Michael F. Hochella

Calcite precipitation mechanisms and inhibition by orthophosphate: In situ observations by Scanning Force Microscopy

Scanning Force Microscopy (SFM) was used to observe near-equilibrium calcite growth processes in solutions of known composition and saturation state. Calcite seeds were reacted in solutions of known saturation state at 25°C and 0.96 atm PCO2 for 1–2 days before transferring to a SFM fluid cell for observations of continued growth in the same solutions. n nWe observed that when solution saturations with respect to calcite were greater than 1–2, precipitation began with the formation of surface nuclei. These nuclei spread, coalesced, and continued growing. Only after nearly two hours was there a transition to a mechanism resembling spiral growth. At these long reaction times, migrating steps assumed individual heights of 1–2 monolayers. We also observed simultaneous growth and dissolution at undersaturated conditions very near equilibrium. n nThe influence of phosphate was also examined and observations suggested two inhibition mechanisms, depending on surface history. Phosphate (6 and 10 μmol PO4) introduced during the nucleation stage results in the formation of nuclei with amorphous shapes. Phosphate introduced during layer growth disrupts the relatively straight steps produced during PO4-free growth to form jagged steps. Both of the phosphate-calcite surface interactions are consistent with mechanisms proposed in previous studies. n nOur findings suggest that when solution saturations with respect to calcite are greater than two, precipitation always begins with the formation of surface nuclei with a later transition to mononuclear growth mechanisms. These observations have implications for carbonate precipitation in natural systems and suggest that calcite growth in environments with frequent wetting and drying cycles begin each wetting event with precipitation by surface nucleation. Results of this study also suggest that experimental investigations of calcite precipitation kinetics and interpretations of growth mechanisms must account for this early stage contribution by nucleation. Otherwise, such rates of calcite growth may not reflect the overall slower rates that occur in continuously wet environments.

Electrochemistry and dissolution kinetics of magnetite and ilmenite

Natural samples of magnetite and ilmenite were experimentally weathered in pH 1–7 anoxic solutions at temperatures of 2–65 °C. Reaction of magnetite is described as [Fe2+Fe23+]O4(magnetite) + 2H+ → γ[Fe23+]O3(maghemite) + Fe2+ + H2O. Dynamic polarization experiments using magnetite electrodes confirmed that this reaction is controlled by two electrochemical half cells, 3[Fe2+Fe23+]O4(magnetite) → 4γ[Fe23+]O3(maghemite) + Fe2+ + 2e− and [Fe2+Fe23+]O4(magnetite) + 8 H+ + 2e− → 3Fe2+ + 4H2O, which result in solid state Fe3+ reduction, formation of an oxidized layer and release of Fe(II) to solution. XPS data revealed that iron is present in the ferric state in the surfaces of reacted magnetite and ilmenite and that the TiFe ratio increased with reaction pH for ilmenite. n nShort-term (<36 h) release rates of Fe(II) were linear with time. Between pH 1 and 7, rates varied between 0.3 and 13 × 10−14 mol · cm−2 · s−1 for magnetite and 0.05 and 12.3 × 10−14 mol · cm−2 · s−1 for ilmenite. These rates are two orders of magnitude slower than electrochemical rates determined by Tafel and polarization resistance measurements. Discrepancies are due to both differences in geometric and BET surface area estimates and in the oxidation state of the mineral surface. In long-term closed-system experiments (<120 days), Fe(II) release slowed with time due to the passivation of the surfaces by increasing thicknesses of oxide surface layers. A shrinking core model, coupling surface reaction and diffusion transport, predicted that at neutral pH, the mean residence time for sand-size grains of magnetite and ilmenite will exceed 107 years. This agrees with long-term stability of these oxides in the geologic record.

The formation of leached layers on albite surfaces during dissolution under hydrothermal conditions

Hydrothermally altered (225°C) albite was compositionally depth-profiled using X-ray photoelectron spectroscopy (XPS)coupled with calibrated Ar ion sputtering. Solution data were collected during dissolution runs for the same crystals which were spectroscopically analyzed. We found that leached zones depleted in Na, Al, and O develop during the initial, incongruent phase of dissolution. Angle resolved XPS (ARXPS) demonstrated that Na and Al are significantly depleted from the upper few monolayers. Depths of leaching, which range from 10 to 900 A, decrease with increasing pH in theacid region and increase with pH in the basic region. Based on calculated dissolution rates the depth of leaching can be roughly correlated with the release rate of Si. From the observation that the equivalents of H+ consumed always exceed the equivalents of Na+ and Al3+ released, hydrolysis cannot be considered to be a simple ion exchange process. The XPS spectra also revealed the presence of Cl− over the entire leaching depth for samples run at pH pHzpc, suggesting electrostatic adsorption of aqueous species at charged sites within the leached layer. The presence of Cl− and Ba2+ also show that preferential leaching creates a porous and open structure which allows for the large-scale influx of solvent molecules. Preliminary evaluations of diffusion transport rates through leached layers suggest that dissolution is not rate limited by diffusion. Instead, the kinetics of dissolution seem to be related to the intrinsic rate of structural hydrolysis. Using the XPS and solution data in conjunction with theoretical and experimental studies in the literature, we propose a dissolution mechanism based on initial ion exchange followed by the hydrolysis of Al and Si, which is modeled as the breakdown of activated complexes formed at bridging oxygen (Obr) sites. Elemental mass balances based on comparisons between the XPS and solution data suggest that dissolution occurs non-uniformly and is probably preferentially constrained to dislocations and macroscopic defects within the structure.

