Activity-composition Relations Research Articles

A revised model for activity–composition relations in solid and molten FePt alloys and a preliminary model for characterization of oxygen fugacity in high-pressure experiments

Abstract. We present new models for the activity of iron (γFe) in solid face-centered cubic (fcc) and liquid FePt alloy at high temperature and pressure to facilitate their use as sliding buffer redox sensors under extreme conditions. Numerous experimental studies of γFe in FePt alloy at 100 kPa have produced a wide spread of values. By favoring high-temperature studies that are more likely to have produced equilibrium measurement and excluding experiments for compositions and temperatures that probably encountered ordered or unmixed low-temperature phases, we regress an asymmetric Margules activity–composition model with parameters WFePtfcc=-121.5±2.1 kJ mol−1 and WPtFefcc=-93.3±4.3 kJ mol−1. These values are close to the widely used model of Kessel et al. (2001), but for Pt-rich compositions they predict larger Fe activities and correspondingly more reduced oxygen fugacities. Activity–composition relations in liquid FePt are calibrated from direct measurements of activities and, most sensitively, from the trace of the Fe–Pt liquidus. Together, these yield asymmetric Margules parameters of WFePtliq=-124.5 kJ mol−1 and WPtFeliq=-94.0 kJ mol−1. The effects of pressure on both fcc and liquid FePt alloy are considered from excess-volume relations. Both solid and liquid alloy display significant positive excess volumes of mixing. Extraction of the excess volume of mixing for fcc FePt alloy requires filtering data for ordered low-temperature phases and corrections for the effects of magnetostriction on Fe-rich compositions which exhibit “Invar” behavior. Applied at high temperatures and pressures, both solid and liquid FePt alloys have strongly negative deviations from ideality at low pressure, which become closer to ideal at high pressure. These models provide a provisional basis for the calculation of aFe in high-temperature, high-pressure experiments that, when combined with estimates of aFeO, allow characterization of fO2 under conditions relevant to magma oceans, core formation, and differentiation processes in the lower mantle of Earth or on other terrestrial planets. Improvements in these models require new constraints on the equation of state of FePt fcc alloy and documentation of the high-pressure melting relations in the system Fe–Pt.

Thermodynamics of ordering and mixing in plagioclase feldspars: atomistic modelling in favour of Landau theory

Plagioclase feldspars are key components of terrestrial planets and deriving their phase diagram and activity–composition relations has been the subject of many studies. Three main approaches have been used for thermodynamic modelling of the behaviour of plagioclase feldspars, and all three disagree in terms of the magnitude and relative importance of mixing energies and ordering energies. The present study uses Monte Carlo simulations based on classical atomistic modelling to provide independent constraints over ordering and mixing for anorthite and its solid solution with albite. Unsurprisingly, Al-avoidance plays a key role. The competition between first-neighbour and second-neighbour interactions is highlighted, providing simple explanations to the high degree of short-range order in macroscopically disordered plagioclase. The mixing properties obtained in this study support strong non-ideality, with negative mixing enthalpy and temperature-dependent mixing entropy. Ordering energies are consistent with studies using Landau theory. Unfortunately, the form of the Landau equations hindered parametrisation of a full thermodynamic model from albite to anorthite, even though the thermodynamic formalism remains simple. Reasons, perspectives, and implications for the phase diagram of ternary feldspars are discussed.

A clean and sustainable strategy to produce bio-lubricant with high-bearing and good anti-oxidation ability from Lanzhou lily

