Chemical and thermodynamic constraints on the hydrothermal transport and deposition of tin: I. Calculation of the solubility of cassiterite at high pressures and temperatures

Estimation of equation of state parameters for Sn⁺⁺ and calculation of the thermodynamic properties of other aqueous species and dissociation constants for various stannous and stannic complexes as a function of temperature permit prediction of the high temperature solution chemistry of tin and calculation of the solubility of cassiterite in hydrothermal solutions. The results of these calculations indicate that in the absence of appreciable chloride and fluoride concentrations, Sn(OH)₂⁰ and Sn(OH)₄⁰ are the predominant tin species in H₂O up to 350°C at ~2 $\text{\$}\text{?}$pH $\text{\$}\text{?}$7.5. The calculations also indicate that chloride complexes of Sn⁺⁺ predominate by several orders of magnitude over their fluoride and hydroxide counterparts in 1–3 molal (m) NaCl solutions, except in the presence of geologically unrealistic concentrations of fluoride or a pH greater than ~3.5 at 250°C or ~5.0 at 350°C. At higher pH values, most of the tin in solution is present as hydroxide complexes, even at concentrations of NaCl as high as 3 m. Calculated values of the solubility of cassiterite at high temperatures compare favorably with experimental data reported in the literature. Depending on the fugacity of oxygen and solution composition, the solubility of cassiterite in hydrothermal solutions may exceed 100 ppm under geologically realistic conditions.     

On the problem of transport species of tungsten by hydrothermal solutions

The solubility of pentatungstate of sodium (PTS) Na₂W₅O₁₆ · H₂O and sodium tungsten bronzes (STB) Na_0.16WO₃ in acid chloride solutions containing 0.026, 0.26, and 3.02m NaCl have been studied at 500°C, 1000 bar, given fO₂ (Co-CoO, Ni-NiO, PTS-STB buffers), and constant NaCl/HCl ratio (Ta₂O₅-Na₂Ta₄O₁₁ buffer). Depending on experimental conditions, the tungsten content in the solutions after experiments varied from 10⁻³ to 2 × 10⁻² mol/kg H₂O. Obtained data were used to calculate the formation constants of predominant tungsten complexes (VI, V): H₃W₃^VIO₁₂³⁻, W₃^VO₉³⁻, [W^VW₄^VIO₁₆]³⁻, for reactions

Pressure-dependent stability of cadmium chloride complexes: Potentiometric measurements at 1–1000 bar and 25°C

Potentiometric measurements were performed in the Cd(NO₃)₂-KCl-H₂O system at 25°C and 1–1000 bar using an isothermal cell with a liquid junction and equipped with a solid contact Cd-selective electrode. At 1 bar, the stepwise equilibrium constant of the fourth cadmium chloride complex CdCl₄²⁻ has been determined (log K₄⁰ = −0.88 ± 0.25). The pressure-dependent stability constants for all cadmium chloride complexes have been experimentally established for the first time. As pressure increases from 1 to 1000 bar, the stability constants for the first, third, and fourth complexes change by less than 0.05 logarithmic units, whereas that for the second complex decreases by 0.33 logarithmic units. On the basis of these data, the partial molar volumes of four cadmium chloride complexes have been determined under standard state conditions: V ⁰(CdCl⁺) = 2.20 ± 3, V ⁰(CdCl_{2 (aq)}) = 42.21 ± 5, V ⁰(CdCl₃⁻) = 63.47 ± 10, and V ⁰(CdCl₄²⁻) = 81.35 ± 15 cm³mol⁻¹. The linear correlation between the nonsolvation contributions of molar volumes and the number of ligands corresponds to the change in coordination from octahedral in Cd²⁺ and CdCl⁺ to tetrahedral in CdCl_{2 (aq)}, CdCl₃⁻, and CdCl₄²⁻ complexes. Using theoretical correlations, the HKF parameters allowing calculation of the volumetric properties of cadmium chloride complexes in a wide range of temperature and pressure have been obtained. The pressure effect on cadmium concentration in sphalerite in equilibrium with the H₂O-NaCl hydrothermal fluid has been estimated. It is shown that the Cd content in sphalerite increases with pressure.     

