Determination of the miscibility gap on the solid solution series paragonite-margarite by means of the infrared spectroscopy

In order to study the miscibility gap of the solid solution series margarite-paragonite oxide mixtures corresponding to intermediate compositions were crystallized at H₂O-pressures between 1 and 6 kb at 400 to 600 °C. Run products were examined by x-ray method and IR-spectroscopy; the latter proved to be the tool which allowed to distinguish between the two phases, the calcic and the sodic mica. A solvus exists in the region between 20 and 50 mole% margarite. Its width is temperature-dependent with a critical temperature above 600 °C.     

Monte Carlo simulation of mixing in Ca3Fe2Ge3O12–Ca4Ge4O12 garnets and implications for the thermodynamic stability of pyrope–majorite solid solution

Static lattice energy calculations, based on empirical pair potentials, were performed for a large set of structures differing in the arrangement of octahedral cations within the garnet 2 × 2 × 2 supercell. The compositions of these structures varied between Ca₃Fe₂Ge₃O₁₂ and Ca₄Ge₄O₁₂. The energies were cluster expanded using pair and quaternary terms. The derived ordering constants were used to constrain Monte Carlo simulations of temperature-dependent mixing properties in the ranges of 1,073–3,673 K and 0–10 GPa. The free energies of mixing were calculated using the method of thermodynamic integration. The calculations predict a wide miscibility gap between Fe-rich (cubic) and Fe-pure (tetragonal) garnets consistent with recent experimental observations of Iezzi et al. (Phys Chem Miner 32:197–207, 2005). It is shown that the miscibility gap arises due to a very strong cation ordering at the Fe-pure composition, driven by the charge difference between Ca²⁺ and Ge⁴⁺ cations. The structural and thermodynamic analogies between Ca–Ge and Mg–Si systems suggest that a similar miscibility gap should exist between pyrope and Mg–Si-majorite.     

Stability properties of the CuS2-FeS2 solid solution series of pyrite type

Summary Experimental investigations on the Cu-Fe-substitution and the formation of a solid solution series in the system CuS₂-FeS₂ were carried out under hydrothermal conditions up to 350°C and 3 kb and by means of a piston cylinder apparatus at higher temperatures and pressures up to 900°C and 45 kb. Under dry conditions at 440°C and above 17 kb the system was found to be binary with a miscibility gap between an iron-rich phase near the FeS₂ end-member and a coexisting copper-rich phase being the solvus composition of a homogeneity region from 75 to 100 mole% CuS₂. This solvus of the copper rich phase was found to be almost independent of temperature and pressure up to 45 kb and 700°C. The solubility of CuS₂ in FeS₂ at 45 kb increases from 0.6 mole% at 700°C to 4.5 mole% at 900°C. Under hydrothermal conditions up to 3 kbars the solvus of metastable (Cu, Fe)S₂ is strongly dependent on pressure only in the Cu-rich part of the system.

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Zusammenfassung Stabilität der CuS₂-FeS₂ Mischreihe des Pyrit-Typs Experimentelle Untersuchungen zur Cu-Fe-Substitution und zur Bildung einer festen Lösung im System CuS₂-FeS₂ wurden mit der Hydrothermalsynthese bis 350°C und 3 kb und mit der Stempelzylindermethode bis 900°C und 45 kb durchgeführt. Unter trockenen Bedingungen bei 440°C und oberhalb 17 kb ist dieses System binär und weist eine Mischungslücke zwischen einer eisenreichen Phase nahe dem FeS₂ Endglied und einer koexistierenden kupferreichen Phase mit der Solvuszusammensetzung eines Homogenitätsbereiches zwischen 75 und 100 mol% CuS₂ auf. Dieser Solvus der kupferreichen Phase wurde bis 45 kb und 700°C nahezu druck- und temperaturunabhängig gefunden. Demgegenüber nimmt die Löslichkeit von CuS₂ in FeS₂ bei 45 kb von 0.6 mol% bei 700°C auf 4.5 mol% bei 900°C zu. Der Solvus der metastabilen (Cu, Fe)S₂-Phasen, die bislang nur unter hydrothermalen Bedingungen synthetisiert werden können, zeigte bis 3 kbar nur im kupferreichen Teil des Systems eine starke Druckabhängigkeit.

