Hþ ! Mg2þ + H2 Mg(M1)Si2 O6ðsurfÞ

ð4Þ

Although, Oelkers and Schott (2001) do not distinguish between the two electrophilic reactions, the slower reaction at M1 yields a different near-surface complex according to: H2 MgSi2 O6ðsurfÞ + 2Hþ ! Mg2þ + H4 Si2 O6ðsurfÞ

ð5Þ

Pyroxenes differ from olivines by the presence of bridging oxygen (BO) atoms (i.e., the Si–O–Si moiety, Fig. 10). As simply and elegantly stated by Budd (1961) for silicate

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V.P. Zakaznova-Herzog et al. / Geochimica et Cosmochimica Acta 72 (2008) 69–86

a

c

b H2O

OH

-

Hydrogen

Silicon Oxygen

surface

Nucleophylic of H2O on Si

H4SiO4(aq) new surface

Ca Mg

M1 Site 2+ (Mg )

M2 Site (Ca2+)

Hydroxylated surface but no electrophylic or nucleophylic reactions

Both electrophylic reactions proceed (dashed lines indicate rxn. fronts) but no nucleophylic reaction has occurred

Two electrophylic and a nucleophylic reactions have occurred

Fig. 10. Schematic illustrating the important aspects of the leaching mechanism proposed for the pyroxenes: (a) idealized schematic of the pyroxene structure illustrating the tetrahedral chain and the M1 and M2 sites. The surface oxygen atom is hydroxylated; (b) idealized schematic illustrating the two electrophilic reactions where reaction at M2 is the more rapid, and the location of the nucleophilic attack by a water molecule; (c) idealized schematic illustrating the effect of the nucleophilic reaction with release of H4SiO4 to solution.

glasses using classical electrophilic and nucleophilic reactions (and also by Oelkers and Schott, 2001 for orthopyroxene, and Xiao and Lasaga, 1994 for feldspars), the bonding in this moiety is likely to be subject to nucleophilic attack in acid where an H2O (or H3O+), Xiao and Lasaga, 1994 attacks a tetrahedrally coordinated Si atom producing a 5coordinated transitional state (Budd, 1961; Xiao and Lasaga, 1994), leading ultimately to rupture of an Si–O bond and release of H4SiO4 to the leaching solution (Fig. 10c). As shown schematically in Fig. 10 for H2O as the nucleophilic agent, two OH groups become bonded to two Si atoms. We can only describe this reaction qualitatively with a schematic diagram because of our rough surfaces with unknown orientation. The nucleophilic reaction apparently proceeds less rapidly than either of the electrophilic reactions as indicated by initial release rates of Ca, Mg and Si to solution in acidic leaching studies, that is the nucleophilic reaction is rate-determining (Oelkers and Schott, 2001). The enhanced OHO 1s intensity in bronzite (12%) relative to olivine (7%) strongly suggests that the nucleophilic attack is on the H4Si2O6(surf) species which has four OH, rather than the H2Mg(M1)Si2O6(surf) species which only has two OH, as for the olivine surface complex H2Mg(M1)SiO4(surf). A diopside leaching mechanism can be obtained by analogy with the enstatite mechanism, and is shown schematically in Fig. 10. All diopside acidic leaching studies (including ours) show Ca2+ to be preferentially leached from the M2 site relative to Mg in the M1 site (eg. Schott et al., 1981; Knauss et al., 1993); the M2 site is more susceptible to electrophilic reaction than is the M1 site. The reaction begins with exchange of Ca2+ for 2H+ to form a near-surface H–O–Si moiety at the expense of a Ca–O–Si

moiety, thus forming a surface complex, H2Mg(M1)Si2O6 (Fig. 10b), as in Eq. (4): CaMgSi2 O6 +2Hþ ! Ca2þ + H2 Mg(M1)Si2 O6ðsurfÞ

ð6Þ

As for bronzite, a slower electrophilic reaction occurs at the M1 site involving the M2–O–Si moiety where H+ is exchanged for Mg2+ so that a second surface complex is formed (Fig. 10b) as in Eq. (5). Mg2+ is released to solution more slowly than Ca during the initial stages of leaching, as observed in experiment (e.g., Knauss et al., 1993). The nucleophilic attack of OH or H2O on the Si–O bond on mainly the H4Si2O6(surf) complex then yields H4SiO4 as shown in Fig. 10c. 4.6. Predictions and XPS results A major difference between the olivine and pyroxene mechanisms is the nucleophilic reaction required to rupture the Si–O–Si bonds and release aqueous silica to solution (Fig. 10c). The two diopside electrophilic reactions are proposed to proceed more rapidly than the nucleophilic reaction (previously proposed for enstatite by Oelkers and Schott, 2001), and they probably must proceed before the nucleophilic reaction is effective, leading to an hydroxylated layer extending below the surface of the mineral (Fig. 10b). The thickness of the hydroxylated layer is determined by the relative rates of the electrophilic and nucleophilic reactions (e.g., Fig. 10b). If, as implied, the nucleophilic reaction is rate-limiting for diopside leaching, but is absent for olivine leaching, the electrophilic fronts should be deeper into the pyroxenes than in olivine. This prediction is consistent with the XPS results; the OH signal (peak) is weak in the leached olivine O 1s spectra (difference spectra

