Modeling ferrous-ferric iron chemistry with application to martian surface geochemistry

The Mars Global Surveyor, Mars Exploration Rover, and Mars Express missions have stimulated considerable thinking about the surficial geochemical evolution of Mars. Among the major recent mission findings are the presence of jarosite (a ferric sulfate salt), which requires formation from an acid-sulfate brine, and the occurrence of hematite and goethite on Mars. Recent ferric iron models have largely focused on 25 °C, which is a major limitation for models exploring the geochemical history of cold bodies such as Mars. Until recently, our work on low-temperature iron-bearing brines involved ferrous but not ferric iron, also obviously a limitation. The objectives of this work were to (1) add ferric iron chemistry to an existing ferrous iron model (FREZCHEM), (2) extend this ferrous/ferric iron geochemical model to lower temperatures (<0 °C), and (3) use the reformulated model to explore ferrous/ferric iron chemistries on Mars.The FREZCHEM model is an equilibrium chemical thermodynamic model parameterized for concentrated electrolyte solutions using the Pitzer approach for the temperature range from <−70 to 25 °C and the pressure range from 1 to 1000 bars. Ferric chloride and sulfate mineral parameterizations were based, in part, on experimental data. Ferric oxide/hydroxide mineral parameterizations were based exclusively on Gibbs free energy and enthalpy data. New iron parameterizations added 23 new ferrous/ferric minerals to the model for this Na-K-Mg-Ca-Fe(II)-Fe(III)-H-Cl-SO₄-NO₃-OH-HCO₃-CO₃-CO₂-O₂-CH₄-H₂O system.The model was used to develop paragenetic sequences for Rio Tinto waters on Earth and a hypothetical Martian brine derived from acid weathering of basaltic minerals. In general, model simulations were in agreement with field evidence on Earth and Mars in predicting precipitation of stable iron minerals such as jarosites, goethite, and hematite. In addition, paragenetic simulations for Mars suggest that other iron minerals such as lepidocrocite, schwertmannite, ferricopiapite, copiapite, and bilinite may also be present on the surface of Mars. Evaporation or freezing of the Martian brine led to similar mineral precipitates. However, in freezing, compared to evaporation, the following key differences were found: (1) magnesium sulfates had higher hydration states; (2) there was greater total aqueous sulfate (SO_4T = SO₄ + HSO₄) removal; and (3) there was a significantly higher aqueous Cl/SO_4T ratio in the residual Na-Mg-Cl brine. Given the similarities of model results to observations, alternating dry/wet and freeze/thaw cycles and brine migration could have played major roles in vug formation, Cl stratification, and hematite concretion formation on Mars.     

Copper sulphide formation chemistry at low temperatures

Summary Covellite, blaubleibender covellite, djurleite and chalcocite have been synthesized by the reaction between Cu₂O with aqueous Na₂S solutions at low temperatures. Digenite was produced by heating the dried products of the aqueous experimentation at 105°C for 50 days, but not directly from aqueous solution. Anilite was formed through air oxidation of chalcocite at room temperature. The pH and Eh conditions of formation of these phases have been studied and it has been shown that the synthetic results broadly reflect the theoretical stability fields for these compounds, inasmuch as more reduced copper sulphides are formed under higher pH and lower Eh conditions. Studies of the changes in these parameters during the course of the experiments have been used to outline the mechanisms involved in the reactions. It has been demonstrated that the form of the copper sulphide produced around neutral pH values is almost independent of the oxidation state of the copper reactant, but highly sensitive to the presence and concentration of elemental sulphur. A detailed discussion of the formation chemistry of copper sulphides in low temperature aqueous solutions is presented.

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Chemie der Kupfersulfidbildung bei niedrigen Temperaturen

Zusammenfassung Covellin, blaubleibender Covellin, Djurleit und Chalkosin wurden durch Reaktion zwischen Cu₂O und wäßrigen Na₂S-Lösungen bei niedrigen Temperaturen synthetisiert. Digenit entstand, wenn die getrockneten Produkte aus wäßrigen Lösungen 50 Tage bei 105°C getempert wurden, aber nicht direkt aus wäßriger Lösung. Anilit bildete sich aus Chalkosin bei Zimmertemperatur durch Oxidation an der Luft. Die pH- und Eh-Bedingungen für die Bildung dieser Phasen wurden untersucht und es wurde gezeigt, daß die Syntheseergebnisse im wesentlichen den theoretischen Stabilitätsfeldern dieser Verbindungen entsprechen, indem sich die stärker reduzierten Kupfersulfide unter höherem pH und kleinerem Eh bilden. Untersuchungen über die Änderung dieser Parameter im Verlauf der Experimente wurden zur Umreißung der Reaktionsmechanismen benützt. Es wurde gezeigt, daß die Art des Kupfersulfides, welches sich um neutrale pH-Werte bildet, fast nicht vom Oxidationszustand des reagierenden Kupfers, aber stark von der Anwesenheit und der Konzentration von elementarem Schwefel abhängt. Die Chemie der Bildung von Kupfersulfiden in wäßrigen Lösungen niedriger Temperatur wird ausführlich diskutiert.