A Brine Evolution Model and Mineralogy of Chemical Sediments in a Volcanic Crater,Lake Kitagata,Uganda

Lake Kitagata, Uganda, is a hypersaline crater lake with Na–SO₄–Cl–HCO₃–CO₃ chemistry, high pH and relatively small amounts of SiO₂. EQL/EVP, a brine evaporation equilibrium model (Risacher and Clement 2001), was used to model the major ion chemistry of the evolving brine and the order and masses of chemically precipitated sediments. Chemical sediments in a 1.6-m-long sediment core from Lake Kitagata occur as primary chemical mud (calcite, magadiite [NaSi₇O₁₃(OH)₃·3H₂O], burkeite [Na₆(CO₃)(SO₄)₂]) and as diagenetic intrasediment growths (mirabilite (Na₂SO₄·10H₂O)). Predicted mineral assemblages formed by evaporative concentration were compared with those observed in salt crusts along the shoreline and in the core from the lake center. Most simulations match closely with observed natural assemblages. The dominant inflow water, groundwater, plays a significant role in driving the chemical evolution of Lake Kitagata water and mineral precipitation sequences. Simulated evaporation of Lake Kitagata waters cannot, however, explain the large masses of magadiite found in cores and the formation of burkeite earlier in the evaporation sequence than predicted. The masses and timing of formation of magadiite and burkeite may be explained by past groundwater inflow with higher alkalinity and SiO₂ concentrations than exist today.     

Deliquescence of NaCl–NaNO<Subscript>3</Subscript>, KNO<Subscript>3</Subscript>–NaNO<Subscript>3</Subscript>, and NaCl–KNO<Subscript>3</Subscript>salt mixtures from 90 to 120°C

We conducted reversed deliquescence experiments in saturated NaCl–NaNO₃–H₂O, KNO₃–NaNO₃–H₂O, and NaCl–KNO₃–H₂O systems from 90 to 120°C as a function of relative humidity and solution composition. NaCl, NaNO₃, and KNO₃ represent members of dust salt assemblages that are likely to deliquesce and form concentrated brines on high-level radioactive waste package surfaces in a repository environment at Yucca Mountain, NV. Discrepancy between model prediction and experiment can be as high as 8% for relative humidity and 50% for dissolved ion concentration. The discrepancy is attributed primarily to the use of 25°C models for Cl–NO₃ and K–NO₃ ion interactions in the current Yucca Mountain Project high-temperature Pitzer model to describe the nonideal behavior of these highly concentrated solutions.     

Calculation of thermodynamic properties of hydrated borates by group contribution method

 General equations to correlate and predict the thermodynamic properties of hydrated borates were developed based on the experimental results according to their structural types. The thermodynamic properties (ΔH _f ⁰ and ΔG _f ⁰) of a hydrated borate phase are the sum of the contributions of the cations in aqueous solution, the borate polyanions, and the structural water to the corresponding thermodynamic properties. This method is called the group contribution method, and it is extensively used to calculate the thermodynamic properties of many kinds of inorganic compounds, such as silicates and clay minerals. Received: 23 November 1998 / Accepted: 11 October 1999     

The Partitioning of Tungsten bwtween Aqueous Fluids and Silicate Melts

An experimental study has been carried out to determine the partition coefficients of tungsten between aqueous fluids and granitic melts at 800 °C and 1.5 kb with natural granite as the starting material. The effects of the solutions on the partition coefficients of tungsten show a sequence of P > CO ₃ ²⁻ > B > H₂O. The effects are limited (generallyK _D < 0.3) and the tungsten shows a preferential trend toward the melt over the aqueous fluid. The value ofK _D increases with increasing concentration of phosphorus; theK _D increases first and then reduces with the concentration of CO ₃ ²⁻ when temperature decreases, theK _D between the solution of CO ₃ ²⁻ and the silicate melt increases, and that between the solution of B₄O ₇ ²⁻ and the silicate melt decreases. The partition coefficients of phosphorus and sodium between fluids and silicate melts have been calculated from the concentrations of the elements in the melts. TheK _D value for phosphorus is 0.38 and that for sodium is 0.56. Evidence shows that the elements tend to become richer and richer in the melts.     

