Thermochemistry of yavapaiite KFe(SO4)2: Formation and decomposition

Yavapaiite, KFe(SO₄)₂, is a rare mineral in nature, but its structure is considered as a reference for many synthetic compounds in the alum supergroup. Several authors mention the formation of yavapaiite by heating potassium jarosite above ca. 400°C. To understand the thermal decomposition of jarosite, thermodynamic data for phases in the K-Fe-S-O-(H) system, including yavapaiite, are needed. A synthetic sample of yavapaiite was characterized in this work by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermal analysis. Based on X-ray diffraction pattern refinement, the unit cell dimensions for this sample were found to be a = 8.152 ± 0.001 Å, b = 5.151 ± 0.001 Å, c = 7.875 ± 0.001 Å, and β = 94.80°. Thermal decomposition indicates that the final breakdown of the yavapaiite structure takes place at 700°C (first major endothermic peak), but the decomposition starts earlier, around 500°C. The enthalpy of formation from the elements of yavapaiite, KFe(SO₄)₂, ΔH°_f = −2042.8 ± 6.2 kJ/mol, was determined by high-temperature oxide melt solution calorimetry. Using literature data for hematite, corundum, and Fe/Al sulfates, the standard entropy and Gibbs free energy of formation of yavapaiite at 25°C (298 K) were calculated as S°(yavapaiite) = 224.7 ± 2.0 J.mol⁻¹.K⁻¹ and ΔG°_f = −1818.8 ± 6.4 kJ/mol. The equilibrium decomposition curve for the reaction jarosite = yavapaiite + Fe₂O₃ + H₂O has been calculated, at pH₂O = 1 atm, the phase boundary lies at 219 ± 2°C.     

Phosphoric acid fractionation factors for smithsonite and cerussite between 25 and 72°C

The intramolecular kinetic oxygen isotope fractionation between CO₂ and CO₃²⁻ during reaction of phosphoric acid with natural smithsonite (ZnCO₃) and cerussite (PbCO₃) has been determined between 25 and 72°C. While cerussite decomposes in phosphoric acid within a few hours at 25°C, smithsonite reacts very slowly with the acid at 25°C providing yields of CO₂ < 25% after 2 weeks. The low yields result in a low precision for oxygen isotope measurements of the acid-liberated CO₂ (±1.65‰, 1σ, n = 9). The yield and reproducibility of oxygen isotope values of the acid-liberated CO₂ from smithsonite can be improved, the latter to ∼±0.15‰, by increasing the reaction temperature to 50°C for 12 h or to 72°C for 1 h. Our new phosphoric acid fractionation factor for natural cerussite at 25°C deviates significantly from a previously published value on synthetic material. The temperature dependence of the oxygen isotope factionation factor, α between acid-liberated CO₂ and carbonate at 25 to 72°C is given by the following equations

     

The transition from pyroxene granulite facies to garnet clinopyroxene granulite facies. Experiments in the system CaO-MgO-Al2O3-SiO2

The pressure-temperature curve for the equilibrium anorthite+2enstatite=pyrope+diopside+quartz has been determined in the system CaO-MgO-Al₂O₂-SiO₂ to be between 13.4 and 14.0 kbars at 900° C. The slope up to 1,240° C is 8.5 bar/K. The entropy change at 1,200 K is 20 kJ. These data, combined with data from the literature, lead to a geobarometer equation which, when applied to rocks from the Agto area (West Greenland), gives pressure estimates of 6–10 kbars at 800° C. The results are consistent for rocks of differing Fe/Mg ratios and are consistent with independent pressure estimates.     

Stability of pyrope-quartz in the system MgO-Al2O3-SiO2

Pyrope and quartz are stable with respect to aluminous enstatite and sillimanite at 1400 °C, 20 kb and at 1100 °C, 16 kb. The phase boundary limiting the coexistence of pyrope and quartz towards lower pressures is probably slightly curved. A slope of 15 bars/°C at 1400° and of 10 bars/°C at 1000 °C has been estimated from the experimental data. Between 1050 and 1100 °C the curve is intersected by the kyanite-sillimanite phase boundary. The calculated slope of the reaction aluminous enstatite + kyanite pyrope + quartz is negative (ca. 18–25 bars/°C). The existence of a negative slope has been demonstrated experimentally. Experimental evidence indicates that the assemblage aluminous enstatite and sillimanite is metastable with respect to sapphirine + quartz at high temperature. The invariant point involving the phases pyrope-sapphirine-aluminous enstatite-sillimanite-quartz is estimated to occur at 1125°±25 °C and 16±1 kb. A model phase diagram for the silicasaturated part of the system MgO-Al₂O₃-SiO₂ has been constructed. The position of three invariant points in this system has been estimated on the basis of presently available data.     

