Effect of water activity on rates of serpentinization of olivine

The hydrothermal alteration of mantle rocks (referred to as serpentinization) occurs in submarine environments extending from mid-ocean ridges to subduction zones. Serpentinization affects the physical and chemical properties of oceanic lithosphere, represents one of the major mechanisms driving mass exchange between the mantle and the Earth’s surface, and is central to current origin of life hypotheses as well as the search for microbial life on the icy moons of Jupiter and Saturn. In spite of increasing interest in the serpentinization process by researchers in diverse fields, the rates of serpentinization and the controlling factors are poorly understood. Here we use a novel in situ experimental method involving olivine micro-reactors and show that the rate of serpentinization is strongly controlled by the salinity (water activity) of the reacting fluid and demonstrate that the rate of serpentinization of olivine slows down as salinity increases and H2O activity decreases

Revisiting geochemical controls on patterns of carbonate deposition through the lens of multiple pathways to mineralization

The carbonate sedimentary record contains diverse compositions and textures that reflect the evolution of oceans and atmospheres through geological time. Efforts to reconstruct paleoenvironmental conditions from these deposits continue to be hindered by the need for process-based models that can explain observed shifts in carbonate chemistry and form. Traditional interpretations assume minerals precipitate and grow by classical ion-by-ion addition processes but are unable to reconcile a number of unusual features contained in Proterozoic carbonates. The realization that diverse organisms produce high Mg carbonate skeletal structures by non-classical pathways involving amorphous intermediates raises the question of whether similar processes are also active in sedimentary environments. This study examines the hypothesis that non-classical pathways to mineralization are the physical basis for some of the carbonate morphologies and compositions observed in natural and laboratory settings. We designed experiments with a series of different solution Mg : Ca ratios and saturation environments to investigate the effects on carbonate phase, Mg content, and morphology. Our observations of diverse carbonate mineral compositions and textures suggest geochemical conditions bias the mineralization pathway by a systematic relationship to Mg : Ca ratio and the abundance of carbonate ions. Environments with low Mg levels produce calcite crystallites with 0–12 mol% MgCO_3. In contrast, the combination of high initial Mg : Ca and rapidly increasing saturation opens a non-classical pathway that begins with extensive precipitation of an amorphous calcium carbonate (ACC). This phase slowly transforms to aggregates of very high Mg calcite nanoparticles whose structures and compositions are similar to natural disordered dolomites. The non-classical pathways are favored when the local environment contains sufficient Mg to inhibit calcite growth through increased solubility—a thermodynamic factor, and achieves saturation with respect to ACC on a timescale that is shorter than the rate of aragonite nucleation—a kinetic factor. Aragonite is produced when Mg levels are high but saturation is insufficient for ACC precipitation. The findings provide a physical basis for anecdotal claims that the interplay of kinetic and thermodynamic factors underlies patterns of carbonate precipitation and suggest the need to expand traditional interpretations of geological carbonate formation to include non-classical pathways to mineralization

Metastability, nanocrystallinity and pseudo-solid solution effects on the understanding of schwertmannite solubility

The role of metastable nanocrystalline precursors, like schwertmannite, in iron and sulfate rich acidic waters is commonly underestimated or even neglected. In addition to schwertmannite metastability, its heterogeneous chemical composition and the current use of disparate solubility products result in an incongruous understanding of this mineral. In order to characterize schwertmannite stability in acid mine drainage settings, we used coincident schwertmannite and solution samples to determine how its solubility product is related to its composition. The solubility products (as log Ksp) for 30 natural samples of this study span a range of log Ksp values from 5.8 to 39.5. These values show a gradual distribution on the pH–pe space from pH 1.93 to 4.71 and pe values from8.5 to 13.7. A set of three predictive equations to select the best schwertmannite solubility product for each new specific case study was obtained. This approximation allows generating an appropriate solubility product for schwertmannite despite the lack of information in certain environments (e.g., absence of former water chemistries on Mars). The trend observed for Fe and S contents in schwertmannite can be interpreted as a pseudo-solid solution ranging from high to low S and Fe concentrations. The polyphasic nature of schwertmannite was studied by means of a thermodynamic model assuming equilibrium between a hydrous ferric oxide (HFO), schwertmannite, and solution. All the results obtained in this study support the understanding of schwertmannite as a polyphasic nanomineral and encourage using a broad log Ksp range to model the solubility of schwertmannite in natureThis study was financed by the Spanish Ministry of Science and Innovation through the project METODICA (Ref. CGL2010-21956-C02-02). M.A.C. was financially supported by the Spanish Ministry of Education and the Post-doctoral International Mobility Sub-programme I+D+i 2008–2011.Departamento de Ingeniería Minera, Mecánica, Energética y de la Construcció