Characterization of leached layers on olivine and pyroxenes using XPS and DFT

below Fig. 6a and b), but is more intense in the leached diopside and bronzite O 1s spectra (difference spectra below Fig. 7b, c, e, and f). These O 1s XPS data demonstrate a greater degree of hydroxylation at the pyroxene surface, hence the electrophilic reactions proceed to greater depth in the pyroxene than in olivine. With respect to the valence band spectra, the Ca 3p peak is less intense relative to the O 2s signal (at 23 eV) in leached samples compared with the pristine sample, as expected where the electrophilic reaction proceeds more rapidly than the nucleophilic reaction. Interestingly, the upper valence bands for diopside, bronzite and olivine change little with leaching (the exception being a weak, broad contribution near 13.5 eV which is discussed subsequently) even though both electrophilic and nucleophilic reactions might be expected to quench some ‘‘molecular orbital contributions’’ and create others as reactions produce new surface complexes. Band theoretical calculations for olivine and diopside (Zakaznova-Herzog et al., 2005, 2006) and earlier X-a calculations (Johnson, 1973) on Na and Li silicates Ching et al., 1983, 1985) demonstrate that Mg–O and Ca–O bonds are highly ionic. With respect to Mg, Ca and O electronic contributions to the valence band, effectively all (more than 95%) are of O 2p character with little Mg 3s/3d or Ca 4s/4d character; that is, effectively all valence electrons of Mg and Ca have been effectively transferred to, and reside on, orbitals of predominantly O 2p character. As a result, exchange of H+ for Ca2+ or Mg2+ have little effect on the upper valence band because O 2p-like electrons are unaffected by the exchange. The same conclusions hold for Na and Li metasilicates (Na2SiO3 and Li2SiO3, Ching et al., 1983, 1985). 4.7. Density functional calculations on surface complexes Previous density functional calculations on pristine (unleached) olivine and pyroxenes are in reasonable agreement with the experimental olivine and pyroxene VB spectra (Zakaznova-Herzog et al., 2005, 2006). In a further attempt to confirm the existence of H-bearing surface complexes at olivine and pyroxene leached surfaces, density functional calculations were preformed on the two Hsubstituted complexes, in which just one of the four Mg2+ in M1 and M2, respectively, in the olivine unit cell are replaced by 2H+. As for the pristine minerals, the theoretical VB spectra are obtained by convoluting the calculated DOS with theoretical XPS cross-sections of orbitals (Fig. 11a, top two spectra). The calculated VB of pristine olivine is shown as the bottom spectrum of Fig. 11a (ZakaznovaHerzog et al., 2005). The calculated density of states (DOS) for these two H-substituted species (somewhat surprisingly) indicates Si 3s intensity in the DOS above 12 eV for both species. The calculated VB for one H-substituted olivine shows a peak at 12.9 eV (2H+ substituted for one Mg2+ on the M2 site) (Fig. 11a, top), and the VB of the other H-substituted olivine (2H+substituted for one Mg2+ on the M1 site) shows a peak at 12.2 eV. (Fig. 11a, middle). The observed peak position of 13.5 eV (Fig. 11b, middle), agrees better with the observed peak position of 12.9 eV when Mg2+ in M2 is replaced by 2H+. Note that the pris-