H2O activity in concentrated NaCl solutions at high pressures and temperatures measured by the brucite-periclase equilibrium

 H₂O activities in concentrated NaCl solutions were measured in the ranges 600°–900° C and 2–15 kbar and at NaCl concentrations up to halite saturation by depression of the brucite (Mg(OH)₂) – periclase (MgO) dehydration equilibrium. Experiments were made in internally heated Ar pressure apparatus at 2 and 4.2 kbar and in 1.91-cm-diameter piston-cylinder apparatus with NaCl pressure medium at 4.2, 7, 10 and 15 kbar. Fluid compositions in equilibrium with brucite and periclase were reversed to closures of less than 2 mol% by measuring weight changes after drying of punctured Pt capsules. Brucite-periclase equilibrium in the binary system was redetermined using coarsely crystalline synthetic brucite and periclase to inhibit back-reaction in quenching. These data lead to a linear expression for the standard Gibbs free energy of the brucite dehydration reaction in the experimental temperature range: ΔG° (±120J)=73418–134.95T(K). Using this function as a baseline, the experimental dehydration points in the system MgO−H₂O−NaCl lead to a simple systematic relationship of high-temperature H₂O activity in NaCl solution. At low pressure and low fluid densities near 2 kbar the H₂O activity is closely approximated by its mole fraction. At pressures of 10 kbar and greater, with fluid densities approaching those of condensed H₂O, the H₂O activity becomes nearly equal to the square of its mole fraction. Isobaric halite saturation points terminating the univariant brucite-periclase curves were determined at each experimental pressure. The five temperature-composition points in the system NaCl−H₂O are in close agreement with the halite saturation curves (liquidus curves) given by existing data from differential thermal analysis to 6 kbar. Solubility of MgO in the vapor phase near halite saturation is much less than one mole percent and could not have influenced our determinations. Activity concentration relations in the experimental P-T range may be retrieved for the binary system H₂O-NaCl from our brucite-periclase data and from halite liquidus data with minor extrapolation. At two kbar, solutions closely approach an ideal gas mixture, whereas at 10 kbar and above the solutions closely approximate an ideal fused salt mixture, where the activities of H₂O and NaCl correspond to an ideal activity formulation. This profound pressure-induced change of state may be characterized by the activity (a) – concentration (X) expression: a _{H 2}O=X _{H 2}O/(1+αX _NaCl), and a _NaCl=(1+α)^(1+α)[X _NaCl/(1+αX _NaCl)]^(1+α). The parameter α is determined by regression of the brucite-periclase H₂O activity data: α=exp[A–B/ϱ_{H 2}O ]-CP/T, where A=4.226, B=2.9605, C=164.984, and P is in kbar, T is in Kelvins, and ϱ_{H 2}O is the density of H₂O at given P and T in g/cm³. These formulas reproduce both the H₂O activity data and the NaCl activity data with a standard deviation of ±0.010. The thermodynamic behavior of concentrated NaCl solutions at high temperature and pressure is thus much simpler than portrayed by extended Debye-Hückel theory. The low H₂O activity at high pressures in concentrated supercritical NaCl solutions (or hydrosaline melts) indicates that such solutions should be feasible as chemically active fluids capable of coexisting with solid rocks and silicate liquids (and a CO₂-rich vapor) in many processes of deep crustal and upper mantle metamorphism and metasomatism. Received: 1 September 1995 / Accepted: 24 March 1996     

The system: Two alkali feldspars-KCl-NaCl-H2O at moderate to high temperatures and low pressures