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With 4 Figures     

Miscibility in the CaSO4·2H2O–CaSeO4·2H2O system: Implications for the crystallisation and dehydration behaviour

SeO₄²⁻ ions can substitute for sulphate in the gypsum structure. In this work crystals of different Ca(SO₄,SeO₄)·2H₂O solid solutions were precipitated by mixing a CaCl₂ solution with solutions containing different ratios of Na₂SO₄ and Na₂SeO₄. The compositions of the precipitates were analysed by EDS and the cell parameters were determined by X-ray powder diffraction. Moreover, a comparative study on dehydration behaviour of selenate rich and sulfate rich Ca(SO₄,SeO₄)·2H₂O solid solutions was carried out by thermogravimetry.The experimental results show that the Ca(SO₄,SeO₄)·2H₂O solid solution presents a symmetric miscibility gap for compositions ranging from X_CaSO4·2H2O = 0.23 to X_CaSO4·2H2O = 0.77. By considering a regular solution model a Guggenheim parameter a₀ = 2.238 was calculated. The solid phase activity coefficients obtained with this parameter were used to calculate a Lippmann diagram for the system Ca(SO₄,SeO₄)·2H₂O–H₂O.     

Pyroxene thermometry in simple and complex systems

Simple mixing models have been applied to ortho- and clinopyroxene solid solutions and a semi-empirical equation of state extracted from the available experimental data for the diopside-enstatite miscibility gap. This equation successfully reproduces the miscibility gap over a temperature range of 800 °C to 1700 °C and is apparently also applicable to aluminous pyroxenes in the system CaSiO₃-MgSiO₃-Al₂O₃. The effect of iron solubility in the pyroxenes has been calibrated empirically using most of the available experimental data for multicomponent pyroxenes. This semi-empirical model reproduces most of the experimental data within 70 °C. Temperatures calculated for naturally equilibrated Mg-rich two-pyroxene assemblages deviate markedly from those estimated using the thermometer of Wood and Banno (1973). These discrepancies can be attributed to large inaccuracies in the thermometer of Wood and Banno (1973) for Mg-rich compositions.     

Thermochemistry of strontium incorporation in aragonite from atomistic simulations

We have investigated the thermodynamics of mixing between aragonite (orthorhombic CaCO₃) and strontianite (SrCO₃). In agreement with experiment, our simulations predict that there is a miscibility gap between the two solids at ambient conditions. All Sr_xCa_1−xCO₃ solids with compositions 0.12 < x < 0.87 are metastable with respect to separation into a Ca-rich and a Sr-rich phase. The concentration of Sr in coral aragonites (x ∼ 0.01) lies in the miscibility region of the phase diagram, and therefore formation of separated Sr-rich phases in coral aragonites is not thermodynamically favorable. The miscibility gap disappears at around 380 K. The enthalpy of mixing, which is positive and nearly symmetric with respect to x = 0.5, is the dominant contribution to the excess free energy, while the vibrational and configurational entropic contributions are small and of opposite sign. We provide a detailed comparison of our simulation results with available experimental data.     

Pressure dependence ot SrSO4 solubility

The solubilities of SrSO₄ in seawater, 0.65 M NaCl and and distilled water were measured as a function of pressure at 2°C. The thermodynamic solubility product was determined from the distilled water measurements and stoichiometric solubility products were determined from the seawater and Nad measurements. The equilibrium quotient for SrSO₄ dissolution at ionic strength of 0.65 was calculated from the NaCl measurements, using the known NaSO₄^? ionpairing association constant. For each of the solubility products values of $\text{Θ V}$ were determined. These experimental values were all $\text{11.0 ± 0.3}\text{ml}$ mole_? lower than the theoretical values based on anhydrous SrSO₄. This difference may be due to the equilibrating solid phase being a hydrated form of SrSO₄.     

