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  • General Geochemistry
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  • 1
    Language: English
    In: Geochimica et Cosmochimica Acta, 2011, Vol.75(2), pp.337-351
    Description: We have compiled time-series concentration data for the biological reduction of manganese(III/IV) published between 1985 and 2004 and fit these data with a simple hyperbolic rate expression or, when appropriate, one of its limiting forms. The compiled data and rate constants are available in . The zero- and first-order rate constants appear to follow a log–normal distribution that could be used, for example, in predictive modeling of Mn-oxide reduction in a reactive transport scenario. We have also included details of the experimental procedures used to generate each time-series data-set in our compilation. These meta-data—mostly pertaining to the type and concentration of micro-organism, electron donor, and electron acceptor—enable us to examine the rate data for trends. We have computed a number of rudimentary, mono-variate statistics on the compiled data with the hope of stimulating both more detailed statistical analyses of the data and new experiments to fill gaps in the existing data-set. We have also analyzed the data with parametric models based on the log–normal distribution and rate equations that are hyperbolic in the concentration of cells and Mn available for reduction. This parametric analysis allows us to provide best estimates of zero- and first-order rate constants both ignoring and accounting for the meta-data.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 2
    Language: English
    In: Geochimica et Cosmochimica Acta, 2011, Vol.75(2), pp.401-415
    Description: The dissolution–precipitation of quartz controls porosity and permeability in many lithologies and may be the best studied mineral-water reaction. However, the rate of quartz-water reaction is relatively well characterized far from equilibrium but relatively unexplored near equilibrium. We present kinetic data for quartz as equilibrium is approached from undersaturation and more limited data on the approach from supersaturated conditions in 0.1 molal NaCl + NaOH + NaSiO(OH) solutions with pH 8.2–9.7 at 398, 423, 448, and 473 K. We employed a potentiometric technique that allows precise determination of solution speciation within 2 kJ mol of equilibrium without the need for to perturb the system through physical sampling and chemical analysis. Slightly higher equilibrium solubilities between 423 and 473 K were found than reported in recent compilations. Apparent activation energies of 29 and 37 kJ mol are inferred for rates of dissolution at two surface sites with different values of connectedness: dissolution at or silicon sites, respectively. The dissolution mechanism varies with Δ such that reactions at both sites control dissolution up until a critical free energy value above which only reactions at sites are important. When our near-equilibrium dissolution rates are extrapolated far from equilibrium, they agree within propagated uncertainty at 398 K with a recently published model by . However, our extrapolated rates become progressively slower than model predictions with increasing temperature. Furthermore, we see no dependence of the postulated reaction rate on pH, and a poorly-constrained pH dependence of the postulated rate. Our slow extrapolated rates are presumably related to the increasing contribution of dissolution at sites far from equilibrium. The use of the potentiometric technique for rate measurement will yield both rate data and insights into the mechanisms of dissolution over a range of chemical affinity. Such measurements are needed to model the evolution of many natural systems quantitatively.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 3
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 May 2013, Vol.108, pp.91-106
    Description: We examined the role of mineral spatial distribution and flow velocity in determining magnesite dissolution rates at different spatial scales. One scale is the column scale of a few to tens of centimeters where dissolution rates are measured. Another is the “local” in situ scale defined as approximately 0.1 mm. The experiments used two columns with the same bulk concentration but different spatial distributions of magnesite. In the “Mixed” column, magnesite was evenly distributed spatially within a quartz sand matrix across the whole column, while in the “One-zone” column, magnesite was distributed in one zone in the middle of the column. The two columns were flushed with the same inlet acidic solution (pH 4.0) under flow velocities varying from 0.18 to 36 m/d. Columns of different lengths (22, 10, and 5 cm) were run to understand the role of length scales. Reactive transport modeling was used to infer local-scale and column-scale dissolution rates. Under the acidic-solution flushing conditions used in this study, local in situ dissolution rates vary by orders of magnitude over a length scale of a few to tens of centimeters. Column-scale rates under different conditions vary between 6.40 × 10 and 1.02 × 10 mol/m /s. The distribution of local-scale rates, which collectively determine the column-scale rates, depend on flow velocity, column length scale, and mineral distribution. A two orders of magnitude difference in flow velocity results in more than two orders of magnitude difference in the column-scale rates. Under the