H.W. Nesbitt

Prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations

Abstract The exposed crust consists mainly of plagioclase (35%), quartz (20%), K-feldspar (11%), volcanic glass (12%), biotite (8%), and muscovite (5%). Quartz is a resistate, thus feldspars and glass represent approximately 75 percent of the labile minerals. The weathering characteristics of these constituents are summarized in the context of thermodynamic, mass balance and kinetic considerations. Experimentally determined release rate constants were used to predict the proportions of Ca, Na and K released by feldspars of plutonic rocks (granites to gabbros) to weathering solutions. The chemical weathering trends of the weathered residues, calculated from the kinetic data, conform closely to the initial trends observed in some recent weathering profiles, demonstrating the accuracy of the predictions. Since the weathering of feldspars is controlled by processes that should not change through geological time, the relative release rates of Ca, Na, and K from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering. Experimentally determined release rate constants are not available for a wide range of volcanic glass compositions, but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks. The kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile.

X-ray photoelectron and Auger electron spectroscopic studies of pyrrhotite and mechanism of air oxidation

Abstract Pyrrhotite (Fe7S8) fractured under high vacuum (10−7 Pa) and reacted with air for 6.5 and fifty hours was analyzed using X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES). XPS iron data from fresh surfaces indicate 32% Fe(III) and 68% Fe(II), both bonded to sulphur. The result agrees closely with stoichiometry which suggests 29% Fe(III) in the pyrrhotite studied. This is the first spectroscopic evidence to indicate Fe(III) in pyrrhotite. Sulphur is present primarily as monosulphide (S2−), with minor amounts of disulphide (S22−) and polysulphide (Sn2−). XPS examination of 6.5 hour air-oxidized surfaces indicates 58% Fe(III) and 42% Fe(II). Fe(III) is bonded to oxygen and most Fe(II) remains bonded to sulphur. XPS iron and oxygen data suggest a Fe(III)-oxyhydroxide to be the species forming. Sulphur spectra demonstrate a range of oxidation states from S2− (monosulphide) to S6+ (sulphate). AES compositional depth profiles of air-oxidized surfaces display three compositional zones. After fifty hours of air oxidation the outermost layer is less than 10 Angstroms, oxygen-rich, and sulphur depleted. Immediately below the O-rich layer exists an Fe-deficient, S-rich layer that displays a continuous, gradual decrease in S Fe from the O-rich zone to that of the unaltered pyrrhotite. Quantification of depth profiles utilizing the sequential layer sputtering model (SLS) indicate alteration trends correspond compositionally to FeO1.5, FeS2, Fe2S3 and Fe7S8. Compositional zones develop by electron and iron migration towards the oxidized surface. Molecular oxygen initially taken onto the surface is reduced to O2− probably by electron transfer from the pyrrhotite interior and is facilitated by rapid electron exchange between Fe(III) and Fe(II) of the bulk solid. Vacancies inherent to nonstoichiometric pyrrhotite probably promote diffusion of iron to the surface resulting in the formation of iron oxyhydroxide species.

X-ray photoelectron spectroscopic study of a pristine pyrite surface reacted with water vapour and air