Garnet-orthopyroxene and orthopyroxene-clinopyroxene relationships in simple and complex systems

Use of simple mixing models of orthopyroxene and garnet solid solutions enables extrapolation of experimentally determined equilibria in the MgSiO₃-Al₂O₃ system to uninvestigated parts of pressure-temperature-composition space. Apparent discrepancies in the experimental data for simple and multicomponent systems may be explained by considering the effect of CaO and FeO on reducing pyrope activity in the garnet solid solutions. Equilibration pressures of natural garnet-orthopyroxene assemblages may be calculated, provided temperatures are known, from a combination of the experimental data on the MgSiO₃-Al₂O₃ system and analyses of coexisting natural phases.Despite the presence of a compositional gap in the system, the solubility of enstatite in diopside coexisting with orthopyroxene can also be approximately treated by an ideal solution model. An empirical approach has been developed to take account of Fe²⁺ on the orthopyroxene-clinopyroxene miscibility gap in natural systems in order to calculate equilibration temperatures of 2-pyroxene assemblages. The model presented reproduces almost all of the available experimental data for multicomponent systems to within 60° C.     

Determination of SrSO04 ion pair formation using conductimetric and ion exchange techniques

The dissociation constant for SrSO⁰₄ ion pair was determined at 25°C using conductance and ion-exchange techniques. Both approaches yield values for pK of SrSO⁰₄ at zero ionic strength in the range 2.28–2.31. Previously reported values range from 2.1 to 3.0. The refinement in the dissociation constant should allow more reliable appraisals of the extent of strontium mineral solubility controls on strontium concentrations in natural water systems.The Lee and Wheaton conductance model was used to interpret the results of the conductivity measurements in strontium sulphate solutions at 25°C. Because of the limitations imposed by the solubility of celestite, a sufficiently-wide concentration range to enable determination of all three of the parameters—dissociation constant, Λ₀, and the distance parameter could not be made. Instead, values are reported for the dissociation constant and Λ₀ using reasonable limiting values for the distance parameter.Dowex-50 was used in the ion-exchange technique to determine the dissociation constant for SrSO⁰₄. This method was used to determine values at other temperatures as well. Although there is considerable scatter in the temperature data, a standard enthalpy for the dissociation reaction: SrSO⁰₄→ Sr²⁺ + SO^2?₄ is computed to be 8.7 ± 2 kJmole^?1 at 25°C.     

An internally consistent model for the thermodynamic properties of Fe−Mg-titanomagnetite-aluminate spinels

A model is developed for the thermodynamic properties of Fe²⁺–Mg²⁺-aluminate-titanate-ferrite spinels of space group Fd3m. The model incorporates an expression for the configurational entropy of mixing which accounts for long-range order over tetrahedral and octahedral sites. Short-range order or departures from cubic symmetry are not considered. The non-configurational Gibbs energy is formulated as a second degree Taylor expansion in six linearly independent composition and ordering variables. The model parameters are calibrated to reproduce miscibility gap constraints, order-disorder phenomena in MgAl₂O₄ and MgFe₂O₄, and Fe²⁺–Mg²⁺ partitioning data between olivine and: (1) aluminate spinels; (2) ferrite spinels; (3) titanate spinels; (4) mixed aluminate-ferrite spinels. This calibration is achieved without invoking non-configurational excess entropies of mixing. The model predicts that the ordering state of FeAl₂O₄ is more normal than that of MgAl₂O₄. It also successfully accounts for heat of solution measurements and activity-composition relations in the constituent binaries. Phase equilibrium constraints require that the structure of Fe₃O₄ is more inverse than random at all temperatures and that Mg²⁺ has a strong tetrahedral site preference with respect to that of Fe²⁺. The analysis suggests that in the titanates short range order on octahedral sites may be significant at temperatures as high as 1300° C. Constraints developed from calibrating the thermodynamic properties of Fe²⁺–Mg²⁺-aluminatetitanate-ferrite spinel solid solutions permit extension of the database of Berman (1988) to include estimates of the end-member properties of hercynite (FeAl₂O₄), ulvöspinel (Fe₂TiO₄), MgFe₂O₄ and cubic Mg₂TiO₄. In constructing these estimates, provision is made for low-temperature magnetic entropy contributions and the energetic consequences of disordering the aluminates and the ferrites. These estimates are consistent with all of the available low-temperature adiabatic calorimetry, high-temperature heat content, and heat of solution measurements on the end-members. The analysis implies that there is a substantial heat capacity anomaly in the range 300°–900° C associated with disordering of the MgAl₂O₄ structure while that in FeAl₂O₄ becomes significant at temperatures above 700° C. The same heat capacity response in the ferrites indicates that the order/disorder transformation is coupled to the antiferromagnetic-paramagnetic transition in MgFe₂O₄ but takes place well above the ferrimagnetic-paramagnetic transition in magnetite. The proposed model is internally consistent with solution theory reported elsewhere for Fe²⁺–Mg²⁺ olivines and orthopyroxenes (Sack and Ghiorso 1989), rhombohedral oxides (Ghiorso 1990a) and the remaining end-member properties of Berman (1988).     

