Henry R. Westrich

The surface chemistry of dissolving labradorite feldspar

Abstract Elastic recoil detection (ERD) analysis was used in conjunction with Rutherford backscattering (RBS) analysis to determine depth profiles of hydrogen, silicon, aluminum and calcium in labradorite crystals reacted under various pH conditions. The inventory of hydrogen in the mineral is strongly affected by solution pH. Hydrogen extensively infiltrates the mineral during reaction for 264 hours with solutions in the pH range 1–3. Infiltration is accompanied by extensive removal of sodium, calcium and aluminum from the mineral. This incongruent reaction proceeds to several hundreds of angstroms of depth and produces a silicon-rich surface which is amorphous to electron diffraction. The amount of hydrogen in the reacted layer is much less than is predicted from knowledge of the quantity of cations leached from the feldspar. These low inventories of hydrogen suggest that hydrogen-bearing groups in the reacted layer repolymermize subsequent to ion exchange and depolymerization reactions. This repolymerization eliminates hydrogen from the layer. At higher pH conditions (pH > 5), hydrogen inventories in the crystals decrease with time relative to an unreacted reference crystal. Hydrogen does not infiltrate beyond the first few unit cells of feldspar. Thus, dissolution in slightly acid, near-neutral, and basic solutions proceeds at the immediate surface of the feldspar. Within the limit of the RBS technique, there is no evidence for incongruent dissolution at these conditions.

What do dissolution experiments tell us about natural weathering

Abstract Much information about the earliest stages of weathering comes from laboratory experiments where the mineral surface chemistry is rigorously controlled and the solution composition is maintained far from equilibrium with the solid. The experiments show that the pathways for dissolution are similar to those for ligand exchange around dissolved metal complexes. One difference is that the rates of mineral dissolution are controlled by the concentrations of surface species while rates of ligand exchange are proportional to bulk concentration. Nevertheless, the correlation is so strong that rates of olivine dissolution are predictable from the rates of solvent exchange around the corresponding divalent metal in solution. However, this level of resolution is achieved by maintaining unnaturally high fluid/mineral ratios and by taking great care to avoid precipitation of secondary minerals. Natural olivine weathers in intimate contact with secondary minerals. Reactions commonly proceed in clay-filled channels only a few tens of Angstroms in diameter and secondary minerals are topotactic with olivine. Primary and secondary minerals are so intimately intergrown in these channels that the olivine surface chemistry is undoubtably influenced by overlapping electrostatic double layers. Smectite growth may have proceeded at near-saturation with the fluid, but the solution is in gross disequilibrium with the olivine. There nevertheless remains some qualitative consistency between weathering in the laboratory and in the field. The ligand-exchange model for bond cleavage, for example, predicts that the presence of ferric iron at the mineral surface will retard weathering rates. Correspondingly, defect planes of oxidized olivine (to produce Fe 3+ and a vacancy) weather at a much slower rate than unoxidized material. Thus, the appropriate level of comparison of field and laboratory weathering is at the scale of reactivity trends; it is unreasonable to expect quantitative similarity in reaction rates.

Vapor saturation and accumulation in magmas of the 1989–1990 eruption of Redoubt Volcano, Alaska