Scanning tunneling microscopy of sulfide surfaces

A fundamental understanding of reactions that occur at mineral surfaces, many of which have bearing on important environmental issues, requires knowledge of atomic surface structures. Scanning tunneling microscopy (STM) is a new technique which can be used to image atomic surface structures in real space. We briefly review STM theory and interpret STM images of galena (PbS) and pyrite (FeS{sub 2}) surfaces by comparing the bias-voltage dependence of the images to the electronic structures of the materials. This approach amounts to a form of tunneling spectroscopy which may ultimately be used to identify individual atoms on mineral surfaces. STM imaging was accomplished on fresh fracture surfaces as well as on surfaces that had been exposed to air for long periods of time. For galena, the Pb and S sites are distinguishable, and the S sites appear to be imaged preferentially. A galena surface which had been oxidized in air for several months was imaged, suggesting either that oxidation products are very thin, occur in local patches on the surface, or are both non-conductive and not coherently bound to the galena surface. Iron appears to be imaged preferentially on fresh fracture surfaces of pyrite. Atomic positions on a pyrite growth surfacemorexa0» were not those expected for a termination of the bulk pyrite structure; it is likely that a surface oxidation product was imaged.«xa0less

Scanning Tunneling Microscopy of Galena (100) Surface Oxidation and Sorption of Aqueous Gold

Scanning tunneling microscopy was used to characterize the growth of oxidized areas on galena (100) surfaces and the formation of gold islands by the reductive adsorption of AuCl4– from aqueous solution. The gold islands and galena substrate were distinguished by atomic resolution imaging and tunneling spectroscopy. Oxidized areas on galena have [110]-trending boundaries; gold islands elongate along [110] directions. However, there are no obvious structural registry considerations that would lead to elongation of gold islands in a [110] direction. Instead, it is probable that a direct coupling of gold reduction and sulfide surface oxidation controls the initial formation of gold islands. Gold islands grow less quickly on preoxidized galena surfaces and show no preferred direction of growth.

Surface chemistry associated with the cooling and subaerial weathering of recent basalt flows

The surface chemistry of fresh and weathered historical basalt flows was characterized using surface-sensitive X-ray photoelectron spectroscopy (XPS). Surfaces of unweathered 1987–1990 flows from the Kilauea Volcano, Hawaii, exhibited variable enrichment in Al, Mg, Ca, and F due to the formation of refractory fluoride compounds and pronounced depletion in Si and Fe from the volatilization of SiF4 and FeF3 during cooling. These reactions, as predicted from shifts in thermodynamic equilibrium with temperature, are induced by diffusion of HF from the flow interiors to the cooling surface. The lack of Si loss and solid fluoride formation for recent basalts from the Krafla Volcano, Iceland, suggest HF degassing at higher temperatures. n nSubsequent short-term subaerial weathering reactions are strongly influenced by the initial surface composition of the flow and therefore its cooling history. Successive samples collected from the 1987 Kilauea flow demonstrated that the fluoridated flow surfaces leached to a predominantly SiO2 composition by natural weathering within one year. These chemically depleted surfaces were also observed on Hawaiian basalt flows dating back to 1801 AD. Solubility and kinetic models, based on thermodynamic and kinetic data for crystalline AlF3, MgF2, and CaF2, support observed elemental depletion rates due to chemical weathering. Additional loss of alkalis from the Hawaiian basalt occurs from incongruent dissolution of the basalt glass substrate during weathering.

The surface chemistry of manganiferous silicate minerals as inferred from experiments on tephroite (Mn2SiO4)

The dissolution rate of tephroite in oxygen-free solutions decreases with increased pH over the interval 2 ≤ pH ≤ 8 and with decreased temperature over the range 25–45°C. The pH-dependence is similar to other orthosilicate minerals at 25°C and presumably relates to variations in the concentration of adsorbed hydrogen ions. The rate order with respect to solution pH increases with temperature and the experimental activation parameters vary strikingly with solution pH. These variations are interpreted to result from contributions of enthalpy to the experimental activation energy (Eexp) from proton adsorption and from coulombic interactions among charged sites on the mineral surface. The dependence of dissolution rate on the logarithm of solution pH is only approximately linear with pH; the dependence is sensitive to changes in temperature via the conditional equilibrium constant that describes the concentrations of charge sites on the mineral surface. n nX-ray photoelectron spectroscopic (XPS) measurements of the reacted mineral surfaces indicate that Mn remains in the divalent valence state at all conditions. The near-surface region of the mineral after acid dissolution has a MnSi ratio which is lower than the unreacted material while the opposite result is observed after reaction at basic-pH conditions.

