Oxidative dissolution of pyritic sludge from the Aznalcóllar mine (SW Spain)

As a result of the collapse of a mine tailing dam, a large extension of the Guadiamar valley was covered with a layer of pyritic sludge. Despite the removal of most of the sludge, a small amount remained in the soil, constituting a potential risk of water contamination. The kinetics of the sludge oxidation was studied by means of laboratory flow-through experiments at different pH and oxygen pressures. The sludge is composed mainly of pyrite (76%), together with quartz, gypsum, clays, and sulphides of zinc, copper, and lead. Trace elements, such as arsenic and cadmium, also constitute a potential source of pollution. The sludge is fine grained (median of 12 μm) and exhibits a large surface (BET area of 1.4±0.2 m² g⁻¹).

The dissolution rate law of sludge obtained is r=10^{−6.1(±0.3)} [O₂(aq)]^0.41(±0.04) a_H+^0.09(±0.06) g_sludge m⁻² s⁻¹ (22 °C, pH=2.5–4.7). The dissolution rate law of pyrite obtained is r=10^{−7.8(±0.3)} [O₂(aq)]^0.50(±0.04) a_H+^0.10(±0.08) mol m⁻² s⁻¹ (22 °C, pH=2.5–4.7). Under the same experimental conditions, sphalerite dissolved faster than pyrite but chalcopyrite dissolves at a rate similar to that of pyrite. No clear dependence on pH or oxygen pressure was observed. Only galena dissolution seemed to be promoted by proton activity. Arsenic and antimony were released consistently with sulphate, except at low pH conditions under which they were released faster, suggesting that additional sources other than pyrite such as arsenopyrite could be present in the sludge. Cobalt dissolved congruently with pyrite, but Tl and Cd seemed to be related to galena and sphalerite, respectively.

A mechanism for pyrite dissolution where the rate-limiting step is the surface oxidation of sulphide to sulphate after the adsorption of O₂ onto pyrite surface is proposed.     

Temperature independent thermal expansivities of sodium aluminosilicate melts between 713 and 1835 K

The thermal expansivities of eight sodium aluminosilicate liquids were derived from the slope of new volume data at low temperatures (713−1072 K) combined with the high temperature (1300−1835 K) volume measurements of Stein et al. (1986) on the same liquids. Melt compositions range from 47−71 wt% SiO₂, 0−31 wt% A1203, and 17−33 wt% Na₂O; the volume of albite supercooled liquid at 1092 K was also determined. The low temperature volumes were derived from measurements of the glass density of each sample at 298 K, followed by measurements of the glass thermal expansion coefficient from 298 K to the respective glass transition interval. This technique takes advantage of the fact that the volume of a glass is equal to the volume of the corresponding liquid at the limiting fictive temperature (T′_f), and that T′_f can be approximated as the onset of the rapid rise in thermal expansion at the glass transition in a heating curve (Moynihan, 1995). No assumptions were made regarding the equivalence of enthalpy and volume relaxation through the glass transition. The propagated error on the volume of each supercooled liquid at T′_f is 0.25%. Combination of these low temperature data with the high temperature measurements of Stein et al. (1986) allowed a constant thermal expansivity of each liquid to be derived over a wide temperature interval (763−1001 degrees) with a fitted 1σ error of 0.6–4.6%; in every case, no temperature dependence to dV/dT^liq could be resolved. Calibration of a linear model equation leads to fitted values ± 1σ (units of cm³/mole) for (26.91 ± .04), (37.49 ± .12), (26.48 ± .06) at 1373 K, and (7.64 ± .08 × 10^-3 cm³/mole-K). The results indicate that neither Si02 nor Al₂O₃ contribute to the thermal expansivity of the liquids, and that dV/dT^liq is independent of temperature between 713–1835 K over a wide range of liquid composition. Calculated volumes based on this model recover both low and high temperature measurements with a standard deviation <0.25%, whereas values of dV/dT^liq can be predicted within 5.6%.     