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With 2 Figures     

An iron isotope signature related to electron transfer between aqueous ferrous iron and goethite

We measured the Fe isotope fractionation during the reactions of Fe(II) with goethite in the presence and absence of a strong Fe(III) chelator (desferrioxamine mesylate, DFAM). All experiments were completed in an O₂-free glove box. The concentrations of aqueous Fe(II) ([Fe(II)_aq]) decreased below the initial total dissolved Fe concentrations ([Fe(II)_total], 2.15 mM) due to fast adsorption within 0.2 day. The concentration of adsorbed Fe(II) ([Fe(II)_ads]) was determined as the difference between [Fe(II)_aq] and the concentration of extracted Fe(II) in 0.5 M HCl ([Fe(II)_extr]) (i.e., [Fe(II)_ads] = [Fe(II)_extr] − [Fe(II)_aq]). [Fe(II)_ads] also decreased with time in experiments with and without DFAM, documenting that fast adsorption was accompanied by a second, slower reaction. Interestingly, [Fe(II)_extr] was always smaller than [Fe(II)_total], indicating that some Fe(II) was sequestered into a pool that is not HCl-extractable. The difference was attributed to Fe(II) incorporated into goethite structure (i.e., [Fe(II)_inc] = [Fe(II)_total] −[Fe(II)_extr]). More Fe(II) was incorporated in the presence of DFAM than in its absence at all time steps. Regardless of the presence of DFAM, both aqueous and extracted Fe(II) (δ^56/54Fe(II)_aq and δ^56/54Fe(II)_extr) became isotopically lighter than or similar to goethite (− 0.27‰) at day 7, implying that the isotope exchange occurred between bulk goethite and aqueous Fe. Consistently, the mass balance indicated that the incorporated Fe is isotopically heavier than extracted Fe. These observations suggested that (i) co-adsorption of Fe(II) with DFAM resulted in more pervasive electron transfer, (ii) the electron transfer from heavy Fe(II) in the adsorbed Fe(II) to light Fe(III) in goethite results in the fixation of heavy adsorbed Fe(III) on the surface and accumulation of Fe(II) within the goethite, and (iii) desorption of the reduced, light Fe from goethite does not necessarily occur at the same surface sites where adsorption occurred.     

A molal-based model for strong acid chemistry at low temperatures (<200 to 298 K)

Geochemical processes occurring in cold environments on Earth, Mars, and Europa have elicited considerable interest in the application of geochemical models to subzero temperatures. Few existing geochemical models explicitly include acid chemistry and those that do are largely restricted to temperatures ≥0°C or rely on the mole-fraction scale rather than the more common molal scale. This paper describes (1) use of the Clegg mole-fraction acid models to develop a molal-based model for hydrochloric, nitric, and sulfuric acids at low temperatures; (2) incorporation of acid chemistry and nitrate minerals into the FREZCHEM model; (3) validation and limitations of the derived acid model; and (4) simulation of hypothetical acidic brines for Europa.The Clegg mole-fraction acid models were used to estimate activities of water and mean ionic activity coefficients that serve as the database for estimating molal Pitzer-equation parameters for HCl (188 to 298 K), HNO₃ (228 to 298 K), and H₂SO₄ (208 to 298 K). Model eutectics for HNO₃ and H₂SO₄ agree with experimental measurements to within ± 0.2°C. In agreement with previous work, the experimental freezing point depression (fpd) data for pure HCl at subzero temperatures were judged to be flawed and unreliable. Three alternatives are discussed for handling HCl chemistry at subzero temperatures. In addition to defining the solubility of solid-phase acids, this work also adds three new nitrate minerals and six new acid salts to the FREZCHEM model and refines equilibria among water ice, liquid water, and water vapor over the temperature range from 180 to 298 K. The final system is parameterized for Na-K-Mg-Ca-H-Cl-SO₄-NO₃-OH-HCO₃-CO₃-CO₂-H₂O.Simulations of hypothetical MgSO₄-H₂SO₄-H₂O and Na₂SO₄-MgSO₄-H₂SO₄-H₂O brines for Europa demonstrate how freezing can convert a predominantly salt solution into a predominantly acid solution at subzero temperatures. This result has consequences for the effects of salinity, acidity, and temperature as limiting factors for potential life on Europa. Strong acidity would limit life-forms to highly acidophilic organisms.     