K-Ar Radiometric Age Determinations of White Micas from the Piemont Zone,French-Italian Western Alps

The olivine-clinopyroxene (Fe-Mg partition) geothermometer of Powell and Powell (1974) was derived on the basis of clinopyroxene mixing parameters which imply a large miscibility gap in CaFeSi₂O₆-CaMgSi₂O₆-CaAl₂SiO₆ clinopyroxenes at 950° C. Application of the Powell and Powell thermometer in an empirical way (ignoring likely errors in mixing parameters) is limited by the fact that almost all natural clinopyroxene-olivine pairs are constrained by the form of the equation to have 1-bar temperatures within the narrow (915–1,060° C) limits of their calibration points.Temperatures obtained from the Powell and Powell thermometer have been compared with those obtained from clinopyroxene-orthopyroxene (miscibility gap) thermometers; agreement with the latter is poor. There are discrepancies between calculated and observed (experimental) temperatures for iron-rich clinopyroxene-olivine pairs. It is concluded that application of the thermometer in its present form is unlikely to produce reliable results.     

Synthesis of barite,celestite and barium-strontium sulfate solid solution crystals

Crystals of synthetic barite (BaSO₄), celestite (SrSO₄), and their solid solutions were recrystallized from brine by thermal cycling. Electron probe microanalyses show that the solid solutions are homogeneous within detection limits. The molar composition of solid solution crystals approximates the ideal value based on the ratio of equilibrium constants for barium and strontium sulfates.     

Characterization,dissolution and solubility of the hydroxypyromorphite–hydroxyapatite solid solution [(Pb<Subscript>x</Subscript>Ca<Subscript>1−x</Subscript>)<Subscript>5</Subscript>(PO<Subscript>4</Subscript>)<Subscript>3</Subscript>OH] at 25 °C and pH 2–9

Background

The interaction between Ca-HAP and Pb²⁺ solution can result in the formation of a hydroxyapatite–hydroxypyromorphite solid solution [(Pb_xCa_1?x)₅(PO₄)₃(OH)], which can greatly affect the transport and distribution of toxic Pb in water, rock and soil. Therefore, it’s necessary to know the physicochemical properties of (Pb_xCa_1?x)₅(PO₄)₃(OH), predominantly its thermodynamic solubility and stability in aqueous solution. Nevertheless, no experiment on the dissolution and related thermodynamic data has been reported.

Results

Dissolution of the hydroxypyromorphite–hydroxyapatite solid solution [(Pb_xCa_1?x)₅(PO₄)₃(OH)] in aqueous solution at 25 °C was experimentally studied. The aqueous concentrations were greatly affected by the Pb/(Pb + Ca) molar ratios (X_Pb) of the solids. For the solids with high X_Pb [(Pb_0.89Ca_0.11)₅(PO₄)₃OH], the aqueous Pb²⁺ concentrations increased rapidly with time and reached a peak value after 240–720 h dissolution, and then decreased gradually and reached a stable state after 5040 h dissolution. For the solids with low X_Pb (0.00–0.80), the aqueous Pb²⁺ concentrations increased quickly with time and reached a peak value after 1–12 h dissolution, and then decreased gradually and attained a stable state after 720–2160 h dissolution.