Biolubricants like natural oil or grease are sustainable and renewable alternative to mineral lubricants. Poor thermo-oxidative stability and load-carrying capacity are the main obstacles limiting the application of bio-lubricants. Striving for a plant resource with plentiful ester analogues as base oil and other lubricating-benefit ingredients as natural additives may be a new and clean strategy to produce biolubricants with high-bearing and good oxidation stability. In this work, the bio-lubricant was extracted as Lanzhou lily crude extract (LLCE) from the widely cultivated crop Lanzhou lily (Lilium davidii var. unicolor) in China. Its lubricity, load-carrying capacity, and activity-composition relationship were investigated. LLCE exhibits good lubricity at temperature above 50 °C and extreme load-carrying capacity above 600 N. The friction coefficient at stable stage is about 0.12 and the wear rate of the steel discs is on the order of 10−7-10−8 mm3/Nm. These intriguing properties are attributed to the specific components in LLCE. Phenol esters or glycosides in LLCE is functional as base oil to provide the boundary lubrication, and other plentiful components like phospholipids, alkaloids, and organic sulfides play as natural additives to improve the lubricity and extreme pressure ability by accelerating the formation of tribo-reaction film. This work provides a sustainable and clean feedstock for high-performance bio-lubricants/additives and simultaneously expands the economic utilization of crops, which can be applied in the open lubricating systems like chainsaw, metal-cutting work and food-processing equipment in agricultural, food and manufacturer industries.

Calculated phase equilibria for high‐pressure serpentinites and compositionally related rocks close to the MgO–Al2O3–SiO2–H2O (MASH) system

AbstractThe metamorphic evolution of serpentinite has a fundamental bearing on geochemical cycles, mechanical behaviour and seismic properties of the Earth. Phase equilibria in the system MgO–Al2O3–SiO2–H2O (MASH) provide a ‘backbone’ for phase equilibria in larger systems that are applicable to serpentinites and also many whiteschists. Phase equilibria for MASH, and departures from MASH, are calculated using the Holland and Powell data set, tc‐ds62, and new activity–composition relations for antigorite and talc. Phase relations are presented using PT projections and compatibility diagrams to highlight the role of P, T and composition on the equilibrium mineral assemblages stable in these chemical systems at high pressure. These phase relations provide the context for pseudosection and other modelling focused on individual rock compositions.

Estimation of self and ternary interaction coefficients in solid Fe–C, Fe–Mn, Fe–Mn–C and Fe–Si–C alloys at 1623 K

ABSTRACT Most reported interaction coefficients of solutes,, and activity coefficients of solutes at their zero concentration,, in Fe-based systems have been measured in liquid solutions. In comparison, the data with solid iron-base alloys are meager. Utilising the Thermo-Calc generated equilibrium compositions of the coexisting liquid iron and austenite in Fe–C and Fe–Mn binaries, and Fe–C–Mn and Fe–C–Si pseudobinaries, and the first-principle activity relationship of each solute in the two phases at equilibrium, seven important thermodynamic data (, , , , , , and ) in solid iron at 1623 K have been generated in this work. A quadratic solution model for the activity coefficient () vs. composition () relation is employed. The published values of the in solid iron, and the interaction coefficients and’s in liquid iron are used, after temperature corrections. For this purpose, the temperature dependences of interaction coefficients and activity coefficients have been formulated under the simplifying condition of ideal entropy of mixing. The reliability of the current method of evaluation of data has been verified by comparing the calculated value of in solid iron with the experimental data. The generated data are as follows: , , , , , and

Activity–Composition Relationships of Fe–Ni–Cu Ternary Nanoparticles Supported on Al2O3 as Three-Way Catalysts for NO Reduction

Activity–Composition Relationships of Fe–Ni–Cu Ternary Nanoparticles Supported on Al₂O₃ as Three-Way Catalysts for NO Reduction

A new activity model for Fe\u2013Mg\u2013Al biotites: I\u2014Derivation and calibration of mixing parameters