Sapphirine and spinel phase relationships in the system FeO-MgO-Al2O3-SiO2-TiO2-O2 in the presence of quartz and hypersthene

Sapphirine and spinel can accommodate significant ferric iron and therefore the mineral equilibria involving these phases must be sensitive to a(O₂). In this paper we examine the theoretical phase relationships involving sapphirine and spinel in addition to sillimanite, garnet, cordierite, rutile, hematite-ilmenite solid solution (henceforth ilmenite), and magnetite-ulvospinel solid solution (henceforth magnetite), in the presence of quartz and hypersthene in the system FeO-MgO-Al₂O₃-SiO₂-TiO₂-O₂ (FMASTO), with particular reference to the topological inversion in P-T postulated by Hensen (Hensen 1986). Documented natural associations suggest that the appropriate topology for assemblages involving magnetite and ilmenite is Hensen's higher a(O₂) one, while, in contrast, the topology for assemblages involving ilmenite and rutile is the lower a(O₂) one. The exact configuration of the inversion between these two topologies remains uncertain because of uncertainties in the ferric/ferrous iron partitioning between sapphirine and spinel-cordierite at high temperatures. By comparison with experimental data and natural occurences, the sillimanite-sapphirine-cordierite-garnet-hypersthene-quartz assemblage is in equilibrium at about 1000°–1020° C and 7–8 kbars, while sapphirine-cordierite-spinel-garnet-hypersthene-quartz occurs at temperatures in excess of those attainable during crustal metamorphism, for ilmenite-rutile buffered assemblages. This implies that sapphirine-rutil-ehypersthene-quartz assemblages, as found in the Napier Complex, Antarctica, can only occur at > 1000° C. Also, spinel-rutile-hypersthene-quartz assemblages should not be found in rocks because temperatures in excess of 1100° C are expected to be involved in their formation. The temperatures of formation of spinel-sillimanite-sapphirine-garnethypersthene-quartz, sapphirine-spinel-cordierite-sillimanite-hypersthene-quartz, and sillimanite-spinel-cordieritegarnet-hypersthene-quartz in assemblages buffered by magnetite and ilmenite are less well constrained, but are likely to be in the range 900°–1000° C. These conclusions apply to rocks with compositions close to FMASTO; the perturbing effects of substantial concentrations of additional components, in particular Ca, mainly in garnet, and Zn and Cr, mainly in spinel, may invalidate these conclusions.     

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.

---

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.

---

---

With 4 Figures     

The stability of merwinite in the system CaO-MgO-SiO2-H2O-CO2 with CO2-poor fluids

The stability of merwinite (Mw) and its equivalent assemblages, akermanite (Ak)+calcite (Cc), diopside (Di)+calcite, and wollastonite (Wo)+monticellite (Mc)+calcite was determined at T=500–900° C and P _f=0.5–2.0 kbar under H₂O–CO₂ fluid conditions with X _{CO 2}=0.5, 0.1, 0.05, and 0.02. Merwinite is stable at P _f=0.5 kbar with T>700° C and X _{CO 2}<0.1. At P _f=2.0 kbar, the assemblage Di+Cc replaces merwinite at all T and X _{CO 2} conditions. At intermediate P _f=1 kbar, the assemblage Ak+Cc is stable above 707° C and Wo+Mc+Cc is stable below 707° C. The univariant curve for the reaction Di+Cc=Wo+Mc+CO₂ is almost parallel to the T axis and shifts to low P _f with increasing X _{CO 2}, with the assemblage Di+Cc on the high-P _f side. The implications of the experimental results in regard to contact metamorphism of limestone are discussed using the aureole at Crestmore, California as an example.     

Experimental determination of the stability and stoichiometry of sulphide complexes of silver(I) in hydrothermal solutions to 400°C