An experimental study of calcite dissolution rates at acidic conditions and 25 °C in the presence of NaPO3 and MgCl2

Dissolution rates of single calcite crystals were determined from sample weight loss using free-drift rotating disk techniques. Experiments were performed at 25 °C in aqueous HCl solutions over the bulk solution pH range −1 to 3 and in the presence of trace concentrations of aqueous NaPO₃ and MgCl₂. These salts were chosen for this study because aqueous magnesium and phosphate are known to strongly inhibit calcite dissolution at neutral to basic pH. Reactive solutions were undersaturated with respect to possible secondary phases. Neither an inhibition or enhancement of calcite dissolution rates was observed in the presence of aqueous MgCl₂ at pH 1 and 3. The presence of trace quantities of NaPO₃, which dissociates in solution to Na⁺ and H₂PO₄⁻, decreased the overall calcite dissolution rate at pH≤2. This contrasting behavior could be attributed to the different adsorption behavior of these dissolved species. As calcite surfaces are positively charged in acidic solutions, aqueous Mg²⁺ may not adsorb, whereas aqueous phosphate, present as either the anion H₂PO₄⁻ or the neutral species H₃PO₄⁰, readily adsorbs on calcite surfaces leading to significant dissolution inhibition.     

Dissolution/precipitation kinetics of boehmite and gibbsite: Application of a pH-relaxation technique to study near-equilibrium rates

The dissolution and precipitation rates of boehmite, AlOOH, at 100.3 °C and limited precipitation kinetics of gibbsite, Al(OH)₃, at 50.0 °C were measured in neutral to basic solutions at 0.1 molal ionic strength (NaCl + NaOH + NaAl(OH)₄) near-equilibrium using a pH-jump technique with a hydrogen-electrode concentration cell. This approach allowed relatively rapid reactions to be studied from under- and over-saturation by continuous in situ pH monitoring after addition of basic or acidic titrant, respectively, to a pre-equilibrated, well-stirred suspension of the solid powder. The magnitude of each perturbation was kept small to maintain near-equilibrium conditions. For the case of boehmite, multiple pH-jumps at different starting pHs from over- and under-saturated solutions gave the same observed, first order rate constant consistent with the simple or elementary reaction: .

This relaxation technique allowed us to apply a steady-state approximation to the change in aluminum concentration within the overall principle of detailed balancing and gave a resulting mean rate constant, (2.2 ± 0.3) × 10⁻⁵ kg m⁻² s⁻¹, corresponding to a 1σ uncertainty of 15%, in good agreement with those obtained from the traditional approach of considering the rate of reaction as a function of saturation index. Using the more traditional treatment, all dissolution and precipitation data for boehmite at 100.3 °C were found to follow closely the simple rate expression:

R_net,boehmite=10^-5.485{m_OH^-}{1-exp(ΔG_r/RT)}, with R_net in units of mol m⁻² s⁻¹. This is consistent with Transition State Theory for a reversible elementary reaction that is first order in OH⁻ concentration involving a single critical activated complex. The relationship applies over the experimental ΔG_r range of 0.4–5.5 kJ mol⁻¹ for precipitation and −0.1 to −1.9 kJ mol⁻¹ for dissolution, and the pH_m ≡ −log(mH⁺) range of 6–9.6. The gibbsite precipitation data at 50 °C could also be treated adequately with the same model:R_net,gibbsite=10^-5.86{m_OH^-}{1-exp(ΔG_r/RT)}, over a more limited experimental range of ΔG_r (0.7–3.7 kJ mol⁻¹) and pH_m (8.2–9.7).     

Gum rosin (colophony): A suitable material for thermomechanical modelling of the lithosphere

We investigate the use of a ductile material with temperature-sensitive viscosity for thermomechanical modelling of the lithosphere. First, we consider the scaling of mechanical and thermal properties. For a normal field of gravity, the balance of stresses and body forces sets the stress scale, in proportion to the linear dimensions and the densities. The equation of thermal conduction sets the time scale. The activation enthalpy for creep sets the temperature scale; but the thermal expansivity provides an additional constraint on this temperature scale.