Because of frequent discrepancies between the available experimental data and the measured composition of alkali chloride aqueous solutions coexisting with two alkali feldspars in high temperatures-low pressures natural systems, a systematic investigation of the system KAlSi₃O₈-NaAlSi₃O₈-KCl-NaCl-H₂O has been undertaken.Experiments have been carried out at temperatures from 300 °C to 660 °C, pressures from 0.2 to 2 kbar and total chloride concentrations ranging from 0.05 to 14 moles/kg H₂O.No effect of pressure on the feldspars solvus could be detected. Smoothing the experimental data on the basis of the regular assymetric solid solution model yields a critical temperature of 661°C and a critical composition of Or_0.36Ab_0.64.The equilibrium constant C = m _KCl/m _NaCl does not depend on total chloride molality, as long as the aqueous solution is homogeneous. But, in the miscibility gap (liquid+vapour) of the fluid, C is always lower in the vapour than in the liquid. The higher the temperature and the lower the pressure, the more striking this effect. For instance, at 500 ° C C _vaqour/C _liquid = 1 above 1 kb, 0.9 at 600 bars, 0.8 at 500 bars, 0.7 at 400–450 bars.The effect of pressure can be neglected in homogeneous fluids and in the liquid phase of unmixed fluids, but it is very important in the vapour phase (dilute solutions at low pressure).The selected values of C _max are (±0.01) 300 ° C0.083; 400 ° C0.139; 500 ° C0.200; 600 ° C0.264; 650 ° C0.298Such a behaviour of the fluid at low pressures explains the abnormally low values of m _KCl/m _NaCl measured in many natural hydrothermal systems. A new mechanism of alkali metasomatism (especially potassic alterations) is also proposed, taking into account the unmixing of alkali chloride aqueous solutions. This model seems particularly interesting in late magmatic hydrothermal processes, such as those occuring in porphyry type deposits.     

H2O activity in concentrated KCl and KCl-NaCl solutions at high temperatures and pressures measured by the brucite-periclase equilibrium

H₂O activities in supercritical fluids in the system KCl-H₂O-(MgO) were measured at pressures of 1, 2, 4, 7, 10 and 15  kbar by numerous reversals of vapor compositions in equilibrium with brucite and periclase. Measurements spanned the range 550–900 °C. A change of state of solute KCl occurs as pressures increase above 2 kbar, by which H₂O activity becomes very low and, at pressures of 4 kbar and above, nearly coincident with the square of the mole fraction (x _H2O). The effect undoubtedly results primarily from ionic dissociation as H₂O density (ρ_H2O) approaches 1 gm/cm³, and is more pronounced than in the NaCl-H₂O system at the same P-T-X conditions. Six values of solute KCl activity were yielded by terminal points of the isobaric brucite-periclase T-x _H2O curves where sylvite saturation occurs. The H₂O mole fraction of the isobaric invariant assemblage brucite-periclase-sylvite-fluid is near 0.52 at all pressures, and the corresponding temperatures span only 100 °C between 1 and 15 kbar. This remarkable convergence of the isobaric equilibrium curves reflects the great influence of pressure on lowering of both KCl and H₂O activities. The H₂O and KCl activities can be expressed by the formulas: a _H2O = γ_H2O[x _H2O+(1 + (1 + α)x _KCl)], and a _KCL = γ_KCl[(1 + α)x _KCl/(x _H2O +(1 + α)x _KCl)]^(1 + α), where α is a degree of dissociation parameter which increases from zero at the lowest pressures to near one at high pressures and the γ's are activity coefficients based on an empirical regular solution parameter W: ln γ_i = (1 − x_i)²W. Least squares fitting of our H₂O and KCl activity data evaluates the parameters: α = exp(4.166 −2.709/ρ_H2O) − 212.1P/T, and W = (−589.6 − 23.10P) /T, with ρ_H2O in gm/cm³, P in kbar and T in K. The standard deviation from the measured activities is only ± 0.014. The equations define isobaric liquidus curves, which are in perfect agreement with previous DTA liquidus measurements at 0.5–2 kbar, but which depart progressively from their extrapolation to higher pressures because of the pressure-induced dissociation effect. The great similarity of the NaCl-H₂O and KCl-H₂O systems suggests that H₂O activities in the ternary NaCl-KCl-H₂O system can be described with reasonable accuracy by assuming proportionality between the binary systems. This assumption was verified by a few reconnaissance measurements at 10 kbar of the brucite-periclase equilibrium with a Na/(Na + K) ratio of 0.5 and of the saturation temperature for Na/(Na + K) of 0.35 and 0.50. At that pressure the brucite-periclase curves reach a lowest x _H2O of 0.45 and a temperature of 587 °C before salt saturation occurs, values considerably lower than in either binary. This double-salt eutectic effect may have a significant application to natural polyionic hypersaline solutions in the deep crust and upper mantle in that higher solute concentrations and very low H₂O activities may be realized in complex solutions before salt saturation occurs. Concentrated salt solutions seem, from this standpoint, and also because of high mechanical mobility and alkali-exchanging potential, feasible as metasomatic fluids for a variety of deep-crust and upper mantle processes. Received: 9 August 1996 / Accepted: 15 November 1996     