The solubility of silver sulphide (acanthite/argentite) has been measured in aqueous sulphide solutions between 25 and 400°C at saturated water vapour pressure and 500 bar to determine the stability and stoichiometry of sulphide complexes of silver(I) in hydrothermal solutions. The experiments were carried out in a flow-through autoclave, connected to a high-performance liquid chromatographic pump, titanium sampling loop, and a back-pressure regulator on line. Samples for silver determination were collected via the titanium sampling loop at experimental temperatures and pressures. The solubilities, measured as total dissolved silver, were in the range 1.0 × 10⁻⁷ to 1.30 × 10⁻⁴ mol kg⁻¹ (0.01 to 14.0 ppm), in solutions of total reduced sulphur between 0.007 and 0.176 mol kg⁻¹ and pH_T,p of 3.7 to 12.7. A nonlinear least squares treatment of the data demonstrates that the solubility of silver sulphide in aqueous sulphide solutions of acidic to alkaline pH is accurately described by the reactions0.5Ag₂S(s) + 0.5H₂S(aq) = AgHS(aq) K_s,1110.5Ag₂S(s) + 0.5H₂S(aq) + HS⁻ = Ag(HS)2− K_s,122Ag₂S(s) + 2HS⁻ = Ag₂S(HS)22− K_s,232where AgHS(aq) is the dominant species in acidic solutions, Ag(HS)2− under neutral pH conditions and Ag₂S(HS)22− in alkaline solutions. With increasing temperature the stability field of Ag(HS)2− increases and shifts to more alkaline pH in accordance with the change in the first ionisation constant of H₂S(aq). Consequently, Ag₂S(HS)22− is not an important species above 200°C. The solubility constant for the first reaction is independent of temperature to 300°C, with values in the range logK_s,111 = −5.79 (±0.07) to −5.59 (±0.09), and decreases to −5.92 (±0.16) at 400°C. The solubility constant for the second reaction increases almost linearly with inverse temperature from logK_s,122 = −3.97 (±0.04) at 25°C to −1.89 (±0.03) at 400°C. The solubility constant for the third reaction increases with temperature from logK_s,232 = −4.78 (±0.04) at 25°C to −4.57 (±0.18) at 200°C. All solubility constants were found to be independent of pressure within experimental uncertainties. The interaction between Ag⁺ and HS⁻ at 25°C and 1 bar to form AgHS(aq) has appreciable covalent character, as reflected in the exothermic enthalpy and small entropy of formation. With increasing temperature, the stepwise formation reactions become progressively more endothermic and are accompanied by large positive entropies, indicating greater electrostatic interaction. The aqueous speciation of silver is very sensitive to fluid composition and temperature. Below 100°C silver(I) sulphide complexes predominate in reduced sulphide solutions, whereas Ag⁺ and AgClOH⁻ are the dominant species in oxidised waters. In high-temperature hydrothermal solutions of seawater salinity, chloride complexes of silver(I) are most important, whereas in dilute hydrothermal fluids of meteoric origin typically found in active geothermal systems, sulphide complexes predominate. Adiabatic boiling of dilute and saline geothermal waters leads to precipitation of silver sulphide and removal of silver from solution. Conductive cooling has insignificant effects on silver mobility in dilute fluids, whereas it leads to quantitative loss of silver for geothermal fluids of seawater salinity.     

Melting and subsolidus reactions in the system K2O-CaO-Al2O3-SiO2-H2O

Beginning of melting and subsolidus relationships in the system K₂O-CaO-Al₂O₃-SiO₂-H₂O have been experimentally investigated at pressures up to 20 kbars. The equilibria discussed involve the phases anorthite, sanidine, zoisite, muscovite, quartz, kyanite, gas, and melt and two invariant points: Point [Ky] with the phases An, Or, Zo, Ms, Qz, Vapor, and Melt; point [Or] with An, Zo, Ms, Ky, Qz, Vapor, and Melt.The invariant point [Ky] at 675° C and 8.7 kbars marks the lowest solidus temperature of the system investigated. At pressures above this point the hydrated phases zoisite and muscovite are liquidus phases and the solidus temperatures increase with increasing pressure. At 20 kbars beginning of melting occurs at 740 °C. The solidus temperatures of the quinary system K₂O-CaO-Al₂O₃-SiO₂-H₂O are almost 60° C (at 20 kbars) and 170° C (at 2kbars) below those of the limiting quaternary system CaO-Al₂O₃-SiO₂-H₂O.The maximum water pressure at which anorthite is stable is lowered from 14 to 8.7 kbars in the presence of sanidine. The stability limits of anorthite+ vapor and anorthite+sanidine+vapor at temperatures below 700° C are almost parallel and do not intersect. In the wide temperature — pressure range at pressures above the reaction An+Or+Vapor = Zo+Ms+Qz and temperatures below the melting curve of Zo+Ms+Ky+Qz+Vapor, the feldspar assemblage anorthite+sanidine is replaced by the hydrated phases zoisite and muscovite plus quartz. CaO-Al₂O₃-SiO₂-H₂O. Knowledge of the melting relationships involving the minerals zoisite and muscovite contributes to our understanding of the melting processes occuring in the deeper parts of the crust. Beginning of melting in granites and granodiorites depends on the composition of plagioclase. The solidus temperatures of all granites and granodiorites containing plagioclases of intermediate composition are higher than those of the Ca-free alkali feldspar granite system and below those of the Na-free system discussed in this paper.The investigated system also provides information about the width of the P-T field in which zoisite can be stable together with an Al₂SiO₅ polymorph plus quartz and in which zoisite plus muscovite and quartz can be formed at the expense of anorthite and potassium feldspar. Addition of sodium will shift the boundaries of these fields to higher pressures (at given temperatures), because the pressure stability of albite is almost 10kbars above that of anorthite. Assemblages with zoisite+muscovite or zoisite+kyanite are often considered to be products of secondary or retrograde reactions. The P-T range in which hydration of granitic compositions may occur in nature is of special interest. The present paper documents the highest temperatures at which this hydration can occur in the earth's crust.     