same conditions of flow velocity and mineral distribution, column-scale rates are higher in short columns and are lower in long columns. Mineral spatial distribution made a maximum difference of 14% in the medium-flow velocity regime where the reaction kinetics of the system operates under mixed-control conditions. Under such mixed-control conditions, the larger difference between the two columns in their spatial variation of pH and saturation state lead to a larger difference in the spatial distribution of local dissolution rates and therefore column-scale rates. In contrast, under slow-flow velocity conditions, the system is mostly at equilibrium without much spatial variation, i.e., the regime of local equilibrium. Under fast-flow velocity conditions, the system is kinetically controlled, the local aqueous geochemistry is everywhere similar to the inlet condition, and is also relatively uniform. Under these two conditions, there is almost no difference between the two columns. Column-scale rates were best understood in terms of the Damkohler number (Da ) that quantifies the relative dominance of advection and dissolution processes. The observations in this study lead us to surmise that rates of weathering and other natural processes may be similarly affected by chemical heterogeneity in natural systems under conditions where reaction rate and flow rate are comparable.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
    Source: ScienceDirect Journals (Elsevier)
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  • 4
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 February 2014, Vol.126, pp.555-573
    Description: We investigate how mineral spatial distribution in porous media affects their dissolution rates. Specifically, we measure the dissolution rate of magnesite interspersed in different patterns in packed columns of quartz sand where the magnesite concentration (v/v) was held constant. The largest difference was observed between a “Mixed column” containing uniformly distributed magnesite and a “One-zone column” containing magnesite packed into one cylindrical center zone aligned parallel to the main flow of acidic inlet fluid (flow-parallel One-zone column). The columns were flushed with acid water at a pH of 4.0 at flow velocities of 3.6 or 0.36 m/d. Breakthrough data show that the rate of magnesite dissolution is 1.6–2 times slower in the One-zone column compared to the Mixed column. This extent of rate limitation is much larger than what was observed in our previous work (14%) for a similar One-zone column where the magnesite was packed in a layer aligned perpendicular to flow (flow-transverse One-zone column). Two-dimensional reactive transport modeling with CrunchFlow revealed that ion activity product (IAP) and local dissolution rates at the grid block scale (0.1 cm) vary by orders of magnitude. Much of the central magnesite zone in the One-zone flow-parallel column is characterized by close or equal to equilibrium conditions with IAP/ 〉 0.1. Two important surface areas are defined to understand the observed rates: the effective surface area ( ) reflects the magnesite that effectively dissolves under far from equilibrium conditions (IAP/ 〈 0.1), while the interface surface area ( ) reflects the effective magnesite surface that lies along the quartz–magnesite interface. Modeling results reveal that the transverse dispersivity at the interface of the quartz and magnesite zones controls mass transport and therefore the values of and . Under the conditions examined in this work, the value of varies from 2% to 67% of the total magnesite BET surface area. Column-scale bulk rates (in units of mol/s) vary linearly with and . Using to normalize rates, we calculate a rate constant (10 mol/m /s) that is very close to the value of 10 mol/m /s under well-mixed conditions at the grid block scale. This implies that the laboratory-field rate discrepancy can potentially be caused by differences in the effective surface area. If we know the effective surface area of dissolution, we will be able to use the rate constant measured in laboratory systems to calculate field rates for some systems. In this work, approximately 60–70% of the is at the magnesite–quartz interface. This implies that in some field systems where the detailed information that we have for our columns is not available, the effective mineral surface area may be approximated by the area of grains residing at the interface of reactive mineral zones. Although it has long been known that spatial heterogeneities play a significant role in determining physical processes such as flow and solute transport, our data are the first that systematically and experimentally quantifies the importance of mineral spatial distribution (chemical heterogeneity) on dissolution.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 5
    Language: English
    In: Geochimica et Cosmochimica Acta, 15 December 2012, Vol.99, pp.159-178