Pristine pyrite fracture surfaces, exposed for 7 h to water vapour at low pressure (10−5 Pa), display no change to their Fe(2p) or S(2p) X-ray photoelectron (XPS) spectrum, but oxygen deposition occurs as H2O, OH− and O2− (74, 19, and 7%, respectively). An additional 24 h exposure to air (80% humidity) causes the proportions of oxygen species to change dramatically, with OH− and O2− increasing to 55 and 25% and H2O decreasing to 20%. These changes are accompanied by development of a broad Fe(III) peak of the Fe(2p) spectrum, produced by oxidation of Fe(II) to Fe(III) and formation of Fe(III)-oxyhydroxide surface species. There is, however, no sulphate peak developed in the S(2p) spectrum during the 24 h exposure to air. The XPS data demonstrate that formation of Fe(III)-oxyhydroxides precedes sulphate formation, hence rates of redox reactions producing sulphate or other oxygen-bearing S species are initially slower than redox reactions leading to the formation of Fe(III)-oxyhydroxide surface species. Exposure to air for an additional 9 days produces no appreciable change to the O(1s) or Fe(2p) spectrum, but small amounts of sulphate are observed in the S(2p) spectrum. After production of sulphate species, Fe(III)-sulphate salts probably form at the surface of pyrite by reaction of sulphate with previously formed Fe(III)-oxyhydroxides. The Fe(2p32) spectrum of the vacuum-fractured pyrite surface reveals a high energy tail on the major Fe(II) peak. The tail may result from the metallic character of Fe in pyrite. The binding energy and shape of the tail, however, are accurately predicted from S-bonded Fe(III) spectral peaks observed in the pyrrhotite Fe(2p) spectrum; consequently, the presence of Fe(III) bonded to sulphur in the near-surface of pyrite is another reasonable explanation for the tail. Disulphide (S2−2), monosulphide (S2−), and polysulphide (S2−n, n>2) are present on the vacuumfractured pyrite surface at 85, 10 and 5%, respectively. Monosulphide decreases, and disulphide increases proportionately, during the first 24 h exposure to air. After a total of 10 days exposure, monosulphide decreases to half its original value, polysulphide increases appreciably, and sulphate and thiosulphate are present at 1.8 and 2.3 at. %, respectively. There is little change to disulphide content during the entire experiment. Two explanations are offered for the presence of the three S species on the vacuum-fractured pyrite surfaces. Approximately 5% of S is present as polysulphide (S2−n) and 10% as monosulphide (S2−). This 1:2 ratio is obtained if some disulphide is “disproportionated” to polysulphide and monosulphide where, on average, four atoms of S are included in the polysulphide species (S2−4). An alternative explanation includes ferric iron; if present, it may give rise to complex charge compensation involving “disproportionation” of disulphide to monosulphide (for charge balance) and polysulphide.

Oxidation of arsenopyrite by air and air-saturated, distilled water, and implications for mechanism of oxidation

Arsenic and sulphur, and possibly iron, exist in multiple oxidation states in the near-surface of unoxidized arsenopyrite. X-ray photoelectron spectroscopy (XPS) of an unoxidized surface reveals that sulphur is present as disulphide (S22−, 78 At.%), monosulphide (S2−, 15%), and as polysulphide (Sn2−, where 2 < n < 8). As1− predominates, but 15% As0 is also observed. Most iron is present as Fe2+ bonded to arsenic and sulphur [Fe(II)-(AsS)], but some Fe3+ may be present, and bonded to AsS [Fe(III)-(AsS)]. Exposure for twenty-five hours to the atmosphere reveals development of Fe(III)-oxyhydroxide species at the surface, with binding energies similar to goethite. Approximately one quarter of the arsenic is present as As5+, As3+, and As1+, although As1− predominates. Minor polysulphides and thiosulphate are produced during oxidation. Surfaces reacted with air-saturated, distilled water for eight hours undergo extensive oxidation. Fe(III)-oxyhydroxides are the dominant Fe surface species. As5+, As3+, and As1− are as abundant as As1−, and sulphate is detectable on the surface. The binding energies and proportions of the oxygen-bearing species of the O1s spectra are effectively the same in the air-oxidized and water-reacted experiments. The striking similarities suggest strongly that the nature of the species produced, and their rates of formation, are the same during reaction with air and with air-saturated distilled water. The mechanisms by which oxygen near-surface species are produced may well be similar in both media. Arsenic is the most readily oxidized species, and sulphur is most slowly oxidized during reaction with the atmosphere. Reaction of arsenopyrite with air-saturated, distilled water oxidizes As1− and Fe2+ at similar rates, and much more rapidly than sulphur is oxidized. Auger depth profiles of oxidized arsenopyrite demonstrate that As diffuses from the interior of the mineral to the surface during oxidation. Diffusion of As to the oxidized surface, combined with the observed production of large amounts of As3+ and As5+ in the near-surface, promotes rapid, selective leaching of arsenites and arsenates. Arsenites (AsO33−) are particularly toxic to biota, and pose a potential risk to water quality where arsenopyrite is abundant.