Computer thermodynamic modeling of the transport and deposition of Sb and Au during the formation of Au-Sb deposits

Using the Chiller computer program, we performed modeling of the mechanisms of the joint transport and deposition of Au and Sb from various ore-forming solutions during the formation of Au-Sb deposits. Three models are considered by the example of the Uderei Au-Sb deposit in the Yenisei Ridge: (1) simple cooling (cooling only), (2) iso-enthalpy boiling (P = f(T)), and (3) solution–rock interaction (rock titration model). The behavior of Sb(III) and Au(I) in the system Au–Sb–Fe–Cu–Pb–Zn–As–H₂O–Cl–H₂S–CO₂ under hydrothermal conditions was studied. It is shown that both weakly alkaline (near-neutral) and reduced acidic Fe_aq²⁺-enriched low-chloride high-CO₂ and high-chloride hydrothermal solutions play a crucial role in the formation of gold parageneses of Au-Sb ores.     

Structure and specification of iron complexes in aqueous solutions determined by X-ray absorption spectroscopy

X-ray absorption spectroscopy, including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) techniques, have been used to determine the structure and speciation of complexes for Fe²⁺ and Fe³⁺ chloride solutions at a variety of pH's, ionic strengths, and chloride/iron ratios.Low intensity K-edge transition features and analysis of modified pair correlation functions, derived from Fourier transformation of EXAFS spectra, show a regular octahedral coordination of Fe(II) by water molecules with a first-shell Fe²⁺-O bond distance, closely matching octahedral Fe²⁺-O bonds obtained from solid oxide model compounds. Solution Fe²⁺-O bond distances decrease with chloride/iron ratio, pH, and total FeCl₂ concentration. A slight intensification of the $\text{1s → 3d}$ transition with increasing FeCl₂ concentration suggests that chloride may begin to mix with water as a nearest-neighbor octahedral ligand. Fe³⁺ solutions show a pronounced increase in the $\text{1s → 3d}$ transition intensities between 1.0 M FeCl₃/7.8 M Cl^? to 1.0 M FeCl₃/ 15 M Cl^?, indicating a coordination change from octahedral to tetrahedral complexes. EXAFS analyses of these solutions show an increase in first-shell Fe³⁺-ligand distances despite this apparent reduction in coordination number. This can be best explained by a change from regular octahedral complexes of ferric iron (either Fe(H₂O)₆³⁺ or trans-Fe(H₂O)₄Cl₂ or both; Fe³⁺-O bond distances of 2.10 Å) to tetra-chloro complexes [Fe³⁺-Cl bond distances of 2.25 Å].     