A new activity model for Fe–Mg–Al biotites is formulated, which extends that of Mg–Al biotites (Dachs and Benisek, Contrib Mineral Petrol 174:76, 2019) to the K2O–FeO–MgO–Al2O3–SiO2–H2O (KFMASH) system. It has the two composition variables XMg = Mg/(Mg + Fe2+) and octahedral Al, and Fe–Mg and Mg–Al ordering variables resulting in five linearly independent endmembers: annite (Ann, K[Fe]M1[Fe]2M2[Al0.5Si0.5]2T1[Si]2T2O10(OH)2, phlogopite (Phl, K[Mg]M1[Mg]2M2[Al0.5Si0.5]2T1[Si]2T2O10(OH)2, ordered Fe–Mg biotite (Obi, K[Fe]M1[Mg]2M2[Al0.5Si0.5]2T1[Si]2T2O10(OH)2, ordered eastonite (Eas, K[Al]M1[Mg]2M2[Al]2T1[Si]2T2O10(OH)2, and disordered eastonite (Easd, K[Al1/3Mg2/3]M1[Al1/3Mg2/3]2M2[Al]2T1[Si]2T2O10(OH)2. The methods applied to parameterize the mixing properties of the model were: calorimetry, analysis of existing phase-equilibrium data, line-broadening in powder absorption infrared (IR) spectra, and density functional theory (DFT) calculations. For the calorimetric study, various biotite compositions along the annite–phlogopite, annite–siderophyllite (Sid, K[Al]M1[Fe]2M2[Al]2T1[Si]2T2O10(OH)2), and annite–eastonite joins were synthesized hydrothermally at 700 °C, 4 kbar and logfO2 of around − 20.2, close to the redox conditions of the wüstite–magnetite oxygen buffer at that P–T conditions. The samples were characterised by X-ray powder diffraction (XRPD), energy-dispersive scanning electron microprobe analysis, powder absorption IR spectroscopy, and optical microscopy. The samples were studied further using relaxation calorimetry to measure their heat capacities (Cp) at temperatures from 2 to 300 K. The measured Cp/T was then integrated to get the calorimetric (vibrational) entropies of the samples at 298.15 K. These show linear behaviour when plotted as a function of composition for all three binaries. Excess entropies of mixing are thus zero for the important biotite joins. Excess volumes of mixing are also zero within error for the three binaries Phl-Ann, Ann-Sid, and Ann-Eas. KFMASH biotite, therefore, has excess enthalpies which are independent of pressure and temperature (WGij = WHij). A least-squares procedure was applied in the thermodynamic analysis of published experimental data on the Fe–Mg exchange between biotite and olivine, combined with phase-equilibrium data for phlogopite + quartz stability and experimental data for the Al-saturation level of biotite in the assemblage biotite–sillimanite–sanidine–quartz–H2O to constrain enthalpic mixing parameters and to derive enthalpy of formation values for biotite endmembers. For Fe–Mg mixing in biotite, the most important binary, this gave best-fit asymmetric Margules enthalpy parameters of WHAnnPhl = 14.3 ± 3.4 kJ/mol and WHPhlAnn = −8.8 ± 8.0 kJ/mol (3-cation basis). The resulting asymmetric molar excess Gibbs free energy (Gex) departs only slightly from ideality and is negative at Fe-rich and positive at Mg-rich compositions. Near-ideal activity–composition relationships are thus indicated for the Ann–Phl binary. The presently used low value of − 2 kJ/mol for the enthalpy change of the reaction 2/3 Phl + 1/3 Ann = Obi is generally confirmed by DFT calculations that gave − 2 ± 3 kJ/mol for this ∆HFe–Mg order, indicating that Fe–Mg ordering in biotite is weak. The large enthalpy change of ∆HMg-Al disorder = 34.5 kJ/mol for the disordering of Mg and Al on the M sites in Eas (Dachs and Benisek 2019) is reconfirmed by additional DFT calculations. In combination with WHPhlEas = 10 kJ/mol, which is the preferred value of this study describing mixing along the Phl–Eas join, Mg–Al disordering over the M sites of biotite is predicted to be only significant at high temperatures > 1000 °C. In contrast, it plays no role in metamorphic P–T settings.

Antimicrobial and Antibiofilm Activity and Machine Learning Classification Analysis of Essential Oils from Different Mediterranean Plants against Pseudomonas aeruginosa.