XPS study of reductive dissolution of 7Å-birnessite by H3AsO3, with constraints on reaction mechanism

Abstract Reductive dissolution of synthetic birnessite (MnO1.7(OH)0.25 or MnO1.95) by arsenious acid (H3AsO3) proceeds in two steps. The first entails reduction of Mn(IV) to Mn(III), with stoichiometry: 2MnO 2 +H 3 AsO 3 =2MnOOH∗+H 3 AsO 4 H3AsO3 then attacks MnOOH∗ according to the stoichiometric reaction: 2MnOOH∗+H 3 AsO 3 =2MnO+H 3 AsO 4 +H 2 O, where MnOOH∗ is an intermediate reaction product. Mn(II) is released ultimately to solution. Most importantly, one electron is transferred to each metal ion per reaction step. A Mn(III) component of the original, synthetic birnessite also undergoes reductive dissolution independently of, and at a different rate than, reduction of MnOOH∗. X-ray Photoelectron Spectroscopy (XPS) demonstrates formation of an intermediate reaction product composed of Mn(III), hydroxyl, and H2O (here represented as MnOOH∗). MnOOH∗ increases to a maximum value and subsequently decreases, as expected of an intermediate reaction product of a consecutive reaction scheme. Seven reactions are required to represent adequately reductive dissolution of birnessite. These include redox and sorption reactions. A Monte Carlo simulation successfully reproduces the major features of both XPS and previously published leach-rate results. Reductive dissolution of birnessite may proceed either via a classic electron transfer mechanism by which a bidentate surface complex forms, or via a substitution reaction mechanism, by which a monodentate surface complex forms. X-ray absorption spectroscopic (XAS) studies may be used to identify the appropriate mechanism.

Sulfur and iron surface states on fractured pyrite surfaces

Abstract Pyrite has a poor {001} cleavage. Unlike most other minerals with a rocksalt-type structure, pyrite typically fractures conchoidally, demonstrating that parting surfaces are not constrained to the {001} crystallographic plane. Cleavage along {001} require rupture of only Fe-S bonds, but pyrite consists of both Fe-S and S-S bonds. Analysis of bond energies indicates that S-S bonds are the weaker bonds and they are likely to be ruptured when pyrite is fractured. With each ruptured S-S bond, two mononuclear species (formally S1-) are produced, one bound to one fracture surface and the second to the opposite fracture surface. This monomer is reduced to S2- (monosulfide) during relaxation through oxidation of surface Fe2+ ions to Fe3+. These surface relaxation processes explain the surface states observed in S(2p) and Fe(2p3/2) X-ray photoelectron spectra (XPS) of pyrite. The S(2p) XPS spectrum is interpreted to include bulk disulfide contributions at 162.6 eV and two surface state contributions at 162.0 and 161.3 eV. The monosulfide (S2-) emission is near 161.3 eV, as observed in S(2p) spectra of pyrrhotite, and the 162 eV peak is interpreted to result from the surface-most sulfur atom of surface disulfide ions. The Fe(2p3/2) XPS spectrum includes three contributions, a bulk Fe2+ emission near 707 eV and emissions from two Fe surface states. One surface state is interpreted to be Fe2+ surface ions. Their coordination is changed from octahedral before fracture to square pyramidal after fracture. The consequent stabilization of the antibonding Fe d2z orbital yields unpaired electrons in the valence band resulting in multiplet peak structure in the Fe(2p3/2) spectrum. Similarly, each surface Fe3+ ion, having contributed a non-bonding 3d electron to the valence band (bonding orbital), contains unpaired 3d electrons, resulting in multiplet splitting of its Fe(2p3/2) signal. The high-energy tail observed in the Fe(2p3/2) spectrum of pyrite is the product of emissions from both surface states with Fe2+ multiplet peaks centered near 708 eV and the surface Fe3+ multiplets spanning the binding energies from 708.75 to about 712 eV.