Nontronite synthesis at low temperatures

The low-temperature synthesis of clay minerals is possible through the aging of freshly prepared hydroxide—silica precipitates. The rapid synthesis of nontronite is only possible at surface temperatures under reducing conditions. Under oxidizing conditions, pure Fe(III)- or pure Al-smectite minerals could not be synthesized at low temperatures. It is only from Fe(II)-containing solutions that nontronite and lembergite, the di-[Fe(III)] and tri-[Fe(II)] octahedral three-layer silicates, are built up in several days at low temperatures. The presence of Fe(II) enables an octahedral layer of the brucite—gibbsite type to be formed. These are necessary for the bidimensional orientation of SiO₄ tetrahedrons, leading to clay-mineral formation. The Fe²⁺ and/or Mg²⁺ ions are necessary for the formation of the Al³⁺- and Fe³⁺-containing three-layer silicate minerals.Under reducing diagenetic conditions, the Fe contents in recent sediments are sufficient to build up Al-rich three-layer minerals under both fresh-water and salt-water conditions.     

低温条件下有机酸对铁、铜的淋滤实验研究

水／岩作用实验研究表明，在低温条件下柠檬酸、草酸和酒石酸对岩石中的铁、铜都有很高的淋滤率。这表明，在表生地质作用过程中有机酸对成矿元素有很强的活化能力。实验研究同时发现，有机酸对成矿元素的活化能力受其分解温度的限制，由于大部分有机酸的分解温度都在２５０℃以下，因而有机酸对成矿元素的活化仅限于低温条件。本次研究还显示，判断一种岩石能否成为矿源层，不能简单的以其中成矿元素的含量高低作为标准，其中更关键的因素是元素在岩石中的存在形式，如果成矿元素在岩石中难以活化的形式存在，那么含量再高也无法提供成矿物质     

Effects of pressure on aqueous chemical equilibria at subzero temperatures with applications to Europa

Pressure plays a critical role in controlling aqueous geochemical processes in deep oceans and deep ice. The putative ocean of Europa could have pressures of 1200 bars or higher on the seafloor, a pressure not dissimilar to the deepest ocean basin on Earth (the Mariana Trench at 1100 bars of pressure). At such high pressures, chemical thermodynamic relations need to explicitly consider pressure. A number of papers have addressed the role of pressure on equilibrium constants, activity coefficients, and the activity of water. None of these models deal, however, with processes at subzero temperatures, which may be important in cold environments on Earth and other planetary bodies. The objectives of this work were to (1) incorporate a pressure dependence into an existing geochemical model parameterized for subzero temperatures (FREZCHEM), (2) validate the model, and (3) simulate pressure-dependent processes on Europa. As part of objective 1, we examined two models for quantifying the volumetric properties of liquid water at subzero temperatures: one model is based on the measured properties of supercooled water, and the other model is based on the properties of liquid water in equilibrium with ice.The relative effect of pressure on solution properties falls in the order: equilibrium constants(K) > activity coefficients (γ) > activity of water (a_w). The errors (%) in our model associated with these properties, however, fall in the order: γ > K > a_w. The transposition between K and γ is due to a more accurate model for estimating K than for estimating γ. Only activity coefficients are likely to be significantly in error. However, even in this case, the errors are likely to be only in the range of 2 to 5% up to 1000 bars of pressure. Evidence based on the pressure/temperature melting of ice and salt solution densities argue in favor of the equilibrium water model, which depends on extrapolations, for characterizing the properties of liquid water in electrolyte solutions at subzero temperatures, rather than the supercooled water model. Model-derived estimates of mixed salt solution densities and chemical equilibria as a function of pressure are in reasonably good agreement with experimental measurements.To demonstrate the usefulness of this low-temperature, high-pressure model, we examined two hypothetical cases for Europa. Case 1 dealt with the ice cover of Europa, where we asked the question: How far above the putative ocean in the ice layer could we expect to find thermodynamically stable brine pockets that could serve as habitats for life? For a hypothetical nonconvecting 20 km icy shell, this potential life zone only extends 2.8 km into the icy shell before the eutectic is reached. For the case of a nonconvecting icy shell, the cold surface of Europa precludes stable aqueous phases (habitats for life) anywhere near the surface. Case 2 compared chemical equilibria at 1 bar (based on previous work) with a more realistic 1460 bars of pressure at the base of a 100 km Europan ocean. A pressure of 1460 bars, compared to 1 bar, caused a 12 K decrease in the temperature at which ice first formed and a 11 K increase in the temperature at which MgSO₄·12H₂O first formed. Remarkably, there was only a 1.2 K decrease in the eutectic temperatures between 1 and 1460 bars of pressure. Chemical systems and their response to pressure depend, ultimately, on the volumetric properties of individual constituents, which makes every system response highly individualistic.     