Gum rosin appears to be a suitable material for lithospheric modelling. We have measured its flow properties, at various temperatures, in a specially designed rotary viscometer with unusually low machine friction. The rosin is almost Newtonian. Strain rate depends upon stress to the power n, where 1.0 <n < 1.14. The viscosity varies over 5 orders of magnitude, from about 10² Pa s at 80°C, to about 10⁷ Pa s at 40°C. The activation enthalphy is thus about 250 kJ/mol. Measured with a needle probe, the thermal conductivity is 0.113 ± 0.001 W m⁻¹K⁻¹; the thermal diffusivity, (6±3) ×10⁻⁷ m² s⁻¹. Calculated from X-ray profiles, the thermal expansivity is about 3 × 10⁻⁴ K⁻¹. These thermal and mechanical properties make gum rosin suitable for thermomechanical models, where linear dimensions scale down by a factor of 10⁶; time, by 10¹¹; viscosity, by 10¹⁷; and temperature change, by 10¹.     

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

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.     

A review and analysis of silicate mineral dissolution experiments in natural silicate melts

We present a database and a graphical analysis of published experimental results for dissolution rates of olivine, quartz plagioclase, clinopyroxene, orthopyroxene, spinel, and garnet in basaltic and andesitic melts covering a range of experimental temperatures (1100–1500°C) and pressures (10⁵ Pa-3.0 GPa). The published datasets of Donaldson (1985, 1990) and Brearly and Scarfe (1986) are the most complete. Experimental dissolution rates from all datasets are recalculated and normalized to a constant oxygen basis to allow for direct comparison of dissolution rates between different minerals. Dissolution rates (ν) range from 5·10⁻¹⁰ oxygen equivalent moles (o.e.m.) cm⁻² s⁻¹ for olivine in a basaltic melt to 1.3·10⁻⁵ o.e.m. cm⁻² s⁻¹ for garnet in a basaltic melt. Values of ln ν are Arthenian for the experiments examined and activation energies range from 118 to 1800 kJ/o.e.m. for quartz and clinopyroxene, respectively.

The relationship between calculated A/RT for the dissolution reactions, where A is the thermodynamic potential affinity, and values of ν is linear for olivine, plagioclase, and quartz. We interpret this as strong evidence in support of using calculated A as a predictor of ν for, at least, superliquidus melt conditions.     

硫化纳米铁对模拟地下水中Cr(Ⅵ)的去除效果及影响因素

硫化纳米铁（S-nZVI）是一种具有壳核结构的新型纳米铁（nZVI）改性材料,在多种污染物的去除上表现出超越nZVI的反应活性。本文采用两步合成法制备了S-nZVI,并采用透射电镜-能量色散X射线（TEM-EDX）、X射线衍射（XRD）和X射线光电子能谱分析（XPS）方法对S-nZVI和nZVI进行表征,探讨了不同硫铁摩尔比（n（S）/n（Fe））、初始pH值、试剂投加量和地下水化学成分对nZVI及S-nZVI去除Cr（Ⅵ）的影响。结果表明：S-nZVI具有明显的壳核结构,其Fe⁰核外层包覆着非晶的硫化亚铁和多硫化物;S-nZVI去除Cr（Ⅵ）的最佳n（S）/n（Fe）为0.14;增加S-nZVI投加量会提高其对Cr（Ⅵ）的去除率,投加量相同时,S-nZVI对Cr（Ⅵ）的去除率显著高于nZVI;提高初始pH值时,S-nZVI和nZVI对Cr（Ⅵ）的去除率均逐渐降低,但在相同pH值条件下,S-nZVI对Cr（Ⅵ）的去除率和去除速率始终高于nZVI,尤其是在pH=5时,S-nZVI仍能去除100%的Cr（Ⅵ）,而nZVI只能去除85%;K⁺、Na⁺、Ca²⁺、Mg²⁺、SO₄^2-、NO₃^-和Cl^-对S-nZVI和nZVI去除Cr（Ⅵ）均有促进作用,但对S-nZVI体系的促进作用更强;HCO₃^-的存在会使溶液的pH值升高从而抑制S-nZVI和nZVI对Cr（Ⅵ）的去除,对nZVI的抑制作用强于S-nZVI。总体来说,S-nZVI对Cr（Ⅵ）的去除率在不同pH值和多种地下水化学组分影响条件下均高于nZVI,因此具有更广泛的应用前景。     