X-ray photoelectron spectroscopic study of water adsorption on iron sulphide minerals

Abstract Samples of natural pyrrhotite and pyrite were fractured within the analytical chamber of an X-ray photoelectron spectrometer. The pristine mineral surfaces were then exposed, in the absence of oxygen, to total doses of 100, 200, 400, 800, 1400, 28,000, and 300,000 Langmuirs (L) of D2O. X-ray photoelectron spectroscopic (XPS) analyses were performed between each water dose, to investigate the interaction of these iron sulphide surfaces with water vapour. Recorded Fe and S photoelectron spectra showed no evidence of oxidation products on either mineral, even at highest D2O doses, nor could an oxide oxygen signal be fitted in the spectra for either mineral. On pyrrhotite, the O 1s spectra are composed of contributions from dominantly hydroxyl (at 532.0 ± 0.2 eV ) and subordinate chemisorbed water (at 533.5 ± 0.2 eV) signals. The main O is peak on pyrite is also formed from hydroxyl (531.0 ± 0.3 eV) and adsorbed water/hydroxyl (at 532.3 eV) signals. However, some O is spectra recorded on pyrite have peaks at anomalously high binding energies (>535 eV ). The anomalous high binding energy species are attributed to electrically-isolated OH/H2O, as reported elsewhere, and to liquid-like water, which has not previously been described in the literature. Pyrrhotite and pyrite interact with water via fundamentally different processes. Pyrrhotite reaction involves the donation of electron charge through Fe vacancies, whereas the water species detected on pyrite interact with the Fe 3d (eg) molecular orbital, and it is suggested that hydrogen bonding with the disulphide moiety may be important.

Synchrotron XPS evidence for Fe2+-S and Fe3+-S surface species on pyrite fracture-surfaces, and their 3D electronic states

Abstract A X-ray photoelectron Fe 2p3/2 spectrum of a pristine pyrite fracture surface was collected using synchrotron radiation with the source tuned to 800 eV. Comparison of this highly surface sensitive Fe 2p spectrum with Fe 2p spectra collected by conventional means (1487 eV AlKa source) reveals that the high binding energy tail of the pyrite Fe 2p3/2 line results primarily from Fe surface state contributions. The three major contributions to the spectrum are interpreted to be: (1) Fe2+ resident on bulk sites; (2) Fe2+ resident on surfaces, edges and corners; (3) Fe3+ surface states produced during fracture by an auto-redox reaction involving Fe and S. The intense main peak is ascribed to the bulk state, whereas the high binding energy tail of the spectrum is composed primarily of Fe2+ and Fe3+ surface state contributions. Fe2+ on bulk sites is octahedrally coordinated (Oh symmetry). All valence electrons of Fe on bulk sites are paired (diamagnetic) and a singlet photopeak at 707 eV is consequently produced. Fracture produces Fe2+ surface states with lower coordination than bulk sites. Fe2+ located at surfaces, edges and corners experiences modified Ligand Field Stabilization Energies (LFSE) which results in stabilization of the dz2 orbital and destabilization of the dxy orbital. Promotion of a dxy electron to the dz2 orbital makes surface Fe2+ surface states paramagnetic resulting in multiplet splitting of their associated photopeaks. The Fe3+ surface state is necessarily paramagnetic and its photoemissions are consequently multiply split. Analysis of photopeak structures and binding energy splittings of Fe2+ and Fe3+ surface states demonstrates that they are located at the appropriate binding energies, and span the appropriate energy range, to satisfactorily explain the high binding energy tail on of the Fe 2p3/2 spectrum.

Formation and evolution of soils from an acidified watershed: Plastic Lake, Ontario, Canada