Modeling aluminum-silicon chemistries and application to Australian acidic playa lakes as analogues for Mars

Recent Mars missions have stimulated considerable thinking about the surficial geochemical evolution of Mars. Among the major relevant findings are the presence in Meridiani Planum sediments of the mineral jarosite (a ferric sulfate salt) and related minerals that require formation from an acid-salt brine and oxidizing environment. Similar mineralogies have been observed in acidic saline lake sediments in Western Australia (WA), and these lakes have been proposed as analogues for acidic sedimentary environments on Mars. The prior version of the equilibrium chemical thermodynamic FREZCHEM model lacked Al and Si chemistries that are needed to appropriately model acidic aqueous geochemistries on Earth and Mars. The objectives of this work were to (1) add Al and Si chemistries to the FREZCHEM model, (2) extend these chemistries to low temperatures (<0 °C), if possible, and (3) use the reformulated model to investigate parallels in the mineral precipitation behavior of acidic Australian lakes and hypothetical Martian brines.FREZCHEM is an equilibrium chemical thermodynamic model parameterized for concentrated electrolyte solutions using the Pitzer approach for the temperature range from <−70 to 25 °C and the pressure range from 1 to 1000 bars. Aluminum chloride and sulfate mineral parameterizations were based on experimental data. Aluminum hydroxide and silicon mineral parameterizations were based on Gibbs free energy and enthalpy data. New aluminum and silicon parameterizations added 12 new aluminum/silicon minerals to this Na-K-Mg-Ca-Fe(II)-Fe(III)-Al-H-Cl-Br-SO₄-NO₃-OH-HCO₃-CO₃-CO₂-O₂-CH₄-Si-H₂O system that now contain 95 solid phases.There were similarities, differences, and uncertainties between Australian acidic, saline playa lakes and waters that likely led to the Burns formation salt accumulations on Mars. Both systems are similar in that they are dominated by (1) acidic, saline ground waters and sediments, (2) Ca and/or Mg sulfates, and (3) iron precipitates such as jarosite and hematite. Differences include: (1) the dominance of NaCl in many WA lakes, versus the dominance of Fe-Mg-Ca-SO₄ in Meridiani Planum, (2) excessively low K⁺ concentrations in Meridiani Planum due to jarosite precipitation, (3) higher acid production in the presence of high iron concentrations in Meridiani Planum, and probably lower rates of acid neutralization and hence, higher acidities on Mars owing to colder temperatures, and (4) lateral salt patterns in WA lakes. The WA playa lakes display significant lateral variations in mineralogy and water chemistry over short distances, reflecting the interaction of acid ground waters with neutral to alkaline lake waters derived from ponded surface runoff. Meridiani Planum observations indicate that such lateral variations are much less pronounced, pointing to the dominant influence of ground water chemistry, vertical ground water movements, and aeolian processes on the Martian surface mineralogy.     

Dislocations in olivine indented at low temperatures

Microhardness experiments have been performed on faces of olivine single crystals oriented at 45° to the [100] and [001] axis. Experiments have been performed between 20°C and 900°C in order to follow the evolution of the slip systems and the evolution of the nature of dislocations with temperature. Slip systems were identified using interferential contrast, decoration and transmission electron microscopy (TEM). Although the two possible Burgers vectors [001] and [100] are acted upon symmetrically, the only activated slip systems are (100) [001] and {110} [001]. The latter system is less developed and appears only at 600°C and above. Dislocations are mainly [001] screws, which are always very straight. Microcracks are observed together with dislocations. Above 600°C there is a clear relationship between both as a narrow plastic zone is developed around the cracks.     