山东省青州市表层土壤硒元素地球化学特征

为研究山东省青州市表层土壤硒元素地球化学特征,在青州市按照平均5.2件/km²的密度采集了8 132件表层土壤样品,分析了硒、有机质、pH等指标。结果表明:青州市表层土壤硒含量范围在0.02~2.77 mg/kg,背景值为0.21 mg/kg(n=7 462)。研究区富硒土壤面积为140.56 km²,占研究区总面积的9.00%;足硒土壤面积为1 248.39 km²,占研究区总面积的79.93%;潜在硒不足土壤面积为148.45 km²,占研究区总面积的9.50%;硒缺乏土壤面积为24.52 km²,占研究区总面积的1.57%。不同的成土母质、土壤类型、地貌类型、土地利用类型区的表层土壤硒含量特征不同,寒武纪—奥陶纪地层成土母质区、钙质粗骨土分布区、溶蚀—切割中山地貌类型区、草地和林地土地利用类型区的表层土壤中硒相对富集。相关分析表明,研究区表层土壤中pH和硒含量无相关性,而有机质与硒含量呈显著正相关关系。     

Liquid immiscibility and its role at the magmatic–hydrothermal transition: a summary of experimental studies

Several types of fluid immiscibility may affect the evolution of volatile-rich magmatic systems at the magmatic–hydrothermal transition. The topology of silicate–salt–H₂O systems implies that three-fluid immiscibility (silicate melt+hydrosaline melt+vapour) should be stable in a broad range of compositions and P–T conditions. The most important factor controlling the immiscibility appears to be the Coulombic properties (electric charges Z and ionic radii r) of the main network-modifying cations and the capacity for immiscibility appears to decrease in the following sequence: Mg>Ca>Sr>Ba>Li>Na>K. Liquid immiscibility is enhanced in peralkaline compositions and in the presence of nonsilicate anions such as F⁻, Cl⁻, CO₃²⁻ and BO₃³⁻. In volatile-rich magmatic systems, the H₂O is likely to react with the chloride, fluoride, borate and carbonate species and the chemical effects of high-temperature hydrolysis may be greatly enhanced by phase separation in systems with multiple immiscible fluid phases. Natural granitic magmas can thus exsolve a range of chemically and physically diverse hydrosaline liquids and the role of these fluid phases is likely to be especially significant in pegmatites and Li–F rare-metal granites.     

Effect of Fe on garnet–melt trace element partitioning: experiments in FCMAS and quantification of crystal-chemical controls in natural systems