Organic compounds on crack surfaces in olivine from San Carlos, Arizona and Hualalai Volcano, Hawaii

Abstract Organic compounds associated with thin carbonaceous films on crack surfaces have been detected by thermal-desorption photoionization mass spectrometry in large single crystals of olivine from San Carlos, Arizona and Hualalai Volcano, Hawaii. Alkalis, silicon, aluminum and halogens are also present in the 3–4 nm thick carbonaceous films. The organics probably were not derived from the upper mantle or lower crust or from environmental biogenic contamination after eruption and cooling. It is likely that the carbonaceous films and organics were deposited or formed on crack surfaces created during eruption and cooling of the host alkali basalts. Whether the organics were produced abiotically by FischerTropsch-like reactions involving volcanic gases and fresh-fractured surfaces where reduced carbon was deposited, or whether the organics represent biogenic material that was assimilated into the magmatic system prior to or during magma ascent, cannot be ascertained at this time due to their low abundance.

The complexity of mineral dissolution as viewed by high resolution scanning Auger microscopy: Labradorite under hydrothermal conditions

Abstract The dissolution of labradorite under hydrothermal conditions has been studied by exposing a packed core of fine-grained labradorite powder to deionized water at 300°C and 300 bars for 57 days using a single-pass flowthrough apparatus. The principal methods used for characterizing the labradorite dissolution were high lateral resolution scanning Auger microscopy (SAM), X-ray photoelectron spectroscopy (XPS), and measurement of the exit solution chemistry. Under these hydrothermal conditions, boehmite and halloysite precipitate throughout the core and the labradorite powder undergoes extensive dissolution. The reacted labradorite surfaces are incongruently modified to levels as deep as 20 A (for Al and Si) to 30 A (for Na and Ca), although the distribution of elements within this depth or deeper is not yet known. The modified surface chemistry not only changes systematically down the length of the core, but is also locally variable. Generally, and relative to the starting composition, the reacted surfaces are enriched in Al and depleted in Na, Ca, and Si, and the Ca surface concentration decreases with distance from the inlet. Locally, some portions of the reacted labradorite surfaces have had Ca (and presumably Na) almost entirely removed. The dissolution mechanism of labradorite under hydrothermal conditions as revealed by SAM is highly complex, and the variability in surface chemistry is probably controlled by the local density and distribution of microcracks and crystal defects, the starting morphology of each labradorite surface, and the local flow patterns of reacting solution over that surface. Our results are contrary to previous XPS studies of dissolving feldspars (at or near room temperature) which do not report dramatic changes in near-surface chemistries. Part of the difference between the XPS and Auger results may be due to the difference in the depth of analysis associated with the characteristic electrons utilized by the two techniques.

Aspects of silicate surface and bulk structure analysis using X-ray photoelectron spectroscopy (XPS)

Abstract X-ray photoelectron spectroscopy (XPS) has been used to study the average local environment and polarizability of oxygen in the near-surface region of crystalline and amorphous SiO2, NaAlSi3O8, NaAlSi2O6, NaAlSiO4, and CaMgSi2O6, as well as crystalline Mg2SiO4 and rhyolitic and basaltic composition glasses. For these materials, the O Is chemical shift due to different oxygen environments is not as large as in previously studied alkali silicate and alkali aluminosilicate systems. However, it was found that the O Is peak width (measured as FWHM) for the minerals is proportional to the number of chemically distinct oxygen environments in each structure. Further, O Is FWHM values for the mineral and rock composition glasses suggest certain structural details that correlate well with previously proposed amorphous structure models. Taken together, these results suggest that the shape and width of the O Is line may be important in monitoring atomic-level structural changes that may occur on silicate mineral and glass surfaces during, for example, reactions with aqueous solutions. In addition, for the O is line to be useful for bulk structural information, approximately the upper 60A of the sample must be representative of the bulk. The O 2s photopeak and X-ray induced O KL2,3L2,3 Auger line were also carefully studied, although they did not yield further structural information beyond that provided by the 0 Is line. However, the oxygen Auger parameters derived using the energy difference between the O Is and O KL2,3L2,3 lines for the crystals and glasses in this study within the Na2O-Al2O3-SiO2 system show a systematic trend with bulk composition. This trend reflects a change in the polarization energy (extra-atomic relaxation energy) for oxygen in these materials.