Genesis of cristobalite and tridymite at low temperatures

It is suggested that an explanation of the observed metastable formation of cristobalite and tridymite in the stability field of quartz lies in the topology of these silica polymorphs and the resulting energies at the time of formation.     

Comments on “Uranium solution-mineral equilibria at low temperatures with applications to sedimentary ore deposits”

The necessity of considering protonation and complexation of weak acid ligands, for proper interpretation of their effect on solution equilibria, has been demonstrated. The solubility of uraninite in presence of σF = 2 ppm has been recalculated to be 0.88 ppb at pH 2.     

Natural fluid phases at high temperatures and low pressures

Gas phases at low pressures and high (magmatic) temperatures have certain peculiar properties. The fluid is mainly water vapour, which is usually observed during discharging of crystal magmatic melts. At >700°C and <100 bar these peculiar properties include: formation of near ‘dry' salt melts as second fluid phase, very strong fractionation of hydrolysis products between vapour and melts, subvalence state of metals during transport processes, and high sensitivity of the gas to conditions of sublimate precipitation. Phase diagram analysis as well as results of field and laboratory experiments are presented in this article. The processes could be a model for industrial technologies to clean wastes from toxic, rare and heavy metals. Transport forms of some elements in volcanic gases are very similar to the species which were formed first in the protosolar nebula.     

Compositions of aqueous fluids in equilibrium with pyroxenes and olivines at mantle pressures and temperatures

Solubility experiments were performed at 30 kbars in the system Mg₂SiO₄-SiO₂-H₂O, and at 20 and 30 kbars on omphacitic pyroxene-water mixtures. They confirm that the solubility of the forsterite component in aqueous fluids remains rather low (up to 5 wt.%), whereas the solubility of the SiO₂ component from solids of appropriate SiO₂-rich compositions in the system Mg₂SiO₄-SiO₂-H₂O increases with temperature up to some 75% at 1,100° C. At this temperature a simplified harzburgite consisting of forsterite and enstatite coexists with a fluid containing about 35% (MgO+SiO₂). Hydrous fluids coexisting with omphacitic clinopyroxenes leach sodium silicate component from the solid leaving less jadeitic pyroxenes behind. Most interestingly, the amount of sodium leached at constant temperature increases with decreasing pressure.Comparison of the results with previous solubility studies in the system K₂O-MgO-Al₂O₃-SiO₂-H₂O indicates that hydrous fluids in the mantle must be alkaline rather than silicanormative. Alkali metasomatism caused by such fluids would lead to potassium enrichment in deeper portions of the upper mantle and to sodium enrichment at shallower levels, where amphiboles become stable. This K/Na fractionation in the upper mantle may explain the generation of K-rich or of Na-rich magmas through partial melting at different depths.     

The mobility of thorium in natural waters at low temperatures

Thermodynamic properties of 32 dissolved thorium species and 9 thorium-bearing solid phases have been collected from the literature, critically evaluated and estimated where necessary for 25°C and 1 atm pressure. Although the data are incomplete, especially for thorium minerals and organic complexes, some tentative conclusions can be drawn. Dissolved thorium is almost invariably complexed in natural waters. For example, based on ligand concentrations typical of ground water (ΣCl = 10 ppm, ΣF = 0.3 ppm, ΣSO₄ = 100 ppm, andΣPO₄ = 0.1 ppm), the predominant thorium species are Th(SO₄)⁰₂, ThF²⁺₂, and Th(HPO₄)₂⁰below pH ≈ 4.5; Th(HPO₄)^2?₃ from about pH 4.5 to 7.5; and Th(OH)⁰₄ above pH 7.5. Based on stability constants for thorium citrate, oxalate and EDTA complexes, it seems likely that organic complexes predominate over inorganic complexes of thorium in organic-rich stream waters, swamp waters, soil horizons, and waterlogged recent sediments. The thorium dissolved in seawater is probably present in organic complexes and as Th(OH)⁰₄. The tendency for thorium to form strong complexes enhances its potential for transport in natural waters by many orders of magnitude below pH 7 in the case of inorganic complexing, and below about pH 8 when organic complexing is important. The existence of complexes in addition to those formed with hydroxyl, is apparent from the fact that measured dissolved thorium in fresh surface waters (pH values generally 5–8) usually ranges from about 0.01 to 1 ppb and in surface seawater (pH = 8.1) is about 0.00064 ppb. This may be contrasted with the computed solubility of thorianite in pure water which is only 0.00001 ppb Th as Th(OH)⁰₄ above pH 5. Although complexing increases the solubility of thorium-bearing heavy minerals below pH 8, maximum thorium concentrations in natural waters are probably limited in general by the paucity and slow solution rate of these minerals and by sorption processes, rather than by mineral-solution equilibria.     