Pseudomonas aeruginosa is a ubiquitous organism and opportunistic pathogen that can cause persistent infections due to its peculiar antibiotic resistance mechanisms and to its ability to adhere and form biofilm. The interest in the development of new approaches for the prevention and treatment of biofilm formation has recently increased. The aim of this study was to seek new non-biocidal agents able to inhibit biofilm formation, in order to counteract virulence rather than bacterial growth and avoid the selection of escape mutants. Herein, different essential oils extracted from Mediterranean plants were analyzed for their activity against P. aeruginosa. Results show that they were able to destabilize biofilm at very low concentration without impairing bacterial viability. Since the action is not related to a bacteriostatic/bactericidal activity on P. aeruginosa, the biofilm change of growth in presence of the essential oils was possibly due to a modulation of the phenotype. To this aim, application of machine learning algorithms led to the development of quantitative activity–composition relationships classification models that allowed to direct point out those essential oil chemical components more involved in the inhibition of biofilm production. The action of selected essential oils on sessile phenotype make them particularly interesting for possible applications such as prevention of bacterial contamination in the community and in healthcare environments in order to prevent human infections. We assayed 89 samples of different essential oils as P. aeruginosa anti-biofilm. Many samples inhibited P. aeruginosa biofilm at concentrations as low as 48.8 µg/mL. Classification of the models was developed through machine learning algorithms.

Mineral equilibria constraints on open‐system melting in metamafic compositions

AbstractThe recent development of activity–composition relations for mineral and melt phases in high‐grade metamafic rocks allows mineral equilibria tools to be used to further aid our understanding of partial melting and the mineralogical consequences of melt segregation in these rocks. We show that bulk compositional data from natural amphibolites cover a wide compositional range, with particular variability in the content and ratios of Ca, Na and K indicating that low‐grade metasomatic alteration can substantially alter the igneous protolith chemistry and potentially affect the volume and composition of melt generated. Mineral equilibria calculations for five samples that span the compositional variability in our data set indicate that melting occurs primarily via the fluid‐absent breakdown of amphibole+quartz to produce a pressure‐sensitive peritectic assemblage of augite, orthopyroxene and/or garnet. The introduction of orthopyroxene at the onset of the amphibolite‐to‐granulite‐facies transition at lower pressure results in an increased rate of melt production until quartz is typically exhausted, and this is similarly seen for the introduction of garnet at higher pressure. Calculated melt compositions are dependent on the protolith composition, but initial solidus melting and biotite breakdown produce 1–3 mol.% of K‐rich granitic melts. As hornblende melting proceeds, 15–20 vol.% of either more granodioritic‐to‐tonalitic or granodioritic‐to‐trondhjemitic melt is produced. Once quartz is exhausted, intermediate to mafic melt compositions are produced at ultrahigh‐temperature conditions. Quartz‐rich lithologies with high Ca coupled to low Na and K are the most fertile under orogenic conditions, yielding up to 25 mol.% of sub‐alkalic granitic melt by 850°C. Such rocks did not experience significant subsolidus alteration. Altered compositions with low Ca and elevated Na and K are not as fertile, yielding less than 15 mol.% of alkalic granitic melt by 850°C. These melt volumes are enough to be segregated, and can make a contribution to granite magmatism and intracrustal differentiation that should not be overlooked.

Simple routes for the improvement of hydrogen evolution activity of Ni-Mo catalysts: From sol-gel derived powder catalysts to graphene supported co-electrodeposits

Development of noble metal-free catalysts for hydrogen production is one of the cores of the sustainable energy economy. Here we present results of systematic analysis of catalytic activity of Ni-Mo alloy powders in alkaline media towards hydrogen evolution reaction (HER). Catalysts were prepared in a wide concentration range (from Ni0.2Mo0.8 to Ni0.9Mo0.1), and resulted with a volcano shaped activity-composition relationship, with maximum catalytic activity achieved for the powder with nominal composition Ni0.6Mo0.4. Improved HER activity is ascribed to reduced deactivation by hydride formation and adequate hydrogen-surface energetics on Ni-Mo catalysts. In the second part, we demonstrate a novel method for electrochemical formation of NiMo@rGO composites. Prepared composite electrodes show improved electrocatalytic activity compared to both pure Ni and Ni@rGO electrodes. Activity was observed to depend on the deposition time and is contributed by two factors: (i) formation of Ni-Mo system and (ii) formation of an interfacial region with rGO. We expect that the provided activity-composition relationship in combination with novel electrochemical NiMo-rGO composite formation procedure will provide a route for the development of new highly efficient noble metal-free HER electrocatalysts.