Synthesis of kosmochlor and phase equilibria in the join CaMgSi2O6-NaCrSi2O6

Kosmochlor (NaCrSi₂O₆) was synthesized by the flux method from melts along the join Na₂O·2 SiO₂-Na₂O·Cr₂O₃·4 SiO₂ at 1000° C in air, and isolated by dissolving the glassy matrix with hydrofluoric and perchloric acids. The join CaMgSi₂O₆-NaCrSi₂O₆ was studied at 1 atmosphere in air by the quenching technique at temperatures between 900° and 1450° C, using mixtures of kosmochlor and diopside crystals or diopside glass as starting materials. The phases are diopside solid solution, kosmochlor, spinel (Mg-chromite), eskolaite (Cr₂O₃) and glass. The maximum solubility of kosmochlor in diopside is 24 wt percent at 1140° C, while diopside is not soluble at all in kosmochlor, resulting in the existence of a wide range of immiscibility. Petrologic significance of the results is discussed.     

Laboratory growth of sector zoned clinopyroxenes in the system CaMgSi2O6-CaTiAl2O6

Sector zoning has been experimentally reproduced in CaMgSi₂O₆-CaTiAl₂O₆ clinopyroxene crystals by isothermal crystallization using seed crystals. Element partitioning in different growth sectors and between the core and rim portions in single crystals was analysed in relation to growth rate R and degree of supercooling T. The TiO₂ and Al₂O₃ contents increase with increase in R and T, but when they are compared between different sectors in a single crystal grown at the same T, they correlate negatively with R. The order of faces in respect of contents of TiO₂ and Al₂O₃ is (100)>(110)(010)(111) at T= 13° C and 18° C but changes to (110)>(100)>(010)>(111) at T= 25° C. The growth mechanism is concluded to be controlled by interface kinetics at T= 13–25° C for all these faces, while at T=45° C this relation holds for (100) and (010) faces, but not for (110) and (111), based on the growth rate versus supercooling relation and surface microtopographic observations. The interface kinetics play the essential role in the formation of sector zoning, when the layer growth mechanism takes place.     

Experimental study of the stability of aluminate-borate complexes in hydrothermal solutions

Formation of aqueous aluminate-borate complexes was characterized at 25°C using ²⁷Al NMR spectroscopy, and at 50-200°C via measurements of gibbsite and boehmite solubility in the presence of boric acid. ²⁷Al spectra performed at pH = 9 in Al-B solution with m(B) = 0.02 show the presence of two peaks at 80.5 and 74.5 ppm which correspond to Al(OH)₄⁻ and a single Al-substituted Q¹_Al dimer, Al(OH)₃OB(OH)₂⁻, respectively. In 0.08 m and 0.2 m borate solution, a third peak appears at 68.5 ppm which can be assigned to the Q²_Al trimer Al(OH)₂O₂(B(OH)₂)₂⁻. These chemical shifts are close to those measured for Al(OH)₃OSi(OH)₃⁻ and Al(OH)₂O₂(Si(OH)₃)₂⁻ (74 and 69.5 ppm, respectively; Pokrovski et al., Min. Mag.62a (1998), 1194) which demonstrates the similar structure of Al-B and Al-Si complexes formed in alkaline solutions. Gibbsite and boehmite solubility were measured in weakly basic solutions as a function of boric acid concentration at 50°C and 78 to 200°C, respectively. Equilibrium was reached within several days at m(B) = 0.01-0.1, but more slowly at higher boron concentrations, and at 50°C and m(B) = 0.2, Al concentration increased continuously during at least 3 months as a result of the sluggish formation of Al-polyborates. The equilibrium constant of the reaction Al(OH)₄⁻ + B(OH)₃⁰_(aq) = Al(OH)₃OB(OH)₂⁻ + H₂O decreases very slowly with increasing temperature to 200°C. The log K values are 1.58 ± 0.10, 1.46 ± 0.10, 1.52 ± 0.15, and 1.25 ± 0.15 at 50, 78, 150 and 200°C, respectively, which result in the following values of the standard thermodynamic properties for this reaction: Δ_rG⁰ = −9.22 ± 3.25 kJ/mol, Δ_rH⁰ = −4.6 ± 2.5 kJ/mol, Δ_rS⁰ = 15.5 ± 6.9 J/mol K. The thermodynamic data generated in this study indicate that Al-B complexes can dominate aqueous aluminum speciation in solutions containing ≥0.7 g/L of boron at temperature to at least 400°C.     