Garnet–melt trace element partitioning experiments were performed in the system FeO–CaO–MgO–Al₂O₃–SiO₂ (FCMAS) at 3 GPa and 1540°C, aimed specifically at studying the effect of garnet Fe²⁺ content on partition coefficients (D^Grt/Melt). D^Grt/Melt, measured by SIMS, for trivalent elements entering the garnet X-site show a small but significant dependence on garnet almandine content. This dependence is rationalised using the lattice strain model of Blundy and Wood [Blundy, J.D., Wood, B.J., 1994. Prediction of crystal–melt partition coefficients from elastic moduli. Nature 372, 452–454], which describes partitioning of an element i with radius r_i and valency Z in terms of three parameters: the effective radius of the site r₀(Z), the strain-free partition coefficient D₀(Z) for a cation with radius r₀(Z), and the apparent compressibility of the garnet X-site given by its Young's modulus E_X(Z). Combination of these results with data in Fe-free systems [Van Westrenen, W., Blundy, J.D., Wood, B.J., 1999. Crystal-chemical controls on trace element partitioning between garnet and anhydrous silicate melt. Am. Mineral. 84, 838–847] and crystal structure data for spessartine, andradite, and uvarovite, leads to the following equations for r₀(3+) and E_X(3+) as a function of garnet composition (X) and pressure (P):

r₀(3+) [Å]=0.930X_Py+0.993X_Gr+0.916X_Alm+0.946X_Spes+1.05(X_And+X_Uv)−0.005(P [GPa]−3.0)(±0.005 Å)

E_X(3+) [GPa]=3.5×10¹²(1.38+r₀(3+) [Å])^−26.7(±30 GPa)

Accuracy of these equations is shown by application to the existing garnet–melt partitioning database, covering a wide range of P and T conditions (1.8 GPa<P<5.0 GPa; 975°C<T<1640°C). D^Grt/Melt for all 3+ elements entering the X-site (REE, Sc and Y) are predicted to within 10–40% at given P, T, and X, when D^Grt/Melt for just one of these elements is known. In the absence of such knowledge, relative element fractionation (e.g. D_Sm^Grt/Melt/D_Nd^Grt/Melt) can be predicted. As an example, we predict that during partial melting of garnet peridotite, group A eclogite, and garnet pyroxenite, r₀(3+) for garnets ranges from 0.939±0.005 to 0.953±0.009 Å. These values are consistently smaller than the ionic radius of the heaviest REE, Lu. The above equations quantify the crystal-chemical controls on garnet–melt partitioning for the REE, Y and Sc. As such, they represent a major advance en route to predicting D^Grt/Melt for these elements as a function of P, T and X.     

Formation of vivianite in holocene clay sediments

Concretions of vivianite attaining 0.5 g occur in the upper 4 m of the bottom sediments of the Åsrum lake, Vestfold, S. Norway. The vivianite-bearing zone is of lacustrine origin. Vivianite is absent in the underlying lagunal and marine sediments. Electro-chemical and chemical data are given for the interstitial water and the clay.

Solubility product of vivianite is determined. The equilibrium constant is K = 10⁻⁴⁰ for the reaction 3 Fe⁺⁺ + 2 HPO₄⁻ + 2 OH⁻ = vivianite. The concretions grew by diffusion from a slightly supersaturated interstitial water phase. The degree of supersaturation is calculated. The stability relations of vivianite and hydroxyl apatite are discussed.     

Dissolution kinetics of experimentally shocked silicate minerals

The effect of lattice strain on mineral dissolution rates was examined by comparing the dissolution rates of shocked and unshocked minerals. Labradorite, oligoclase and hornblende were explosively shocked at mean pressures ranging from 4 to 22 GPa. The labradorite was examined with transmission electron microscopy to estimate the density of dislocations produced by the shock-loading experiment. Subsamples of the labradorite were then thermally annealed to remove some of the dislocations, and to evaluate the importance of such thermal pre-treatment in preparing mineral surfaces for experiments. The dissolution rates of these minerals were measured in batch experiments at pH-values of 2.7 and 4.0.

Shock-loading did not produce extremely high dislocation densities in the labradorite. The density of dislocations in the unshocked labradorite is ≤ 10¹⁰ m⁻². After shocking, the density increases to 10¹²-10¹³ m⁻². The distribution of dislocations is heterogeneous, and the amount of deformation does not increase substantially with shock pressure. These results are highly atypical of shock-modified minerals, where relatively low shock pressures usually induce high ( 10¹⁵ m⁻²) densities of dislocations. Thermal annealing for 1 hr. at 900°C in a dry furnace removes many dislocations from the shocked labradorite.