Distribution Behavior of Aluminum and Titanium Between Nickel-Based Alloys and Molten Slags in the Electro Slag Remelting (ESR) Process

The equilibrium reaction between Ni alloys and CaO-Al2O3-CaF2-TiO2 system electroslag remelting (ESR) slags was investigated in the temperature range of 1773 K to 1873 K (1500 °C to 1600 °C) at p(O2) = 10−16 atm in order to obtain the optimized composition of the slags for producing Ni alloys with various Al and Ti ratios. In addition, the temperature dependence of the reaction equilibria between the ESR slags and Ni alloys was also evaluated. The stable ionic species of titanium in the ESR slag under the present experimental conditions was experimentally confirmed to be mainly Ti4+ (i.e., TiO2) by X-ray photoelectron spectroscopy analysis of the quenched samples. The activity-composition relationship of TiO2 and Al2O3 in the ESR slag was determined as a function of the Al/Ti ratio of the alloys and the CaF2 content of the slags in conjunction with the activity ratio of Al to Ti in the alloys calculated from the FactSageTM 7.0 software. The temperature dependence of the activity-composition relationship of TiO2 and Al2O3 in the slag showed good linear correlations, and the equilibrium content ratio of TiO2 to Al2O3 at a fixed activity ratio increased with increasing temperature, which was expected based on the standard enthalpy change of the reaction. Thus, higher amounts of TiO2 should be added at higher operation temperatures in the ESR process. A 120 kg scale pilot ESR test (2000 A and 16 V) was performed to produce a commercial grade Ni-based superalloy based on the activity-composition relationship of the slag components obtained in the present study. Consequently, the contents of Al and Ti in the solidified ESR ingot were nearly the same as that of the original electrode throughout the entire length (280 mm) after the ESR process.

Thermodynamic Modeling of Fe–C–S Ternary System

A CALPHAD type thermodynamic modeling of the Fe–C–S system was carried out, in particular, in order to provide accurate description of activity-composition relationship of Fe–C–S liquid phase. The liquid may be used as a solvent of ferrous scrap recycling. All available and reliable experimental data including phase equilibria and activity of S in the liquid Fe–C–S were analyzed, and a self-consistent set of Gibbs energy equations for the stable phases were obtained. In addition to the Gibbs energy equations, interaction parameters between S and C in the liquid were additionally estimated as functions of temperature:which were derived from the optimized Gibbs energy of the liquid phase using the Modified Quasichemical Model in the pair approximation. Moreover, the present thermodynamic modeling can be used to calculate various phase diagrams in the Fe–C–S system, and can be further extended in order to develop a multicomponent thermodynamic database.

Activity–composition relations for the calculation of partial melting equilibria in metabasic rocks

AbstractA set of thermodynamic models is presented that, for the first time, allows partial melting equilibria to be calculated for metabasic rocks. The models consist of new activity–composition relations combined with end‐member thermodynamic properties from the Holland & Powell dataset, version 6. They allow for forward modelling in the system NaO–CaO–KO–FeO–MgO–AlO–SiO–HO–TiO–FeO. In particular, new activity–composition relations are presented for silicate melt of broadly trondhjemitic–tonalitic composition, and for augitic clinopyroxene with Si–Al mixing on the tetrahedral sites, while existing activity–composition relations for hornblende are extended to include KO and TiO. Calibration of the activity–composition relations was carried out with the aim of reproducing major experimental phase‐in/phase‐out boundaries that define the amphibolite–granulite transition, across a range of bulk compositions, at ≤13 kbar.