Stable Isotope Fractionation during Experimental Formation of Norsethite (BaMg[CO3]2): A Mineral Analogue of Dolomite

Stable oxygen and carbon isotopefractionation during the experimental formation ofordered norsethite (BaMg[CO₃]₂) from thereaction of anhydrous BaCO₃ (witherite) withrelatively low concentrated sodium-magnesiumbicarbonate solutions has been studied between20° and 135 °C. In the investigatedtemperature range, ¹⁸O and ¹³C are enrichedin norsethite with respect to water and gaseous carbondioxide, respectively. Whereas ¹⁸O/¹⁶Opartitioning is intermediate between those of theBaCO₃–H₂O and MgCO₃–H₂O systems,¹³C/¹²C partitioning is more similar to thatfor BaCO₃–CO₂. Between 20° and90°C, the temperature dependences of the¹⁸O/¹⁶O and ¹³C/¹²C fractionationfactors are represented by the equations (T in °K):10³ ln _BaMg[CO₃]₂-H_2O = 2.83 10⁶T^--2.85, and 10³ln_BaMg[CO₃]₂-CO₂(gas) = 1.78 10⁶T^--10.16. The later equation considers carbon isotope fractionationbetween the dissolved carbonate ion and carbon dioxide measured by Halaset al. (1997). Under standard state conditions (25 °C) the fractionation factors in the system BaMg[CO₃]₂-CO₂-H₂O are: Oxygen isotopes: _BaMg(CO₃)₂-H_2O = 1.02941, _BaMg(CO₃)₂-OH-(aq) = 1.07059,_BaMg(CO₃)₂-CO₂(gas) = 0.98868, and_BaMg(CO₃)₂-H₂CO₃ ^* = 0.98843; carbon isotopes:_BaMg(CO₃)₂-CO₂(gas) = 1.00992,_BaMg(CO₃)₂-H₂CO₃ ^* = 1.01099,_BaMg(CO₃)₂-HCO₃ ^- = 1.00194,_BaMg(CO₃)₂-CO₃ ^2- = 1.00491 or 1.00150.The spontaneous precipitation of aBaMg[CO₃]₂ gel at 20 °C,followed by the alteration of the products at20° or 60°C for 31 days,demonstrated isotope exchange reactions betweensolids and mother solutions dueto recrystallization. Isotope equilibrium, wasnot reached within run time.     

Empirical prediction of the Pitzer’s interaction parameters for cationic Al species with both SiO2(aq) and CO2(aq): Implications for the geochemical modelling of very saline solutions

The Pitzer’s interaction parameters, λ_N–M, involving the Mth cationic Al species Al³⁺ or AlOH²⁺ or AlO⁺ and the Nth neutral species SiO_2(aq) (at temperatures of 25–300 °C) or CO_2(aq) (at temperatures of 25–150 °C), have been evaluated through empirical linear relationships between λ_N–M and the surface electrostatic field of the ionic species of interest. These relationships have been obtained starting from the known λ_N–M for both SiO_2(aq) and CO_2(aq) with the main dissolved cations. The Pitzer’s interaction parameter thus estimated for the pair CO_2(aq)–Al³⁺ at 25 °C, 0.327, is 20–40% higher than the corresponding values obtained from CO₂ solubilities in concentrated solutions of AlCl₃, 0.272 ± 0.010 (2σ), and Al₂(SO₄)₃, 0.232 ± 0.002 (2σ), partly corroborating the empirical approach adopted in this study. To test the Pitzer’s interaction coefficients for cationic Al species with aqueous SiO₂, the log K values of the kaolinite dissolution reaction have been computed starting from available experimental data at 23–25 °C and ionic strengths of 0.0001–0.12 mol/kg adopting, alternatively, the Pitzer’s equations and the Debye–Hückel equation. A satisfactory agreement has been found between the log K values obtained through these two approaches, with maximum deviations of 0.11–0.12 log units. This good convergence of results is encouraging as it represents a necessary condition to prove the reliability of the Pitzer’s interaction coefficients estimated in this work. These results are a first step to take into account specific interactions among solutes in concentrated electrolyte solutions, such as those hosted in sedimentary basins or geothermal waters, for instance through the Pitzer’s equations. However, experimental or field data at higher ionic strengths are absolutely necessary to validate the reliability of the Pitzer’s interaction coefficients determined in this study.     