Abstract The 1989–1990 eruption of Redoubt Volcano, Alaska, provided an opportunity to compare petrologic estimates of SO 2 and Cl emissions with estimates of SO 2 emissions based on remote sensing data and estimates of Cl emissions based on plume sampling. In this study, we measure the sulfur and chlorine contents of melt inclusions and matrix glasses in the eruption products to determine petrologic estimates of SO 2 and Cl emissions. We compare the results with emission estimates based on COSPEC and TOMS data for SO 2 and data for Cl/SO 2 in plume samples. For the explosive vent clearing period (December 14–22, 1989), the petrologic estimate for SO 2 emission is 21,000 tons, or ~12% of a TOMS estimate of 175,000 tons. For the dome growth period (December 22, 1989 to mid-June 1990), the petrologic estimate for SO 2 emission is 18,000 tons, or ~3% of COSPEC-based estimates of 572,000–680,000 tons. The petrologic estimates give a total SO 2 emission of only 39,000 tons compared to an integrated TOMS/COSPEC emission estimate of ~1,000,000 tons for the whole eruption, including quiescent degassing after mid-June 1990. Petrologic estimates also appear to underestimate Cl emissions, but apparent HCl scavenging in the plume complicates Cl emission comparisons. Several potential sources of ‘excess sulfur’ often invoked to explain petrologic SO 2 deficits are concluded to be unlikely for the 1989–1990 Redoubt eruption — e.g., breakdown of sulfides, breakdown of anhydrite, release of SO 2 from a hydrothermal system, degassing of commingled infusions of basalt in the magma chamber, and syn-eruptive degassing of sulfur from melt present in non-erupted magma. Leakage and/or diffusion of sulfur from melt inclusions do not provide convincing explanations for the petrologic SO 2 deficits either. The main cause of low petrologic estimates for SO 2 is that melt inclusions do not represent the total sulfur content of the Redoubt magmas, which were vapor-saturated magmas carrying most of their sulfur in an accumulated vapor phase. Almost all the sulfur of the SO 2 emissions was present prior to emission as accumulated magmatic vapor at 6–10 km depth in the magma that supplied the eruption; whole-rock normalized concentrations of gaseous excess S in these magmas remained at ~0.2 wt.% throughout the eruption, equivalent to ~0.7 vol.% at depth. Data for CO 2 emissions during the eruption indicate that CO 2 at whole-rock concentrations of ~0.6 wt.% in the erupted magma was a key factor in creating the vapor saturation and accumulation condition making a vapor phase source of excess sulfur possible at depth. When explosive volcanism involves magma with accumulated vapor, melt inclusions do not provide a sufficient basis for predicting SO 2 emissions. Thus, petrologic estimates made for SO 2 emissions during explosive eruptions of the past may be too low and may significantly underestimate impacts on climate and the chemistry of the atmosphere.

the surface of labradorite feldspar after acid hydrolysis

Abstract After reaction with a pH Both the surface area of the reacted feldspar and the porosity increase with acid hydrolysis. Modeling of nitrogen sorption onto the surface suggests that the pores have a nominal radius of ∼ 20–80 A or less. This distribution of pore sizes resembles other acid-reacted silicate materials, such as glass, chrysotile and kaolinite. Although the mineral surface clearly becomes more porous during acid hydrolysis, the increase in powder area also does not coincide with an increase in the flux of dissolved Si from the powder. We thus attribute most of this increase in area to spallation of the silica-rich surface from the feldspar upon drying.

The effect of Fe-oxidizing bacteria on Fe-silicate mineral dissolution

Abstract Acidithiobacillus ferrooxidans are commonly present in acid mine drainage (AMD). A. ferrooxidans derive metabolic energy from oxidation of Fe2+ present in natural acid solutions and also may be able to utilize Fe2+ released by dissolution of silicate minerals during acid neutralization reactions. Natural and synthetic fayalites were reacted in solutions with initial pH values of 2.0, 3.0 and 4.0 in the presence of A. ferrooxidans and in abiotic solutions in order to determine whether these chemolithotrophic bacteria can be sustained by acid-promoted fayalite dissolution and to measure the impact of their metabolism on acid neutralization rates. The production of almost the maximum Fe3+ from the available Fe in solution in microbial experiments (compared to no production of Fe3+ in abiotic controls) confirms A. ferrooxidans metabolism. Furthermore, cell division was detected and the total cell numbers increased over the duration of experiments. Thus, over the pH range 2–4, fayalite dissolution can sustain growth of A. ferrooxidans. However, ferric iron released by A. ferrooxidans metabolism dramatically inhibited dissolution rates by 50–98% compared to the abiotic controls. Two sets of abiotic experiments were conducted to determine why microbial iron oxidation suppressed fayalite dissolution. Firstly, fayalite was dissolved at pH 2 in fully oxygenated and anoxic solutions. No significant difference was observed between rates in these experiments, as expected, due to extremely slow inorganic ferrous iron oxidation rates at pH 2. Experiments were also carried out to determine the effects of the concentrations of Fe2+, Mg2+ and Fe3+ on fayalite dissolution. Neither Fe2+ nor Mg2+ had an effect on the dissolution reaction. However, Fe3+, in the solution, inhibited both silica and iron release in the control, very similar to the biologically mediated fayalite dissolution reaction. Because ferric iron produced in microbial experiments was partitioned into nanocrystalline goethite (with very low Si) that was loosely associated with fayalite surfaces or coated the A. ferrooxidans cells, the decreased rates of accumulation of Fe and Si in solution cannot be attributed to diffusion inhibition by goethite or to precipitation of Fe–Si-rich minerals. The magnitude of the effect of Fe3+ addition (or enzymatic iron oxidation) on fayalite dissolution rates, especially at low extents of fayalite reaction, is most consistent with suppression of dissolution by interaction between Fe3+ and surface sites. These results suggest that microorganisms can significantly reduce the rate at which silicate hydrolysis reactions can neutralize acidic solutions in the environment.