Conclusions

The dissolution process of the solids with high X_Pb (0.89–1.00) was different from that of the solids with low X_Pb (0.00–0.80). The average K _sp values were estimated to be 10^?80.77±0.20 (10^?80.57–10^?80.96) for hydroxypyromorphite [Pb₅(PO₄)₃OH] and 10^?58.38±0.07 (10^?58.31–10^?58.46) for calcium hydroxyapatite [Ca₅(PO₄)₃OH]. The Gibbs free energies of formation (ΔG _f ^o ) were determined to be ?3796.71 and ?6314.63 kJ/mol, respectively. The solubility decreased with the increasing Pb/(Pb + Ca) molar ratios (X_Pb) of (Pb_xCa_1?x)₅(PO₄)₃(OH). For the dissolution at 25 °C with an initial pH of 2.00, the experimental data plotted on the Lippmann diagram showed that the solid solution (Pb_xCa_1?x)₅(PO₄)₃(OH) dissolved stoichiometrically at the early stage of dissolution and moved gradually up to the Lippmann solutus curve and the saturation curve for Pb₅(PO₄)₃OH, and then the data points moved along the Lippmann solutus curve from right to left. The Pb-rich (Pb_xCa_1?x)₅(PO₄)₃(OH) was in equilibrium with the Ca-rich aqueous solution.

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Graphical abstractLippmann diagrams for dissolution of the hydroxypyromorphite–hydroxyapatite solid solution [(Pb_xCa_1?x)₅(PO₄)₃OH] at 25??C and an initial pH of 2.00.

     

Thermodynamic properties of hematite — ilmenite — geikielite solid solutions

A solution model is developed for rhombohedral oxide solid solutions having compositions within the ternary system ilmenite [(Fe ₂₊ ^s Ti ₄₊ ^1–s ) ^A (Fe ₂₊ ^1–s Ti ₄₊ ^s ) ^B O₃]-geikielite [(Mg ₂₊ ^t Ti ₄₊ ^1–t ) ^A (Mg ₂₊ ^1–t Ti ₄₊ ^t ) ^B O₃]-hematite [(Fe³⁺) ^A (Fe³⁺) ^B O₃]. The model incorporates an expression for the configurational entropy of solution, which accounts for varying degrees of structural long-range order (0s, t1) and utilizes simple regular solution theory to characterize the excess Gibbs free energy of mixing within the five-dimensional composition-ordering space. The 13 model parameters are calibrated from available data on: (1) the degree of long-range order and the composition-temperature dependence of the transition along the ilmenite-hematite binary join; (2) the compositions of coexisting olivine and rhombohedral oxide solid solutions close to the Mg–Fe²⁺ join; (3) the shape of the miscibility gap along the ilmenite-hematite join; (4) the compositions of coexisting spinel and rhombohedral oxide solid solutions along the Fe²⁺–Fe³⁺ join. In the course of calibration, estimates are obtained for the reference state enthalpy of formation of ulvöspinel and stoichiometric hematite (–1488.5 and –822.0 kJ/mol at 298 K and 1 bar, respectively). The model involves no excess entropies of mixing nor does it incorporate ternary interaction parameters. The formulation fits the available data and represents an internally consistent energetic model when used in conjuction with the standard state thermodynamic data set of Berman (1988) and the solution theory for orthopyroxenes, olivines and Fe–Mg titanomagnetite-aluminate-chromate spinels developed by Sack and Ghiorso (1989, 1990a, b). Calculated activity-composition relations for the end-members of the series, demonstrate the substantial degree of nonideality associated with interactions between the ordered and disordered structures and the dominant influence of the miscibility gap across much of the ternary system. The predicted shape of the miscibility gap, and the orientation of tie-lines relating the compositions of coexisting phases, display the effects of coupling between the excess enthalpy of solution and the degree of long-range order. One limb of the miscibility gap follows the composititiontemperature surface corresponding to the ternary second-order transition.     

Atomistic model of diopside-K-jadeite (CaMgSi<Subscript>2</Subscript>O<Subscript>6</Subscript>-KAlSi<Subscript>2</Subscript>O<Subscript>6</Subscript>) solid solution