High‐grade metamorphism and partial melting of basic and intermediate rocks

AbstractRocks of basic and intermediate bulk composition occur in orogenic terranes from all geological time periods and are thought to represent significant petrological components of the middle and lower continental crust. However, the former lack of appropriate thermodynamic models for silicate melt, amphibole and clinopyroxene that can be applied to such lithologies at high temperature has inhibited effective phase equilibrium modelling of their petrological evolution during amphibolite‐ and granulite facies metamorphism. In this work, we present phase diagrams calculated in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2(NCKFMASHTO) compositional system for a range of natural basic and intermediate bulk compositions for conditions of 2–12 kbar and 600–1050 ∘C using newly parameterized activity–composition relationships detailed in a companion paper by Green et al. in this issue. Particular attention is given to mid‐ocean ridge basalt (MORB) and diorite protolith bulk compositions. Calculated subsolidus mineral assemblages in all basic and intermediate rock types are modally dominated by hornblende and plagioclase, with variable proportions of epidote, clinopyroxene, garnet, biotite, muscovite, quartz, titanite or ilmenite present at different pressures. The H2O‐saturated (wet) solidus has a negativeP−Tslope and occurs between ∼620–690∘C at mid‐ to lower‐crustal pressures of 5–10 kbar. The lowest‐Tmelts generated close to the wet solidus are calculated to have granitic major‐element oxide compositions. Melting at higher temperature is attributed primarily to multivariate hydrate‐breakdown reactions involving biotite and/or hornblende. Partial melt compositions calculated at 800–1050∘C for MORB show good correlation with analysed compositions of experimental glasses produced via hydrate‐breakdown melting of natural and synthetic basic protoliths, with Niggli norms indicating that they would crystallize to trondhjemite or tonalite. Diorite is shown to be significantly more fertile than MORB and is calculated to produce high‐Tmelts (>800 ∘C) of granodioritic composition. Subsolidus and suprasolidus mineral assemblages show no significant variation between different members of the basalt family, although theP−Tconditions at which orthopyroxene stabilizes, thus defining the prograde amphibolite–granulite transition, is strongly dependent on bulk‐rock oxidation state and water content. The petrological effects of open‐ and closed‐system processes on the mineral assemblages produced during prograde metamorphism and preserved during retrograde metamorphism are also examined via a case‐study analysis of a natural Archean amphibolite from the Lewisian Complex, northwest Scotland.

Thermometry of the magma ocean: Controls on the metal–silicate partitioning of gold

We have performed experiments to investigate the solubility and metal–silicate partitioning of gold as a function of metal sulphur content (XS), silicate melt polymerization (NBO/T) and pressure (P). These experiments show that Au becomes less siderophile both with increasing pressure and as the metal phase becomes more sulphur-rich. For the studied range of compositions, melt polymerization has no effect on the solubility of Au. The reduction in the siderophile tendency of gold with increasing metal sulphur content is greater than expected on the basis of activity–composition relationships in the metal phase. This suggests a significant role for complexing between Au and S in the silicate melt. Our new experimental results are combined with literature data to yield a parameterisation for the exchange coefficient of Au (KdAuMet/Sil) as a function of P, T and XS:logKdAuMet/Sil=-1.10(0.04)+1.11(0.01)104T(K)-67.8(10.9)P(GPa)T(K)+5.81(0.24)log(1-XS)Using this relationship, alongside similar parameterisations for Ni and Co selected from the literature, we performed continuous accretion models to delineate regions of parameter space in which Au and moderately siderophile element (MSE) abundances in the primitive upper mantle (PUM) could be reproduced. These models suggest that for metal–silicate equilibrium at very high pressures, Au will be overabundant in the PUM if equilibrium also occurs at extreme temperatures. Instead, most successful models are found when equilibrium occurs at high pressure but sub-liquidus temperatures. This result is in keeping with the physical conditions expected for a scenario where core-forming metal ponds and equilibrates at the rheological base of a magma ocean (e.g. Wade and Wood, 2005).