Experimental determination of the spinel peridotite to garnet peridotite reaction in the system MgO-Al2O3-SiO2 in the range 900 °–1100 °C and Al2O3 isopleths of enstatite in the spinel field

The univariant high-pressure reaction of aluminous enstatite and spinel to pyrope and forsterite in the MgO-Al₂O₃-SiO₂ system has been determined in the temperature range 900 °–1100 °C by hydrothermal reversals in the piston-cylinder apparatus using the low-friction NaCl pressure medium. A mixture of synthetic minerals, including an enstatite with 6 wt% Al₂O₃, with product and reactant assemblages in nearly equal amounts, was the starting material. The equilibrium pressure of 19.3±0.3 kbar at 1000 ° C and average dP/dT slope of 8.0 bars/ ° C confirm the strong curvature of the equilibrium below 1200 ° C deduced by Obata (1976) from a theoretical study of experimental Al₂O₃ isopleths of enstatite in the garnet field. His prediction of an absolute minimum pressure near 18 kbar of the garnet peridotite assemblage in the ternary system is undoubtedly correct.Three reversed determinations of the equilibrium Al₂O₃ content of enstatite in the presence of spinel +forsterite were made at points adjacent to the univariant curve. The points are 5.5 wt% Al₂O₃ at 950 ° C and 20 kbar, 6.2 wt% at 1000 ° C and 20 kbar and 7.2 wt% at 1080 ° C and 20 kbar. These values are somewhat higher than given by the MacGregor (1974) isopleth set and quite close to those predicted by Fujii (1976) from experimental synthesis data at higher temperatures, using the Wood and Banno (1973) model of ideal solution of the Mg₂Si₂O₆ and MgAl₂SiO₆ components in enstatite to reduce the data.All of the available spinel-field isopleth data can be systematized with the use of the ideal solution model. A value of H ⁰ of 9000 cal fits the reduced data well, and is in agreement with the calorimetrically determined value of 8500±1900 calories. An accurate calculation of the dP/dT slope of the univariant equilibrium at 1000 ° C based on calorimetry gives 7±2bars/ °C, also in good agreement with experiment. Thus, all of the available experimental and calorimetric data are consistent with the ideal-solution aluminous enstatite model.The dP/dT slopes of the spinel-field isopleths are too large to permit their use as an accurate geobarometric scale. They do have considerable potential as a thermometric indicator for certain natural peridotites, however. The southwestern Oregon overthrust peridotite masses of Cretaceous age have enstatite of 5.6 wt% Al₂O₃ with spinel of nearly 80 mole% MgAl₂O₄. The present reduced isopleth data directly give 930 ° C for the equilibration, assuming 12 kbar pressure. A first order correction based on ideal solution departures from the ternary system, as suggested by Stroh (1976) gives 1000 ° C. Thus, the high temperatures deduced by Medaris (1972) are confirmed. The pressure cannot be deduced independently from the pyroxene Al₂O₃ contents.     

The Blosseville Coast basalts of East Greenland: Their occurrence,composition and temporal variations