The difference in observed dissolution rates between shocked and unshocked minerals appears to have a weak correlation with the increase in the density of dislocations on the mineral surface. The unshocked and shocked oligoclase and hornblende samples exhibit limited dissolution enhancement at pH 4.0. Increasing the density of dislocations by several orders of magnitude with shock-loading causes a relatively small increase in dissolution rates for these silicate minerals. These results suggest that the surface dislocations produced by the shock treatment are not the primary sites for dissolution reactions.     

Confined fluids and their role in pressure solution

The process of pressure solution is defined as the dissolution of materials under high stress at grain-to-grain contacts and precipitation at interfaces under low stress. The kinetics of this process are still poorly understood mainly because of the large timescales involved. In this research, the Surface Forces Apparatus (SFA) technique was coupled with an optical interference technique for in situ visualization of the nanoscale deformations and thickness changes. The SFA was used to measure the forces (or pressures) and distances between two solid surfaces pressed together with a thin film between them. Using the SFA, combined with geological observations, we are studying the short-range colloidal forces between surfaces of mica and silica at the nanoscale such as van der Waals, electrostatic, and hydration forces.

This study involves two cases, the symmetric case of mica in contact with mica and the asymmetric case of a quartz surface in contact with mica. Our results reveal highly subtle effects depending on the nature and concentration of the counterions present in the solution either of Na⁺, Ca²⁺, or mixtures of these ions, as well as on the pH. For the symmetric case, the equilibrium interactions of force F or pressure P versus fluid film thickness T have been measured between the mica surfaces across aqueous films in the thickness range from T = 25 Å down to contact separations around T = 0 Å, and depend on the solution conditions and applied lithostatic pressure. Measurements have also been made of the rates of diffusion of ions through such ultra-thin films and on the precipitation and growth of ionic crystallite layers on the surfaces. Our results show that the diffusion coefficient of hydrated sodium is two orders of magnitude lower than the diffusion of water into mica–mica cleavage and a factor of 40 lower than the coefficient of sodium ions in bulk water.

For the asymmetric case, the dissolution of the quartz surface was observed to be dependent on the interfacial fluid composition and pH, the externally applied ‘lithostatic’ pressure, and the type of crystalline structure exposed to the mica surface. Our experiments also show that there is an initial stage after fresh solution is added in which the spacing between the surfaces increases, however, the thickness started decreasing steadily after approximately 4 h of exposure independently of the crystallinity of the quartz surface. For a particular set of conditions, the process eventually slows down and reaches equilibrium after some time, but a further increase in pressure restarts the dissolution process. This is also true for the addition of fresh interfacial solution during the experiment after a period of thickness fluctuation. These results are consistent with the observation that pressure solution of quartz is greatly enhanced with the presence of mica.     

Kinetics and mechanisms of kaolinite dissolution: effects of organic ligands

The effect of protons, low molecular weight organic ligands, soil humic acid (HA), and stream water dissolved organic matter (DOM) on the rate of dissolution of kaolinite was examined. In acid solution (no ligands present) the rate of dissolution increased with increasing [H⁺] and the rate of Si dissolution was generally faster than Al. Low molecular weight organic ligands markedly increased the dissolution rates of both Al and Si in the following order: oxalate > malonate ≈ salicylate > o-phthalate. In the presence of organic ligands, the rate of Al dissolution was generally much greater than Si. Soil HA and stream water DOM did not promote the dissolution of kaolinite under the experimental conditions examined in this study.

The dissolution kinetics of Al were interpreted in terms of a surface complexation model and the rate equations described in terms of the concentrations of specific (i.e. inner sphere) surface complexes.     