Partitioning of Chlorine Between Nacl Brines and Ferro-Pargasite: Implications For the Formation of Chlorine-Rich Amphiboles In Mafic Rocks

Determining the partitioning of chloride between fluid and rock has relevance to a wide range of geological processes, including high-grade metamorphism, economic deposit formation, and seawater-oceanic crust interactions. Calcium amphiboles, particularly those that are iron-rich, are a common host for chlorine and have the potential to reveal information about the ambient fluid composition, if the crystal-chemical controls on chlorine incorporation are known. In this study we investigated the incorporation of chlorine into synthetic ferro-pargasite [NaCa_2(Fe_4Al)(Al_2Si_6)O_(22)(Cl,OH)_2] formed mostly at 700 °C, 0.2 GPa, and oxygen fugacities (f_(O2)) between Co–CoO and wustite–magnetite (−20.2 < log(f_(O2)) < −17.5) for durations of 3–28 days. Starting mixtures consisted of reagent oxides and metallic iron to which chlorine was added as either a stock solution of NaCl brine up to 5 molal (m) (equivalent to mole fraction of NaCl, X_(NaCl), up to 0.082), or by mixing NaCl ± water directly with the starting mixture to achieve higher concentration brines by homogenization at elevated pressures and temperatures. Good yields of amphibole were obtained, with the most common additional phases being plagioclase, hedenbergite, and halite. At low salinities the Cl content of the amphibole, determined by electron microprobe analysis (EMPA) of individual grains, increases linearly to about 0.045 anions per formula unit (apfu) as the brine concentration increases to about 1.7 m NaCl (X_(NaCl) = 0.03). Thermodynamic modeling of the partitioning of Cl– between brine and the amphibole yields a constant value for the equilibrium constant (K_a) of 0.78 ± 0.09 at 700 °C and 0.2 GPa over the range 0.1–2.0 m NaCl with the empirically derived relationship for the activity of Cl– in amphibole (ɑ^(ɑmph)_(Cl)_-) being ɑ^(ɑmph)_(Cl)_- ═ (X^(o(3)_(Cl))^(2/3). This activity-composition relationship suggests much less random mixing of Cl^–, OH^–, and O2^– at the O(3) site. At salinities above 1.7 m NaCl, the Cl content of the amphibole is fairly insensitive to the brine concentration. Only at halite saturation (83 m NaCl, or X_(NaCl) = 0.60, at 700 °C and 0.2 GPa) is there a noticeable increase in the Cl content of the amphibole, with a bimodal distribution centered at 0.2 and 0.7 Cl apfu observed in the highest salinity synthesis. The implication of this study is that (1) amphiboles with Cl contents above about 0.1 apfu (∼0.4 wt.% Cl) require ambient brines at halite saturation, and (2) oceanic crust reacting with homogeneous seawater could incorporate 50–200 ppm Cl via the presence of 8–31 wt.% ferro-pargasite in the oceanic crust, respectively.

P33] Activity-Composition Relations and Thermodynamic Properties in Pt-V and Pt-Cr Alloys at 1500oC

P33] Activity-Composition Relations and Thermodynamic Properties in Pt-V and Pt-Cr Alloys at 1500oC

A model for the activity of silica along the carbonatite–kimberlite–mellilitite–basanite melt compositional joint

Carbon dioxide and water, being present in the Earth's mantle at concentration levels of tens to hundreds of ppm, greatly lower the peridotite solidus temperature and drastically modify the composition of produced melts. The presence of CO2 produces silica-poor, carbonate-rich liquids at the onset of melting, and these liquids shift toward silica rich compositions as the degree of melting increases. Numerous geochemical observations and experimental studies have revealed the complexity of the transition between carbonate-rich and silicate-rich melts. It is characterized by a strongly non-linear evolution and, under specific conditions, by immiscibility. To better constrain this transition, we have used the thermodynamic activity of silica as a probe of the mixing properties between molten carbonate and molten silicate. The activity of silica aSiO2l was calculated for a large number of experimental liquids from two equilibria: olivine–orthopyroxene–melt and immiscible silicate-rich melt–carbonate-rich melt (491 data points ranging from 1 to 14GPa and 1090 to 1800°C). We modelled aSiO2l during incipient melting of the peridotite in presence of CO2 with a generalized Margules function. Our model reproduces well the silica activity–composition relationships of the experimental database, and can be used to predict the silica content of the melts coexisting with olivine and orthopyroxene. We show that water content and Ca/Mg ratio in the melts have an important influence on the aSiO2l. In contrast to a recent empirical model (Dasgupta et al., 2013), the analysis of the experimental database reveals that the transition from carbonate to silicate melt with decreasing depth should occur abruptly in oceanic mantle. Our model predicts that carbonatitic melts with ~5wt.% SiO2 can be stabilized from ~150km depth, at the onset of incipient melting by “redox melting”, up to ~75km, above which the liquid evolves abruptly to a carbonated silicate composition (> ~25wt.% SiO2). In the cratonic mantle lithosphere, our model predicts that carbonatitic melts are prevailing up to shallow depth, and conflicts the recent model (Russell et al., 2012) of CO2-saturation triggered by orthopyroxene assimilation during kimberlite ascent.