Acid mine drainage

When metal sulfi de minerals are exposed to air and water, they break down and give rise to acidic, sulfate-rich waters contaminated with dissolved metals, particularly iron. Most commonly, this exposure is due to mining, and the waters are called acid mine drainage (AMD). Sulfi des may also be exposed by natural processes or construction proj-ects, and then the resulting contaminated waters are called acid rock drainage. In the USA alone, over 15,000 kilometers of rivers are polluted by AMD, a legacy of the mining of metals and coal. AMD reduces water quality, kills aquatic organisms, and makes receiving waters unsuit-able for domestic and industrial use. By far the most important sulfi de mineral contributing to AMD is pyrite (FeS2). In this article we discuss the processes involved in AMD generation and in the management of AMD, focusing on pyrite as the source of contamination, and we dem-onstrate the role of “mineralogy ” in our understanding of this problem. PYRITE AND AMD GENERATIO

Estimating the thermodynamic properties (Δ Gof and Δ Hof ) of silicate minerals at 298 K from the sum of polyhedral contributions

Many physical properties of silicate minerals can be modeled as a combination of basic polyhedral units (Hazen, 1985, 1988). It follows that their thermodynamic properties could be modeled as the sum of polyhedral contributions. We have determined, by multiple regression, the contribution of the [4]A12O3,[6]A12O3, [6]Al(OH)3, [4]SiO2, [6]MgO, [6]Mg(OH)2,[6]CaO, [8-z]CaO, [6−8]Na2O, [8−12]K2O, H2O, [6]FeO, [6]Fe(OH)2, and [6]Fe2O3 components to the total ΔG0f and ΔH0f of a selected group of silicate minerals. Using these data we can estimate the ΔG0f and ΔH0f of other silicate minerals from a weighted sum of the contribution of each oxide and hydroxide component: ΔG0f = Σnigi ,  and ΔH0f = Σni hi, where ni is the number of moles of the oxide or hydroxide per formula unit and gi and hi, are the respective molar free energy and enthalpy contribution of 1 mol of each oxide or hydroxide component. The technique outlined here can be used to estimate the thermodynamic properties of many silicate phases that are too complex or too impure to give reliable calorimetric measurements. Experimentally measure ΔG0f and ΔH0f vs. predicted ΔG0f and ΔH0f for the minerals used in the model have associateda verager esiduals of 0.26% and 0.24% respectively. Thermodynamic properties of minerals not used in the model but for which there are experimentally determined calorimetric data have average differences between measured and predicted values of 0.25% for ΔG0f or 18 minerals and 0.22% for ΔH0f for 20 minerals

Measuring reaction rates at equilibrium with the isotope doping method

Since the time of J. H. van’t Hoff [1], it has been known that chemical equilibrium is dynamic, meaning that at equilibrium, chemical reactions do not cease, but instead the forward and backward reaction rates are equal. The constant concentrations at equilibrium preclude the use of concentrations to measure reaction rates at equilibrium. Therefore, with the exception of a few special cases, no reaction rates at equilibrium have been published in the literature of chemistry, physics, or chemical engineering. Here we report dissolution and precipitation rates at equilibrium for quartz and barite with the isotope-doping method. Experimental data show that dissolution and precipitation rates are equal at equilibrium, indicating the principle of detailed balance (PDB) appear to be applicable at these experimental conditions. The PDB has been a cornerstone for irreversible thermodynamics and chemical kinetics for a long time, and its wide application in geochemistry has mostly been implicit and without experimental testing of its applicability. Nevertheless, many extrapolations based on PDB without experimental validation have far reaching impacts on society’s mega environmental enterprises. The isotope doping method appears to able to test its applicability for a variety of minerals at a wide range of conditions