Distribution of Fe and Mg between coexisting garnet and hornblende in synthetic and natural systems: an empirical calibration of the garnet–hornblende Fe–Mg geothermometer

Multiple regression analysis of a compilation of the Fe²⁺–Mg distribution between garnet and hornblende from experimental runs on basaltic to intermediate compositions (n=22) and coexisting garnet–clinopyroxene–hornblende from natural (intermediate to basaltic) rocks (n=43) has been performed to define ln K_{D(Fe²⁺/Mg)}^Grt–Hbl as a function of temperature and garnet composition. The regression of data covering a large span in pressure (5–16 kbar), temperature (515–1025°C) and composition yields the ln K_{D(Fe²⁺/Mg)}^Grt–Hbl–P–T compositional relationship (r²=0.93):

where

Application of this expression to natural garnet–hornblende pairs in intermediate to basaltic and semipelitic rock types from various settings gives temperatures that are consistent with other methods.     

In situ production of terrestrial cosmogenic helium and some applications to geochronology

The concentrations of cosmogenic ³He have been measured in a series of basaltic drill core samples from Hawaiian volcanoes Haleakala and Mauna Loa. The ³He concentration in the surface of a radiocarbon dated Mauna Loa flow (20,000 years) gives reasonable agreement with a theoretical production rate of 140 atoms g−1yr−1 and suggests that the uncertainty in this rate is roughly 10%. The results illustrate the feasibility of using ³He to measure exposure ages of young basaltic lava flows and for measuring erosion rates. Erosion rates calculated from the three Haleakala cores range from 7 to 11 meters/million years. The drill core data demonstrate that accurate depth control is crucial to the use and evaluation of cosmogenic helium. Depth profiles from several of the older cores display a non-exponential depth dependence of ³He_c below 170 g cm⁻², which is attributed to the contribution from ⁶Li(n, )T, where the neutrons are from stopped muons. This has important implications for depth dependence of cosmogenic ³He because muons are weakly attenuated compared to the nucleonic component that produces spallation.     

稀有金属矿物溶解度对花岗伟晶岩成矿作用的制约

花岗伟晶岩型矿床是稀有金属矿床重要的类型之一。在花岗伟晶岩中,稀有金属元素Li、Be、Nb和Ta主要以独立矿物的形式存在,前人对稀有金属独立矿物在硅酸盐熔体中的溶解度及其影响因素展开了系统研究。本文综合分析了已有的实验数据,其结果表明,影响稀有金属独立矿物溶解度最为重要的2个参数是温度(T)和铝饱和指数(ASI)。因此本文建立了稀有金属独立矿物,尤其是铌锰矿和钽锰矿溶解度,与温度(T)和铝饱和指数(ASI)之间的定量关系: lg [w(Li)/10^-6]=-0.37×[1 000/(T/K)]+4.56,R²=0.44 lg [w(BeO)/10^-6]=-4.21×[1 000/(T/K)]+6.86,R²=0.91 lg [K_sp(Nb)/(mg²·kg^-2)]=-(2.86±0.14)×ASI_(Mn+Li)-(4.95±0.31)×[1 000/(T/K)]+(4.20+0.28),R²=0.86 lg [K_sp(Ta)/(mg²·kg^-2)]=-(2.46±0.11)×ASI_(Mn+Li)-(4.86±0.30)×[1 000/(T/K)]+(4.00+0.30),R²=0.80 式中,温度T为热力学温度,ASI_(Mn+Li)(ASI=Al₂O₃/(CaO+Na₂O+K₂O+Li₂O+MnO),摩尔分数比)和T的适用范围分别为0.6~1.2和1 073~1 373 K的范围内。上述公式为估算硅酸盐熔体中稀有金属含量提供了便利,为量化花岗伟晶岩成矿模型提供了基础。 稀有金属独立矿物溶解度随温度降低和铝饱和指数的增加而急剧降低,因此,在岩浆演化过程中,由岩浆侵位、分离结晶以及流体作用等因素引起的岩浆温度降低和铝饱和指数的增加,是导致稀有金属独立矿物结晶的主要机制。     