    Description: This paper demonstrates a method for systematic analysis of published mineral dissolution rate data using forsterite dissolution as an example. The steps of the method are: (1) identify the data sources, (2) select the data, (3) tabulate the data, (4) analyze the data to produce a model, and (5) report the results. This method allows for a combination of of data, based on expert knowledge of theoretical expectations and experimental pitfalls, and of the data using statistical methods. Application of this method to all currently available forsterite dissolution rates (0 〈 pH 〈 14, and 0 〈 〈 150 °C) normalized to geometric surface area produced the following rate equations: For pH 〈 5.6 and 0° 〈 〈 150 °C, based on 519 data For pH 〉 5.6 and 0° 〈 〈 150 °C, based on 125 data The values show that ∼10% of the variance in is not explained by variation in 1/ and pH. Although the experimental error for rate measurements should be ± ∼30%, the observed error associated with the log values is ∼0.5 log units (±300% relative error). The unexplained variance and the large error associated with the reported rates likely arises from the assumption that the rates are directly proportional to the mineral surface area (geometric or BET) when the rate is actually controlled by the concentration and relative reactivity of surface sites, which may be a function of duration of reaction. Related to these surface area terms are other likely sources of error that include composition and preparation of mineral starting material. Similar rate equations were produced from BET surface area normalized rates. Comparison of rate models based on geometric and BET normalized rates offers no support for choosing one normalization method over the other. However, practical considerations support the use of geometric surface area normalization. Comparison of Mg and Si release rates showed that they produced statistically indistinguishable dissolution rates because dissolution was stoichiometric in the experiments over the entire pH range even though the surface concentrations of Mg and Si are known to change with pH. Comparison of rates from experiments with added carbonate, either from CO partial pressures greater than atmospheric or added carbonate salts, showed that the existing data set is not sufficient to quantify any effect of dissolved carbonate species on forsterite dissolution rates.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 6
    Language: English
    In: Earth and Planetary Science Letters, 15 February 2017, Vol.460, pp.29-40
    Description: O and CO , the two essential reactants in weathering along with water and minerals, are important in deep regolith development because they diffuse to weathering fronts at depth. We monitored the dynamics of these gas concentrations in the hand-augerable zone on three ridgetops—one on granite and two on diabase—in Virginia (VA) and Pennsylvania (PA), U.S.A. and related the gas chemistry to regolith development. The VA granite and the PA diabase protoliths were more deeply weathered than the VA diabase. We attribute this to high protolith fracture density. The pO and pCO measurements of these more fractured sites displayed the characteristics of aerobic respiration year round. In contrast, the relation of pO versus pCO on the more massive VA diabase is consistent with seasonal changes in the dominant electron acceptor from O to Fe(III), likely regulated by the expansion/contraction of nontronite in the soil BC horizon. These observations suggest that the fracture density is a first order control on deep regolith gas chemistry. However, fractures can be present in protolith but also can be caused by oxidation of ferrous minerals. We propose that subsurface pO and weathering-induced fracturing can create positive feedbacks in some lithologies that cause regolith to thicken while nonetheless maintaining aerobic respiration at depth. In contrast, in the absence of weathering-induced fracturing and depletion of pO , a negative feedback that may be modulated by soil micro-biota ultimately results in thin regolith. These feedbacks may have been important in weathering systems over much of earth's history.
    Keywords: Soil Po2 and Pco2 ; Mafic Vs. Felsic Bedrocks ; Weathering-Induced Fracturing ; Fe Redox Cycling ; Regolith Development ; Geology ; Physics
    ISSN: 0012-821X
    E-ISSN: 1385-013X
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  • 7
    Language: English
    In: Geochimica et Cosmochimica Acta, 2011, Vol.75(14), pp.3973-3981
    Description: In this study, we applied time-resolved synchrotron X-ray diffraction (TRXRD) to develop kinetic models that test a proposed two-stage reaction pathway for cation exchange in birnessite. These represent the first rate equations calculated for cation exchange in layered manganates. Our previous work has shown that the substitution of K, Cs, and Ba for interlayer Na in synthetic triclinic birnessite induces measurable changes in unit-cell parameters. New kinetic modeling of this crystallographic data supports our previously postulated two-stage reaction pathway for cation exchange, and we can correlate the kinetic steps with changes in crystal structure. In addition, the initial rates of cation exchange, (Å min ), were determined from changes in unit-cell volume to follow these rate laws: = 1.75[ ] , = 41.1[ ] , = 1.15[ ] . Thus, the exchange rates for Na in triclinic birnessite decreased in the order: Cs ≫ K 〉 Ba. These results are likely a function of hydration energy differences of the cations and the preference of the solution phase for the more readily hydrated cation.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 8
    Language: English
    In: Geochimica et Cosmochimica Acta, 2008, Vol.72(11), pp.2587-2600