In the system CaO-MgO-A1₂O₃-SiO₂ the tie lines connecting anorthite with other phases are sequentially broken down with increasing pressure according to the following univariant reactions: anorthite+ enstatite_ss+sillimanite pyrope-grossular_ss+quartz (3), anorthite+enstatite_ss pyrope-grossular_ss+diopside_ss+quartz (2), anorthite+pyrope-grossular_ss+ quartz diopside_ss+kyanite (4) and anorthite+diopside_ss grossular-pyrope_ss +kyanite+quartz (8). At 1,200 ° C these reactions occur at 14.5± 0.5, 15.5±0.5, 19.5±0.5 and 26.4±1 kilobar and have positive slopes (dP/dT) of 1±0.5, 2.8±0.5, 13.3±0.5 and 24±2bars/°C respectively. An invariant point involving kyanite rather than sillimanite, occurs at 850 °C±25 °C and 14.5±0.5kbar at the intersection of reactions (3), (2) and (4). Reaction(4) exhibits significant curvature with an increase in dP/dT from 13.3±0.5 to 18.5± 0.5 bars/°C between 1,050° and 850° C. The pressure at which the complete grossular-pyrope join is stable with quartz is estimated at 41 ± 1 kbar at 1,200 ° C. The pressure at which garnet appears according to reaction (2) is lowered by 5 kbar for a composition with anorthite and orthopyroxene (En_0.5Fs_0.5). Enstatite and plagioclase (An_0.5Ab_0.5) first produce garnet at 2 kbar higher pressure than enstatite and pure anorthite (reaction (2)). The calcium content of garnet in various divariant assemblages is relatively insensitive to temperature but very sensitive to pressure, it is therefore a useful geobarometer. At metamorphic temperatures of 700–850 °C pressures of 8–10 kbar are required for the formation of quartz-bearing garnet granulites containing calcic plagioclase and with (Mg/Mg+Fe) bulk = 0.5.     

A spectrophotometric study of neodymium(III) complexation in chloride solutions

The formation constants of neodymium complexes in chloride solutions have been determined spectrophotometrically at temperatures of 25 to 250°C and a pressure of 50 bars. The simple ion, Nd³⁺, is dominant at 25°C, whereas NdCl²⁺ and NdCl₂⁺ are the dominant species at elevated temperatures. Equilibrium constants were calculated for the following reactions:Nd³⁺ + Cl⁻ = NdCl²⁺ β₁,Nd³⁺ + 2 · Cl⁻ = NdCl⁺₂ β₂.The values of β₁ were found to be identical within experimental error to the values reported by Gammons et al. (1996) but substantially different from those proposed by Stepanchikova and Kolonin (1999). The values of β₂ obtained in this study agree relatively well with those of Gammons et al. (1996); differences are greatest at intermediate temperature and reach a maximum of one half an order of magnitude at 200°C.Theoretical estimates of β₁ and β₂ by Haas et al. (1995) using the revised Helgeson-Kirkham-Flowers (HKF) equation of state predict lower stability of NdCl²⁺ and NdCl₂⁺ at temperatures above 150°C than determined in this study. A new fit to the HKF equation of state is therefore proposed, which yields values for β₁ and β₂ similar to those obtained experimentally.Using the formation constants reported in this study, we predict that typical seafloor hydrothermal vent fluids will contain a maximum concentration of Nd of ∼2 ppb. This value is several orders of magnitude lower than would be required to explain the levels of Nd mobility commonly reported for seafloor hydrothermal systems and suggests that other ligands may be more important than Cl in transporting rare earth elements in the Earth’s crust.     

Transformation of synthetic U3O8 into different uranium oxide hydrates

Summary Referring to the natural formation of secondary uranium minerals, the primary transformation of U₃O₈ into schoepite has been investigated. The transformation is realized in a continuous system with O₂, CO₂ and H₂O. At 100°C schoepite III, UO₃ · zH₂O (z 1), is formed (a = 14.12; b = 16.83; c = 15.22 Å) with a density of 4.460 g/cm³. At 25°C a mixture of schoepite II (UO₃ · yH₂O, 1 < y < 2; a = 13.99; b = 16.72; c = 14.73 Å) and schoepite I (UO₃ · xH₂O, x 2; a = 14.33; b = 16.79; c = 14.73 Å) is obatined. From thermogravimetric analysis the activation energy of dehydration for schoepite III is determined as 49(3) · 10³ J/mole.

---

Umwandlung von synthetischem U₃O₈ in verschiedene Uranoxidhydrate

Zusammenfassung In Hinblick auf die natürliche Bildung sekundärer Uranminerale wurde die primäre Umwandlung von U₃O₈ in Schoepit untersucht. Die Umwandlung wurde in einem kontinuierlichen System mit O₂, CO₂ und H₂O bewerkstelligt. Bei 100°C bildet sich Schoepit III (UO₃ · zH₂O, z 1; a = 14.12, b = 16.83, c = 15.22 Å; Dichte: 4.460 g/cm³). Bei 25°C wird eine Mischung von Schoepit II (UO₃ · yH₂O, 1 < y < 2; a = 13.99, b = 16.72, c = 14.73 Å) und Schoepit I (UO₃ · xH₂O, x 2; a = 14.33, b = 16.79, c = 14.73 Å) erhalten. Aus der thermogravimetrischen Analyse wurde die Aktivierungsenergie der Dehydratation von Schoepit III mit 49(3) · 10³ J/mole berechnet.