Pre-eruptive water content of rhyolitic magmas as determined by ion microprobe analyses of melt inclusions in phenocrysts

Abstract The ion microprobe was used to analyze trapped melt inclusions in phenocrysts from two rhyolitic eruptions for H2O, F and incompatible trace elements. Eleven melt inclusions from phenocrysts in air-fall tephra from Obsidian Dome near Long Valley, USA gave an average of 4.1±1.2 wt .% H2O and showed large variations in F and incompatible trace elements reflecting a complex history reported to involve mixing of at least two magma types. Eight inclusions in phenocrysts from the Taupo “ultraplinian” event, New Zealand gave 4.5 ± 0.8 wt .% H2O with small variations in the other elements analyzed. Measured water contents are similar to earlier estimates of the pre-eruptive water contents of these and similar rhyolites and major- and trace-element analyses of inclusions compare closely with bulk analyses suggesting that the analyses of inclusions represent the chemistry of the magma at the time of entrapment.

Determination of water in volcanic glasses by Karl-Fischer titration

Abstract A method is described for the routine measurement of dissolved water in crushed and sieved volcanic glasses utilizing Karl-Fischer titration and sample pyrolysis without the addition of a flux. Sample fragments between 75 and 150 μm in size proved to be desirable for rapid measurements at low water contents because complete degassing is achieved without significant water adsorption. However, adsorbed water was found in all ground samples after long-term storage, illustrating the need for prompt water analyses after crushing. Estimates of analytical precision were based upon replicate measurements of natural rhyolitic and basaltic glasses. Results indicate a relative standard deviation for this method of 1.5 wt.%) water contents. The ease of sample analysis and good analytical precision make this technique quite useful for geologic interpretation of volcanic processes. Detection limits depend upon sample size but were estimated to be 60 ppm for a 1-g sample.

Ca self-diffusion in grossular garnet

Abstract Use of a thin-film technique and an ion microprobe make it possible to conduct cation self-diffusion experiments of 44Ca in near end-member grossular garnet at temperatures of 800-1000 °C. The experiments were conducted at 1 atm and under quartz + fayalite + magnetlte fO2 conditions. The resulting activation energy (Ea = 155 ± 10 kJ/mol) and the frequency factor (Do = 7.2 × 10-16 m2/s; log Do = -15.1 ± 0.4) were obtained from the temperature dependence of the diffusion data. Comparison of these data with a comparable study of 25Mg self-diffusion coefficients in pyrope confirmed that Ca diffuses more slowly through garnet than Mg under identical conditions.

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. X-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.

Degassing of the 1912 Katmai magmas

Pre- and post-eruptive H2O, F, Cl, and S contents of the three 1912 Katmai magmas were inferred from analyses of melt inclusions and matrix glasses in tephra samples. With increasing silica content (andesite⇒rhyolite), pre-emptive melt H2O increases from ≥1.0 to 3.8 wt.%, Cl increases slightly from 1700 to 1900 ppm, S decreases from 170 to ≤65 ppm, and F remains constant at 550 ppm. These variations are not consistent with a simple crystal fractionation relationship. For plausible chamber depths, the magmas were vapor undersaturated during storage and fragmented during the last few hundred meters of ascent, consistent with geologic evidence for excavation of the vent funnel within the upper 1 km. Vitrophyres of welded intravent fallback tephra ejected late in the eruption show that extensive degassing and complete welding could take place in less than the 60-hour eruptive period. Release of HCl was twice that of the 1980 eruption of Mount St. Helens while H2SO4 output was comparable to that of the 3.5 ka Santorini eruption. Significant retention of Cl and F, which would be released along with residual H2O during high-temperature devitrification, may explain the important vapor transport that occurred in the Valley of Ten Thousand Smokes fumaroles following emplacement of the ignimbrite.