83

tine olivine spectrum (Fig. 11b, bottom) shows no sign of a peak in the 12.5–14 eV region, as expected because the pristine surface has no OH. In an attempt to simulate a VB spectrum from these theoretical spectra with one monolayer of the surface complex H2Mg(M1)SiO4(surf) on bulk olivine, consider that the intensity of the surface complex will be about 10% (previously observed for pyrite, Nesbitt and Muir, 1994) of the overall VB signal at these large kinetic energies (1480 eV). Since the calculation is done for with only 1/4 of the Mg2+ in a unit cell substituted by 2H+, the overall contribution of the surface complex H2Mg(M1)SiO4(surf) to the VB spectrum will be somewhat larger than 10% depending on the crystal face analyzed; and we arbitrarily choose 15% for our theoretical spectrum. The spectrum obtained by using 15% of the intensity of Fig. 11a, top and 85% of the intensity of Fig. 11a, bottom is given in Fig. 11b, top. The overall intensity of the 12.9 eV feature in this spectrum (Fig. 11b, top) is very similar to the intensity of the 13.5 eV peak in the experimental spectrum; in both cases the peak is very weak. The experimental peak appears to be broader than the theoretical, and this broadening could be due to rapid ‘‘hopping’’ of H among oxygen atoms on the M sites (Dawn and Tse, 2007). The 13.5-eV peak therefore confirms the essential veracity of the calculations, the presence of an hydroxylated surface complex on olivine leached surfaces, and supports previous suggestions that H-exchange occurs in the nearsurface of olivine and pyroxenes (Pokrovsky and Schott, 2000a; Rosso and Rimstidt, 2000; Liu et al., 2006). To summarize, the combination of the O 1s, and VB spectra with theoretical calculations, provides good evidence for the surface complex H2Mg(M1)SiO4(surf) on olivine as suggested by Rosso and Rimstidt (2000) and Liu et al. (2006). Our pyroxene results re-enforce the model for enstatite leaching proposed by Oelkers and Schott (2001), and are consistent with the first stages of diopside leaching proposed by Schott et al., 1981. Both the O 1s and VB spectra strongly suggest that the near-surface hydroxylated complex never increases to greater than the equivalent of a monolayer or two, as indicated in the mechanism given by Eqs. (2) and (3) and as shown in Fig. 10, again consistent with the thicknesses proposed by Pokrovsky and Schott (2000a). 5. CONCLUSIONS Using high-resolution XPS spectra, we report clear new spectroscopic evidence for the existence of surface complexes after leaching of fractured non-conductor olivine and pyroxene surfaces. All core and valence band XPS spectra for the three leached minerals are of very similar quality to the spectra from fractured in-vacuum surfaces. The O1s spectra of leached surfaces show a new weak peak at 532.2 eV due to monolayer OH coverage to give surface complexes such as H2Mg(M1)SiO4(surf) for olivine, and mainly H4Si2O6(surf) for pyroxenes by the electrophilic reaction of 2H+ in solution with cations such as Mg2+ (and Ca2+) in the M2 sites. These surface complexes appear to be stable in the high vacuum XPS. The relative intensities for the OH signals in the O 1s spectra in the olivine and

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a H2MgSiO4

M2 substitution

M1 substitution

Intensity

Mg2SiO4 14

12

10

8

6

4

2

0

b 15% of calc. H2MgSiO4 and 85% of calc. Mg2SiO4

sum of all leached olivine VB

Vac. fract. olivine VB

14

12

10

8

6

4

2

0

Binding energy (eV) Fig. 11. (a) Theoretical calculations of the three VBXPS spectra of an olivine with one of four Mg2+ in M2 in the unit cell substituted by 2H+ (top), an olivine with one of four Mg2+ in M1 in the unit cell substituted by 2H+ (middle) and pristine Mg2SiO4 (bottom) (Zakaznova-Herzog et al., 2005). (b) Theoretical plot of 85% of the intensity of the calculated Mg2SiO4 VB spectrum plus 15% of the intensity of the calculated VB spectrum (intensities again normalized to the intense peak at 10.5 eV) for the H+-substituted olivine VB (top), along with experimental VB spectrum of all leached olivines added together (middle); and the VB spectrum of vacuum-fractured olivine shown for comparison (bottom).

pyroxenes are consistent with the above two complexes being formed. The rate of overall leaching of pyroxenes is

given by the nucleophilic reaction of the surface complex H4Si2O6(surf) with or H2O (or H3O+).

Characterization of leached layers on olivine and pyroxenes using XPS and DFT

The evidence for these surface complexes is greatly increased by the experimental and theoretical valence band spectra of the leached surfaces. An initial calculation of the valence band spectrum for complexes in which one Mg2+ is substituted by 2H+ revealed a new VB feature at about 12.5 eV. The theoretical prediction of this peak pushed us to look more closely at our VB spectra, and to characterize a peak at 13.5 eV after much multi-scanning of many leached samples. The intensity of this peak is consistent again with the predicted intensity from monolayer coverage. Our study shows that the Kratos charge neutralizer should be very useful for high-resolution XPS ex-situ studies of solution reactions on non-conductors of all types. The use of variable energy synchrotron radiation, as already successfully used on sulphides (Schaufuss et al., 1998) and oxides (Liu et al., 1998) will be especially useful to increase the surface sensitivity of XPS dramatically to better characterize surface complexes on non-conductor silicates. ACKNOWLEDGMENTS This study was funded by the Natural Science and Engineering Research Council (NSERC) of Canada. We thank Surface Science Western at the University of Western Ontario for all their technical help with the Kratos XPS. We also acknowledge valuable comments from R. Hellman and two other reviewers.

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