Atomistic model was proposed to describe the thermodynamics of mixing in the diopside-K-jadeite solid solution (CaMgSi₂O6-KAlSi₂O₆). The simulations were based on minimization of the lattice energies of 800 structures within a 2 × 2 × 4 supercell of C2/c diopside with the compositions between CaMgSi₂O₆ and KAlSi₂O₆ and with variable degrees of order/disorder in the arrangement of Ca/K cations in M2 site and Mg/Al in Ml site. The energy minimization was performed with the help of a force-field model. The results of the calculations were used to define a generalized Ising model, which included 37 pair interaction parameters. Isotherms of the enthalpy of mixing within the range of 273–2023 K were calculated with a Monte Carlo algorithm, while the Gibbs free energies of mixing were obtained by thermodynamic integration of the enthalpies of mixing. The calculated T-X diagram for the system CaMgSi₂O₆-KAlSi₂O₆ at temperatures below 1000 K shows several miscibility gaps, which are separated by intervals of stability of intermediate ordered compounds. At temperatures above 1000 K a homogeneous solid solution is formed. The standard thermodynamic properties of K-jadeite (KAlSi₂O₆) evaluated from quantum mechanical calculations were used to determine location of several mineral reactions with the participation of the diopside-K-jadeite solid solution. The results of the simulations suggest that the low content of KalSi₂O₆ in natural clinopyroxenes is not related to crystal chemical factors preventing isomorphism, but is determined by relatively high standard enthalpy of this end member.     

Molecular simulations of interfacial and thermodynamic mixing properties of grossular–andradite garnets

Experimental observations using transmission electron microscopy (TEM) indicate that Fe³⁺-rich grossular–andradite solid solutions with oscillatory zoning tend to occur as separate lamellae of andradite and intermediate compositions (Hirai and Nakazawa 1986; Pollok et?al. 2001). From one lamella to the next, the Fe³⁺ concentration can change significantly within a few nm. In order to understand the Fe³⁺ and Al content of each phase and the thermodynamics, chemistry, structure, and stability at the interfaces, Monte Carlo simulations were performed. According to our calculations, there is an ordered structure with a 1:1 ratio of Al and Fe³⁺ with alternating Al and Fe octahedra along the main cubic crystallographic axes. Even though this ordered grandite is more energetically favorable than a 1:1 mixture of the end members grossular and andradite [by ≈1.6?kJ (mol exchangeable cations)^?1], this structure is stable only at temperatures below ≈500?K. Enthalpies, free energies, configurational and vibrational entropies of mixing, and the long-range order parameter are influenced by the formation of ordered grandite below 500?K. These data also explain why interfaces are stable only between grossular and grandite or between andradite and grandite but not between the end members. The interface energies between the end members and ordered grandite are comparably low [0.16?meV?Å^?2∥(1?0?0), 0.55?meV?Å^?2∥(1?1?0), 0.63?meV?Å^?2∥(1?1?1)] and, therefore, do not hinder the formation of lamellae. Our calculations on the free energies of mixing indicate that there are miscibility gaps between grossular and grandite and between grandite and andradite only below ≈430?K. Since most of these solid solutions are formed at higher temperatures for which we did not find evidence of a miscibility gap, the formation of compositional oscillations is probably due to kinetic hindering of thermodynamically stable complete solid solutions. ?A new methodological aspect is the incorporation of zero-point energies of vibrations and the vibrational entropies into the calculation of the free energy of mixing. In case of the grossular–andradite solid solution, these vibrational effects change the free energy of mixing by only a few percent.     

The role of sulfate groups in controlling CaCO3 polymorphism

The nucleation and growth of CaCO₃ phases from aqueous solutions with SO₄²⁻:CO₃²⁻ ratios from 0 to 1.62 and a pH of ∼10.9 were studied experimentally in batch reactors at 25 °C. The mineralogy, morphology and composition of the precipitates were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy and microanalyses. The solids recovered after short reaction times (5 min to 1 h) consisted of a mixture of calcite and vaterite, with a S content that linearly correlates with the SO₄²⁻:CO₃²⁻ ratio in the aqueous solution. The solvent-mediated transformation of vaterite to calcite subsequently occurred. After 24 h of equilibration, calcite was the only phase present in the precipitate for aqueous solutions with SO₄²⁻:CO₃²⁻ ? 1. For SO₄²⁻:CO₃²⁻ > 1, vaterite persisted as a major phase for a longer time (>250 h for SO₄²⁻:CO₃²⁻ = 1.62). To study the role of sulfate in stabilizing vaterite, we performed a molecular simulation of the substitution of sulfate for carbonate groups into the crystal structure of vaterite, aragonite and calcite. The results obtained show that the incorporation of small amounts (<3 mole%) of sulfate is energetically favorable in the vaterite structure, unfavorable in calcite and very unfavorable in aragonite. The computer modeling provided thermodynamic information, which, combined with kinetic arguments, allowed us to put forward a plausible explanation for the observed crystallization behavior.     