    Description: In natural weathering systems, both the chemistry and the topography of mineral surfaces change as rocks and minerals equilibrate to surface conditions. Most geochemical research has focused on changes in solution chemistry over time; however, temporal changes in surface topography may also yield information about rates and mechanisms of dissolution. We use stochastic dissolution simulations of a regular 2-D lattice with reaction mechanisms defined in terms of nearest neighbor interactions to elucidate how the surface area and reactivity of a crystal evolve during dissolution. Despite the simplicity of the model, it reproduces key features observed or inferred for mineral dissolution. Our model results indicate that: (i) dissolving surfaces reach a steady-state conformation after sufficient dissolution time, (ii) linear defects cause surface area and dissolution rate to vary in concert with one another, (iii) sigmoidal and non-sigmoidal rate vs. free-energy of reaction (Δ ) behavior can be rationalized in terms of the multiple steps occurring during dissolution, and (iv) surface roughness as a function of Δ is highly sensitive to the reaction mechanism. When simulated times to reach steady-state are compared to published time series rate data using suitable scaling, good agreement is found for silicate minerals while the model significantly over-predicts the duration of the transient for Fe and Al oxides. The implication of our simple model is that many aspects of mineral dissolution behavior, including approach to steady-state, sigmoidal vs. non-sigmoidal rate vs. Δ behavior, and development of rougher surfaces in conditions further from equilibrium can be explained by nearest neighbor interactions and simple Kossel-type models where reactivity of a surface is defined in terms of perfect surface, step, and kink sites.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 9
    In: Geology, Oct, 1995, Vol.23(10), p.933(4)
    Description: Airborne measurements of CO (sub 2) released from Oldoinyo Lengai, the only carbonatite-erupting volcano in the world, reveal a CO (sub 2) flux of 0.055 X 10 (super 12) mol/yr. This flux is substantially smaller than that of Mount Etna (1X10 (super 12) mol/yr), which accounts for over half of the global carbon flux attributed to subaerial volcanoes (1-2X10 (super 12) mol/yr). We propose that the subaerial flux distribution may be a power-law distribution (fractal) rather than Gaussian. Fitting the limited available volcanic flux data to a fractal distribution yields a power-law exponent of 〈1. Although rigorous testing of the power-law nature of the flux distribution is impossible, the skewed nature of the distribution and low value of the power-law exponent suggest that simultaneous measurement of the 20 largest-flux volcanoes could yield an accurate assessment of the volcanic CO (sub 2) flux. Summation over the power-law distribution predicts a maximum global subaerial passive volcanic flux of 2-3X10 (super 12) mol/yr and 2-3X10 (super 11) mol/yr for CO (sub 2) and SO (sub 2) , respectively. Normalizing the emission flux by scaling per unit crater area (instead of per volcano) to investigate the extension of the power-law behavior to geothermal areas with lower gas fluxes yields a power-law exponent of approximately 1 and predicts a subaerial volcanic-metamorphic CO (sub 2) flux of 6X10 (super 12) mol/yr.
    Keywords: Volcanoes -- Research ; Volcanism -- Research ; Metamorphism (Geology) -- Research ; Carbon Dioxide -- Environmental Aspects
    ISSN: 0091-7613
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  • 10
    Language: English
    In: Chemical Geology, 2010, Vol.278(1), pp.1-14
    Description: The primary objective of this research was to investigate the effects of aliphatic and aromatic low molecular weight organic acids (LMWOAs) on rare earth element and yttrium (REY) release from the phosphate minerals apatite and monazite. Since prior studies have shown that redox status can affect REY partitioning during incongruent dissolution, a secondary objective was to assess the influence of dissolved O concentration. Increasing LMWOA concentrations from 0 to 10 mM resulted in enhanced REY release. In general, REY release increased in the order: no ligand ≈ salicylate 〈 phthalate ≈ oxalate 〈 citrate. REY–ligand stability constants were only useful for predicting REY release for oxalate reacted with apatite and phthalate reacted with monazite. The role of dissolved oxygen in dissolution of the phosphate minerals was mixed and inconsistent. Mineral type was observed to significantly affect REY pattern development. REY release patterns for apatite range from nearly flat to those exhibiting the lanthanide contraction effect (radius-dependent fractionation); whereas, monazite REY release patterns are best described as exhibiting an M-type lanthanide tetrad effect (radius-independent fractionation). Weathering of apatite in the presence of aliphatic LMWOAs resulted in development of the lanthanide contraction effect fractionation pattern, and the aliphatic LMWOAs further developed MREE and radius-independent fractionation during monazite dissolution. Geochemical and mineral-specific REY signatures may, therefore, have utility for distinguishing the impacts of biota on soil weathering processes on early Earth. The development of such signatures may be mitigated, in part, by accessory mineral composition, the types and concentration of LMWOAs present, and precipitation of secondary minerals.
    Keywords: Apatite ; Dissolution ; Monazite ; Organic Acids ; Rare Earth Elements ; Yttrium ; Geology
    ISSN: 0009-2541
    E-ISSN: 1872-6836
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