Comprehensive chemical analyses of natural cordierites: implications for exchange mechanisms

Cordierite samples from pegmatites and metamorphic rocks have been analysed for major [electron microprobe analysis (EMPA)] and trace elements [inductively coupled plasma mass spectrometry (ICP-MS), secondary ion mass spectrometry analyses (SIMS)] as well as for H₂O and CO₂ (coulometric titration), and the results evaluated in conjunction with published data in order to determine which exchange mechanisms are significant. Apart from the homovalent substitutions FeMg₋₁ and MnMg₋₁ on the octahedral site, some minor KNa₋₁ on the Ch0 channel site, and Fe³⁺Al₋₁ on the T₁1 tetrahedral site, the three most important substitution mechanisms are those for the incorporation of Li on the octahedral sites (NaLi□₋₁Mg₋₁), and of Be and other divalent cations on the tetrahedral T₁1 site (NaBe□₋₁Al₋₁ and Na(Mg,Fe²⁺)□₋₁Al₋₁). The dominant role of the last vector is clearly demonstrated. We propose a new generalized formula for cordierite: ^Ch(Na,K)_0–1 ^VI(Mg,Fe²⁺,Mn,Li)₂ ^IVSi₅ ^IVAl₃ ^IV(Al, Be, Mg, Fe²⁺, Fe³⁺)O₁₈ *x^Ch(H₂O, CO₂…). Our results show that the population of (Mg, Fe²⁺) on the T₁1-site is limited to about 0.08 a.p.f.u. Other exchange mechanisms that were encountered in experiments operate only under P–T conditions or in bulk compositions that are rarely realized in nature. Routine analyses by electron microprobe in which Li and Be are not determined can be plotted as (Mg+Fe+Mn) versus (Si+Al) to assess whether significant amounts of Li and Be could be present. These amounts can be calculated as Li (a.p.f.u.)=Al+Na–4 and Be (a.p.f.u.)=10–2Al–M²⁺–Na.     

Evolution of mineral–fluid interfaces studied at pressure with synchrotron X-ray techniques

In situ measurements of mineral surface evolution during the process of pressure solution are possible with the high brightness of synchrotron X-ray sources. This capability has been explored through the use of newly developed reaction vessels that allow transmission of the incident and scattered X-ray beam through a low atomic weight piston. Several new vessels are described, along with details of computational algorithms that are used to simulate X-ray scattering in this unconventional geometry. Results using calcite (CaCO₃) and halite (NaCl) as reactant crystals are presented and compared to other atomic-scale measurements of surface dissolution processes. Calcite was reacted with an unsaturated fluid at 30 bars of pressure for approximately 24 h. During reaction the root mean square surface roughness (σ) evolved from 13.7 Å (± 0.5 Å) to 19.5 Å (± 1.0 Å), giving a roughening rate of: dσ/dt = +6.3 × 10^− 5 Å s^− 1. This is consistent with other measurements made with free calcite surfaces and is driven almost entirely by chemical disequilibrium. Analysis of the surface ex situ post-reaction gives an identical σ value, showing that the in situ measurements are well-constrained. Experiments also at 30 bars but in a saturated solution indicate that the calcite surface does not significantly roughen, giving the result that pressure solution of calcite at this pressure cannot be monitored in experiments of several days duration. Experiments with halite, a much more reactive phase, in saturated solutions showed the reflectivity profile to be dynamic on a time scale of hours. This experiment was left to reach equilibrium over 108 days and then re-analyzed, showing that σ had increased from 34 Å (± 2 Å) to 41 Å (± 2 Å), giving a roughening rate of: dσ/dt ≤ +6.4 × 10^− 7 Å s^− 1. This is two orders of magnitude smaller than the calcite roughening rate caused by chemical disequilibrium and provides the first direct in situ atomic-scale measurement of the rate of surface roughening due to pressure solution.