Reversed pyroxene phase equilibria in CaO-MgO-SiO2 from 925° to 1,175° C at one atmosphere pressure

Experiments using V₂O₅ as a high-temperature solvent have produced compositional reversals defining the miscibility gap between enstatite and diopside on the join Mg₂Si₂O₆-CaMgSi₂O₆ between 925° and 1,175° C at atmospheric pressure. These experiments locate an equilibrium near 1,000° C among diopside, protoenstatite, and orthoenstatite; they verify the stable coexistence of diopside and protoenstatite above 1,000° C and disprove the hypothesis that orthoenstatite has a stability field which is continuous from temperatures below 1,000° C to the solidus. The phase relations suggest that the orthorhombic low-Ca pyroxene on the solidus in this system (formerly identified as orthoenstatite) is a phase distinct from the orthoenstatite stable with diopside at low subsolidus temperatures. Data locating the orthoenstatite-diopside miscibility gap validate the use at low pressures of symmetric orthopyroxene and asymmetric clinopyroxene solution models in this system.     

Fluid inclusion geochemistry of the early Proterozoic Pirilä gold deposit in Rantasalmi,Southeastern Finland

The small Pirilä gold deposit, which is located in the southeastern part of the Svecofennian complex near the Archean/Proterozoic boundary, is hosted by quartz veins and lenses occurring in mica schist. The rocks of the area were metamorphosed under conditions of amphibolite facies. Gold is invariably associated with sulphides. Microthermometry of fluid inclusions in quartz indicates four types of inclusions: (1) weakly saline H₂O-CO₂ (< 4.0 eq.wt% NaCl) with small amounts of CH₄ (< 10 mole% CH₄); (2) CO₂ (< 10 mole% CH₄); (3) CH₄; and (4) H₂O (< 25 eq.wt% NaCl) with less than 0.85 mole% CO₂ in the vapour phase. Texturally these inclusion types are classified as primary (H₂O-CO₂) and secondary (H₂O, CO₂ and CH₄). Leachate analysis shows that, in addition to Na, the aqueous fluids contain Ca and Fe with minor amounts of K and Mg. The primary H₂O-CO₂ and the secondary H₂O inclusions contain sulphide and unidentified opaque grains, respectively. The secondary CH₄ inclusions are often associated with short trails of arsenopyrite grains. Fluid inclusion and geological data suggest ore mineral mobilization, crystallization of host quartz, and deposition of sulphides controlled by the D₂ and D₃ structures in the presence of a H₂O-CO₂ fluid mainly during the plastic D₃ deformation and during the amphibolite facies metamorphism (i.e. 3.4 kbars/540–670°C). During ductile-brittle deformation (probably D₄), precipitation of tectonic remobilized gold from sulphides in fractures occurred in the presence of CH₄ and H₂O fluids at lowered temperature (< 440°C) and pressure (< 2 kbars).     

57Fe-Moessbauer spectra,electronic and crystal structure of members of the CuS2-FeS2 solid solution series

Members of the (Cu, Fe)S₂ solid solution crystallize in the pyrite structure type, space group Pa 3, Cu and Fe being statistically distributed on the metal sites. Within this series, a semiconductor to metal transition can be detected between 25 and 38 mole% CuS₂. Compositional dependent ⁵⁷Fe-Moessbauer spectra reveal Fe²⁺ in low-spin configuration. A minimum of the quadrupole splitting and the slope in the ⁵⁷Fe-isomer shift in the intermediate part of the system, near 30 mole% CuS₂, can be correlated with the onset of metallic conductivity, whereas the structural parameters are not influenced by this transition. The analysis of the compositional dependency of the quadrupole splitting, in comparison to the isotypic system (Co, Fe)S₂, leads to the conclusion that Cu in solid (Cu, Fe)S₂ compounds is Cu⁺ with an Ar -3 d¹⁰ electronic configuration.