Thermodynamic mixing behavior of synthetic Ca-Tschermak–diopside pyroxene solid solutions: II. Heat of mixing and activity–composition relationships

The heat of mixing for the binary solid solution diopside–Ca-Tschermak was investigated at T = 980 K by lead borate solution calorimetry. A new statistical technique was applied to overcome the problem of using experimental data of various precisions. A two-parameter Margules model was fitted to the calorimetric data leading to W _CaTs–Di^H = 31.3 ± 3.4 kJ mol⁻¹ and W ^H _Di–CaTs = 2.4 ± 4.3 kJ mol⁻¹. The results are in good agreement with calorimetric data given in the literature. They agree also with enthalpy data that were extracted from phase equilibrium experiments. With configurational entropy values taken from the literature, the volume and the vibrational entropy, presented in Part I of this work, and the enthalpy data of this study, the activity–composition relationships of the diopside–Ca-Tschermak binary were calculated.     

Melting of albite and dehydration of brucite in H2O–NaCl fluids to 9 kbars and 700–900°C: implications for partial melting and water activities during high pressure metamorphism

 The melting reaction: albite_(solid)+ H₂O_(fluid) =albite-H₂O_(melt) has been determined in the presence of H₂O–NaCl fluids at 5 and 9.2 kbar, and results compared with those obtained in presence of H₂O–CO₂ fluids. To a good approximation, albite melts congruently at 9 kbar, indicating that the melting temperature at constant pressure is principally determined by water activity. At 5 kbar, the temperature (T)- mole fraction (X _(H2O) ) melting relations in the two systems are almost coincident. By contrast, H₂O–NaCl mixing at 9 kbar is quite non-ideal; albite melts ∼70 ^°C higher in H₂O–NaCl brines than in H₂O–CO₂ fluids for X _(H2O) =0.8 and ∼100 ^°C higher for X _(H2O) =0.5. The melting temperature of albite in H₂O–NaCl fluids of X _(H2O)=0.8 is ∼100 ^°C higher than in pure water. The P–T curves for albite melting at constant H₂O–NaCl show a temperature minimum at about 5 kbar. Water activities in H₂O–NaCl fluids calculated from these results, from new experimental data on the dehydration of brucite in presence of H₂O–NaCl fluid at 9 kbar, and from previously published experimental data, indicate a large decrease with increasing fluid pressure at pressures up to 10 kbar. Aqueous brines with dissolved chloride salt contents comparable to those of real crustal fluids provide a mechanism for reducing water activities, buffering and limiting crustal melting, and generating anhydrous mineral assemblages during deep crustal metamorphism in the granulite facies and in subduction-related metamorphism. Low water activity in high pressure-temperature metamorphic mineral assemblages is not necessarily a criterion of fluid absence or melting, but may be due to the presence of low a _(H2O) brines. Received: 17 March 1995/Accepted: 9 April 1996     

A modern, guano-related occurrence of foggite, CaAl(PO4)(OH)2 · H2O and churchite-(Y), YPO4 · 2H2O in Cioclovina Cave, Romania

Summary This study reports foggite and churchite-(Y) from two spatially separate locations in the guano-related phosphate deposit from the Cioclovina Cave, Romania. Optical microscope observations, powder X-ray diffraction, electron microprobe analyses, and FTIR were used in the analysis of the two minerals. The chemical composition of foggite was determined to be Ca_0.925(Al_0.91Fe²⁺_0.016)_Σ0.926(P_0.991Si_0.043)_Σ1.034O_3.74(OH)_2.26 · H₂O and churchite-(Y) [(Y_0.830Dy_0.043Er_0.033Gd_0.029Yb_0.022)_Σ0.957Ca_0.009]P_1.023O_4.00 · 2H₂O. Chemical analyses of Cioclovina churchite-(Y) clearly revealed enrichment in lanthanides of even atomic number. The refined unit-cell parameters are for foggite (orthorhombic) a = 9.264(1) ?, b = 21.334(8) ?, c = 5.197(7) ?, and V = 1027.13(8) ?³ (Z = 8); for churchite-(Y) (monoclinic): a = 5.578(8) ?, b = 15.013(6) ?, c = 6.277(8) ?, β = 117.94(4)°, and V = 464.38(5) ?³ (Z = 4). FTIR spectrum of churchite-(Y) exhibits all the bands assigned to the vibrations of PO₄, OH, and water groups. Unlike other documented occurrences of foggite and churchite-(Y), in Cioclovina Cave, the occurrence of these minerals are related to a process that phosphatized subjacent limestone and various cave sediments (sand, clay, and limy mud) to form a complex phosphate assemblage. The minerals are presumably derived from phosphate-rich solutions that reacted with clay earth while moving downward through the sediments. Foggite was formed at the expense of the originally precipitated crandallite. Locally concentrated yttrium, REE, and dissolved phosphate are probably responsible for the precipitation of churchite-(Y). Present address: Department of Geology, University of South Florida, Tampa, FL, USA     