Abstract The Plastic Lake watershed contains podzols developed on glacial tills deposited 12,000 years ago. Present-day, cationic fluxes from the soils are greater by a factor of 2 than long-term fluxes averaged over the age of the tills. The high rates of present-day chemical weathering may be a result of increased input of anthropogenic acids into the Plastic Lake watershed. Time-averaged proportions of cations leached from the soils are strikingly different from the proportions of cations now being leached, indicating that the character of chemical weathering has changed over time. Weathering was and is dominated by mineral dissolution, but cation exchange has become increasingly important as the soils have matured. The large amount of Ca now released to the soil solutions probably is derived from exchange substrates, promoted by preferential uptake of Al onto the substrates. Aluminum is derived primarily through dissolution of feldspars. Bulk compositional analyses of soil profiles demonstrate that feldspars of the AE horizon release base cations (Na, K, Ca) and Al to solution in near-stoichiometric proportions, just as is observed experimentally for feldspar dissolution in acidic solutions. Furthermore, surface area normalized dissolution rates of plagioclase, K-feldspar, quartz, and micas of Plastic Lake soils are similar to rates obtained experimentally. The favourable comparison between natural and experimental dissolution rates, and near-stoichiometric release of base cations and Al from feldspars to acidic solutions in both natural and experimental settings, suggests that recent laboratory release rates can be applied to dissolution of feldspars within the AE horizon of the Plastic Lake Podzols. Surface area-normalized, time-averaged, dissolution rates of primary minerals of Plastic Lake soils are significantly greater than present-day rates measured for mature soil profiles. Rapid release from Plastic Lake soils probably results from reaction of acidic soil solutions with highly reactive biotite and ultrafine grains of feldspars produced by comminution during glaciation. Exhaustion of these reactive phases may well explain the low leach rates observed for mature profiles. After just 12,000 years of weathering, biotite has been effectively exhausted in the Plastic Lake soils and much of the fine-grained feldspar probably has been dissolved. Time-averaged concentrations of elements removed from the soil, recast into essential mineralogy, indicate that vermiculite weathers most rapidly, followed closely by plagioclase. K-feldspar weathers less rapidly than plagioclase but more rapidly than quartz or hornblende. Al-silicate, possibly imogolite, and goethite are added to the soils during weathering. Plagioclase dissolution consumes more acid than all other reactions, accounting for 60% of the total acid consumed. Combined with K-feldspar, they account for more than 80% of the total acid neutralized. The dissolution rate of quartz is one-half that of plagioclase; furthermore, the rate is consistent with the experimentally determined rates. The combination of high abundance and dissolution rate results in large amounts of quartz being dissolved from the soils of Plastic Lake; it may also contribute significantly to total elemental fluxes from other catchments.

X-ray photoelectron and Auger electron spectroscopy of air-oxidized pyrrhotite: Distribution of oxidized species with depth

Abstract Angle resolved X-ray photoelectron spectroscopy (ARXPS) of air-oxidized pyrrhotite (Fe 7 S 8 ) surfaces reveals two distinctive compositional zones. The outer most zone is composed of iron oxyhydroxide, whereas the underlying zone is sulphur-rich and depleted of Fe relative to bulk pyrrhotite. Underlying this sulphur-rich zone is bulk pyrrhotite. Auger compositional depth profiles confirm that the outer most iron-oxyhydroxide layer is approximately 5 A thick. A sharp interface separates this layer from the underlying sulphur-rich layer (approx. 30 A thick), in which the Fe:S ratio approaches 1:2 and contains minor iron thiosulphate and iron sulphate. ARXPS and Auger data provide insight into the mechanism of incipient pyrrhotite oxidation. Monosulphide of the sulphur-rich underlayer is oxidized to disulphide and polysulphides primarily. The likely reduction reaction is conversion of molecular oxygen to oxide at the mineral surface. Iron diffuses from the interior to the surface where it combines with oxide oxygen, hydroxide, and water to form ferric oxyhydroxides. Although Fe diffuses from the interior to the surface, sulphur species do not migrate appreciably from the subsurface giving rise to the sulphur-rich zone. There is no evidence that oxygen diffuses from the oxyhydroxide layer.into the sulphur-rich layer during the initial stages of oxidation. The angle resolved S 2 p XPS spectrum demonstrates clearly that the disulphide signal is derived from the sulphur-rich zone beneath the oxyhydroxide layer. X-ray diffraction studies of pyrrhotite conversion to marcasite have shown that removal of Fe atoms from the pyrrhotite structure produces marcasite (compositionally and structurally) on a macroscopic scale. The same conversion probably occurs in the sulphur-rich zone of pyrrhotite, where diffusion of Fe to the oxidized surface results in formation of marcasite-like composition and structure in the sulphur-rich layer of oxidized pyrrhotite.