Brine substitute liquids for soil freezing at very low temperatures

The strength of a frozen soil increases with decreasing temperature. Furthermore, the speed it takes to form a frozen wall increases on lowering the temperature of the freezing liquid.

With the traditional freezing systems using brine it is difficult to work with temperatures below −30°C. To go lower than this limit, it is necessary to substitute the brine by using freezing liquids that maintain good hydraulic and thermal characteristics at much lower temperatures.

Different organic liquids have been tested and good results have been obtained with some aromatic hydrocarbon mixes from the terpene family.

As a result of the research, for practical purposes a by-product ofa distilling citrus fruit skins has been selected. This liquid solidifies at −100°C approx. and maintains a low viscosity rate below −30°C.

The present paper describes the thermal and hydraulic properties of this product as a function of the temperature concerned and compares them to the same properties of classic brines of CaCl₂.     

Unexpected phase assemblages in inclusions with ternary H2O-salt fluids at low temperatures

Phase assemblages and temperatures of phase changes provide important information about the bulk properties of fluid inclusions, and are typically obtained by microthermometry. Inclusions are synthesized in natural quartz containing an aqueous fluid with a composition in the ternary systems of H₂O-NaCl₂-CaCl₂, H₂O-NaCl-MgCl₂, and H₂O-CaCl₂-MgCl₂. This study reveals that these fluid inclusions may behave highly unpredictably at low temperatures due to the formation of metastable phase assemblages. Eutectic temperatures cannot be detected in most of the fluid inclusions containing these ternary systems. Nucleation of a variety of solid ice and salt-hydrate phases in single fluid inclusions is often partly inhibited. Raman spectroscopy at low temperatures provides an important tool for interpreting and understanding microthermometric experiments, and visualizing stable and metastable phase assemblages. Final dissolution temperatures of ice, salt-hydrates, and salt must be treated with care, as they can only be interpreted by purely empirical or thermodynamic models at stable conditions.     

Phonon density of states in iron at high pressures and high temperatures

The phonon density of states (DOS) in iron has been measured in situ by nuclear resonance inelastic X-ray scattering (NRIXS) at high pressures and high temperatures in a resistively heated diamond anvil cell. The DOS data provide a variety of thermodynamic and elastic parameters essential for characterizing iron at depth in the Earth interior, such as average sound velocity, Debye temperature, atomic mean square displacement, average kinetic energy, vibrational entropy and specific heat. The NRIXS data were collected at 6, 20, and 29 GPa and at temperatures up to 920 K. Temperatures were directly determined from the measured spectra by the ratio of intensities of the phonon creation/annihilation side bands that are determined only by the Boltzmann factor. The change of the DOS caused by the structural transition from -Fe to -Fe is small and not resolvable within the experimental precision. However, the phonon energies in -Fe are clearly shifted to lower values with respect to - and -Fe. The temperature dependence of derived thermodynamic parameters is presented and compared with those obtained by Debyes model. The Debye temperatures that best describe the data decrease slightly with increasing temperature.     

Electrical resistivity of fcc phase iron hydrides at high pressures and temperatures

We measured the electrical resistivity of face-centered-cubic (fcc) structured iron hydrides at high pressures up to 65 GPa and high temperatures in a laser-heated diamond anvil cell. The results indicate that the resistivity of stoichiometric fcc FeH_x (x ～ 1.0) is smaller than that of fcc Fe at the same pressure and temperature conditions. The same behavior was also observed in fcc FeNiH_x (x ～ 1.0). On the other hand, hydrogen-poor fcc FeH_x (x < 0.77) showed a resistivity comparable to that of the fcc phase of pure iron. Therefore, we conclude that the stoichiometric fcc Fe (–Ni) hydride is more conductive than Fe (–Ni) with the same crystal symmetry, and the impurity resistivity of hydrogen in Fe is vanishingly small. Even if hydrogen is the major light element in the Earth's core, it would have little influence on the electrical and thermal conductivity of Fe–Ni alloys, and hence the thermal evolution of the core.     

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.