Feasibility and cost of creating an iron-phosphate coating on pyrrhotite to prevent oxidation

 Acid mine drainage (AMD) occurs when sulfide minerals are exposed to an oxidizing environment. Most of the methods for preventing AMD are either short-term or high cost solutions. Coating with iron phosphate is a new technology for the abatement of AMD. It involves treating the sulfide with a coating solution composed of H₂O₂, KH₂PO₄, and sodium acetate as a buffer agent. The H₂O₂ oxidizes the sulfide surface and produces Fe³⁺ so that iron phosphate precipitates as a coating on the sulfide surface. Experiments performed under laboratory conditions prove that an iron phosphate coating can be established on pyrrhotite surfaces with optimal concentrations of the coating solution in the range of: 0.2M/0.01M H₂O₂, 0.2M KH₂PO₄, and 0.2M sodium acetate NaAc, depending on the experimental scale. Iron phosphate coating may be a long-term solution to the problem of AMD. The method would be easy to implement; the reagent cost, however, is not low enough, although it is lower than the conventional treatment with lime. Received: 30 March 1995 · Accepted: 6 September 1995     

Halogen-bearing minerals in syenites and high-grade marbles of Dronning Maud Land,Antarctica: monitors of fluid compositional changes during late-magmatic fluid-rock interaction processes

Meta-sedimentary rocks including marbles and calcsilicates in Central Dronning Maud Land (CDML) in East Antarctica experienced a Pan-African granulite facies metamorphism with peak metamorphic conditions around 830 ± 20 °C at 6.8 ± 0.5 kbar which was accompanied by the post-kinematic intrusion of huge amounts of syenitic (charnockitic) magmas at 4.5 ± 0.7 kbar. The marbles and calcsilicates may represent meta-evaporites as indicated by the occurrence of metamorphic gypsum/anhydrite and Cl-rich scapolite that formed in the presence of saline fluids with X _NaCl in the range 0.15–0.27. The marbles and calcsilicates bear biotite, tremolite and/or hornblende and humite group minerals (clinohumite, chondrodite and humite) which are inferred to have crystallized at about 650 °C and 4.5 kbar. The syenitic intrusives contain late-magmatic biotite and amphibole (formed between 750 and 800 °C) as well as relictic magmatic fayalite, orthopyroxene and clinopyroxene. Two syenite and two calcsilicate samples contain fluorite. Corona textures in the marbles and calcsilicates suggest very low fluid-rock ratios during the formation of the retrograde (650 °C) assemblages. Biotite in all but two syenite samples crystallized at log(f _{H 2 O}/f _HF) ratios of 2.9 ± 0.4, while in the calcsilicates, both biotite and humite group minerals indicate generally higher log(f _{H 2 O}/f _HF) values of up to 5.2. A few samples, though, overlap with the syenite values. Log(f _{H 2 O}/f _HCl) derived from biotite covers the range 0.5–2.6 in all rock types. Within a single sample, the calculated values for both parameters vary typically by 0.1 to 0.8 log units. Water and halogen acid fugacities calculated from biotite-olivine/orthopyroxene-feldspar-quartz equilibria and the above fugacity ratios are 1510–2790 bars for H₂O, 1.3–5.3 bars for HF and 7–600 bars for HCl. The results are interpreted to reflect the reaction of relatively homogeneous magmatic fluids [in terms of log(f _{H 2 O} /f _HF)] derived from the late-magmatic stages of the syenites with both earlier crystallized, still hotter parts of the syenites and with adjacent country rocks during down-temperature fluid flow. Fluorine is successively removed from the fluid and incorporated into F-bearing minerals (close to the syenite into metamorphic fluorite). In the course of this process log(f _{H 2 O} /f _HF) increases significantly. Chlorine preferably partitions into the fluid and hence log(f _{H 2 O} /f _HCl) does not change markedly during fluid-rock interaction. Received: 28 November 1997 / Accepted: 27 April 1998     