Thermodynamic Modeling of Poorly Complexing Metals in Concentrated Electrolyte Solutions: An X-Ray Absorption and UV-Vis Spectroscopic Study of Ni(II) in the NiCl2-MgCl2-H2O System

Knowledge of the structure and speciation of aqueous Ni(II)-chloride complexes is important for understanding Ni behavior in hydrometallurgical extraction. The effect of concentration on the first-shell structure of Ni(II) in aqueous NiCl2 and NiCl2-MgCl2 solutions was investigated by Ni K edge X-ray absorption (XAS) and UV-Vis spectroscopy at ambient conditions. Both techniques show that no large structural change (e.g., transition from octahedral to tetrahedral-like configuration) occurs. Both methods confirm that the Ni(II) aqua ion (with six coordinated water molecules at R Ni-O = 2.07(2) Å) is the dominant species over the whole NiCl2 concentration range. However, XANES, EXAFS and UV-Vis data show subtle changes at high salinity (> 2 mol∙kg-1 NiCl2), which are consistent with the formation of small amounts of the NiCl+ complex (up to 0.44(23) Cl at a Ni-Cl distance of 2.35(2) Å in 5.05 mol∙kg-1 NiCl2) in the pure NiCl2 solutions. At high Cl:Ni ratio in the NiCl2-MgCl2-H2O solutions, small amounts of [NiCl2]0 are also present. We developed a speciation-based mixed-solvent electrolyte (MSE) model to describe activity-composition relationships in NiCl2-MgCl2-H2O solutions, and at the same time predict Ni(II) speciation that is consistent with our XAS and UV-Vis data and with existing literature data up to the solubility limit, resolving a long-standing uncertainty about the role of chloride complexing in this system.

MgAl2O4-Al8/3O4 spinels: Formulation and calibration of the low-pressure thermodynamics of mixing

A thermodynamic model for the mixing properties of MgAl2O4–Al8/3O4 spinels is formulated, calibrated, and used to develop a closed-form polynomial for the excess Gibbs energy of mixing. The model is formulated on the assumptions that Mg, Al and vacancies exhibit long-range-ordering (LRO) between tetrahedral and octahedral sites, and that the vibrational Gibbs energy may be described by a Taylor expansion of second degree in the composition variable X ≡ 3 2 (2XAlOCT + XAlTET − 2) = (3XVacTET + 6XVacOCT) and in LRO variables s1 ≡ XAlOCT − 1 2 XAlTET and s2 ≡ 3XVacTET − 6XVacOCT. Calibration of the parameters produces a model that, in addition to being consistent with spectroscopic constraints on Mg-Al ordering in MgAl2O4-rich spinels, successfully reproduces the brackets on the compositions of MgAl2O4–Al8/3O4 spinels coexisting with corundum over the temperature range 1400 to 2200 K, employing only a few mixing parameters and making physically plausible predictions, such as random distribution of vacancies and Al between octahedral and tetrahedral sites in the metastable, cubic γ-Al8/3O4 endmember. The resulting activity–composition relations for MgAl2O4–Al8/3O4 spinels are extremely simple and show minimal (Al8/3O4) to negligible (MgAl2O4) temperature dependence.