Partitioning of strontium between calcite,dolomite and liquids: An experimental study under higher temperature diagenetic conditions,and a model for the prediction of mineral pairs for geothermometry

The partitioning of Sr between calcite, dolomite and liquids is essentially independent of temperature between 150° and 350° C. The partition coefficients corrected for number of cation sites are b _calc=0.096 and b _dol= 0.048 for 1 mol cations/6 mol H₂O liquid. Upon dilution the partition coefficients increase, but their ratio stays constant at about 2∶1. This ratio is due to the fact that calcite has twice as many Ca-sites for Sr-substitution as dolomite. The 2∶1 relationship is also observed in natural calcite and dolomite which have undergone diagenesis. The temperature independence of partitioning is caused by the relatively small thermal expansion of calcite and dolomite. Thermal expansion between 25° and 400° C was found to follow the equations V _calc=7.0·10⁻⁴ T(°C)+36.95 and V _dol=6.9·10⁻⁴ T(°C)+32.24, V: cm³/mol. Therefore calcite and dolomite cannot serve as a temperature indicator. To have an ideal geothermometer a mineral pair with high and low thermal expansion is required. Literature date demonstrate that wurtzite, sphalerite, and galena are such minerals.     

The passivation of calcite by acid mine water. Column experiments with ferric sulfate and ferric chloride solutions at pH 2

Column experiments, simulating the behavior of passive treatment systems for acid mine drainage, have been performed. Acid solutions (HCl or H₂SO₄, pH 2), with initial concentrations of Fe(III) ranging from 250 to 1500 mg L⁻¹, were injected into column reactors packed with calcite grains at a constant flow rate. The composition of the solutions was monitored during the experiments. At the end of the experiments (passivation of the columns), the composition and structure of the solids were measured. The dissolution of calcite in the columns caused an increase in pH and the release of Ca into the solution, leading to the precipitation of gypsum and Fe–oxyhydroxysulfates (Fe(III)–SO₄–H⁺ solutions) or Fe–oxyhydroxychlorides (Fe(III)–Cl–H⁺ solutions). The columns worked as an efficient barrier for some time, increasing the pH of the circulating solutions from 2 to 6–7 and removing its metal content. However, after some time (several weeks, depending on the conditions), the columns became chemically inert. The results showed that passivation time increased with decreasing anion and metal content of the solutions. Gypsum was the phase responsible for the passivation of calcite in the experiments with Fe(III)–SO₄–H⁺ solutions. Schwertmannite and goethite appeared as the Fe(III) secondary phases in those experiments. Akaganeite was the phase responsible for the passivation of the system in the experiments with Fe(III)–Cl–H⁺ solutions.     

Geochemical hypothesis for hydrated magnesium borate deposit in Salt Lake,NW China

Chloropinnoite, 2MgO·2B₂O₃·MgCl₂·14H₂O, was a new borate, obtained from the natural concentrated salt lake brine in Qinghai–Xizang Plateau, P. R. China. The phase transitions from chloropinnoite dissolved in water and boric acid solution would strongly consist of magnesium borate minerals deposited in a salt lake of China. The obtained results from phase relations and kinetic mechanism of chloropinnoite–water/boric acid system would further propose a new geochemical hypothesis for hydrated magnesium borate deposition. The chloropinnoite was diluted by rich boric acid or solution, and it would accelerate the phase transition of chloropinnoite into other borates, which would explain well the geochemical formation of hydrated borate minerals deposited in Qinghai–Xizang Plateau, China.