---

---

Who wishes to dedicate the paper to the memory of his father, Hendrik Vochten.

---

With 3 Figures     

Relationships between silicate melts and carbonate-precipitating melts in CaO-MgO-SiO2-CO2-H2O at 2 kbar

Summary Phase fields intersected by three joins in the System CaO-MgO-SiO₂-CO₂-H₂O at 2 kbar were investigated experimentally to determine the melting relationships and the sequences of crystallization of liquids co-precipitating silicate minerals and carbonates. These joins connect SiO₂ to three mixtures of CaCO₃-MgCO₃-Mg(OH)₂ with compositions on the primary îield for calcite, between the composition CaCO₃ and the low-temperature (650°C eutectic liquid co-precipitating calcite, dolomite and periclase. In the pseudo-quaternary tetrahedron calcite-magnesite-brucite-diopside, two of the significant reactions found are: (1) a eutectic at 650°C, calcite + dolomite + periclase + forsterite + vapor = liquid, and (2) a peritectic at 1038°Cwhich is either calcite + åkermanite + forsterite + vapor = monticellite + liquid calcite + monticellite + forsterite + vapor = åkermanite + liquid. The eutectic liquid has high MgO/CaO and CO₂/H₂O and only 2–3% SiO₂ (estimated 15–20% MgCO₃, 35–40% CaCO₃, 40–45% Mg(OH)₂, and 5–6% Mg₂SiO₄). The composition joins intersect a thermal maximum for åkermanite + forsterite + vapor = liquid, which separates high-temperature liquids precipitating silicates together with a little calcite, from low-temperature liquids precipitating carbonates with a few percent of forsterite; there is no direct path between the silicate and synthetic carbonatite liquids on these joins, but it is possible that fractionating liquid paths diverging from the joins may connect them. More complex relationships involving the pprecipitatioon off monticellite and åkermanite are also outlined. Magnetite-magnesioferrite may replace periclase in natural magmatic systems. The results indicate that the assemblage calcite-dolomite-magnetite-forsterite represents the closing stages of crystallization of carbonatites, whereas assemblages such as calcite-magnetite-forsterite and dolomite-magnetite-forsterite span the whole range of carbonatite evolution in terms of temperature and composition, and provide the link between liquids precipitating silicates and those precipitating carbonates.

---

Die Beziehungen zwischen silikarischen Schmelzen und karbonatbildenden Schmelzen im System CaO-MgO-SiO₂-CO₂-H₂O bei 2 kbar

Zusammenfassung Phasenfelder, die durch den Schnitt von drei Verbindungslinien im System CaO-MgO-SiO₂-CO₂-H₂Odefiniert werden, wurden experimentell bei 2 kbar untersucht, um die Schmelzbeziehungen und die Kristallisationsfolge von Schmelzen, die gleichzeitig silikatische und karbonatische Minerale ausscheiden, zu bestimmen. Diese Linien verbinden SiO₂ mit drei Mischungen von CaCO₃-M9CO₃-Mg(OH)₂ mit Zusammensetzungen im primären Calcitfeld, zwischen der Zusammensetzung CaCO₃ und der tieftemperierten (650°C Calcit-, Dolomit- und Periklasbildenden eutektischen Schmelze. Zwei wichtige im ppseudo-quaternären Tetraeder Calcit-Magnetit-Brucit-Diopsid gefundene Reaktionen sind: (1) Ein Eutektikum bei 650°C Calcit + Dolomit + Periklas + Forsterit + Vapor = Liquid und (2) ein Peritektikum bei 1038°C mit entweder Calcit + Åkermanit + Forsterit + Vapor = Monticellit + Liquid oder Calcit + Monticellit + Forsterit + Vapo = Åkermanit + Liquid Die eutektische Schmelze zeigt hohe MgO/CaO und CCO₂H₂O Verhältnisse und nur 2–3% SiO₂(geschätzter Anteil an MgCO₃15–20%, CaCO₃ 35–40%, Mg(OH)₂ 40–50% und Mg₂SiO₄ 5–6%). Die Verbindungslinie schneidet ein thermisches Maximum von Åkermanit + Forsterit + Vapor = Liquid, das höher temperierte Schmelzen, die Silikate gemeinsam mit etwas Clacit ausscheiden, von tiefer temperierten Schmelzen trennt, aus denen sich Karbonate gemeinsam mit wenigen Prozenten Forsterit abscheiden. Es existiert keine direkte Verbindung zwischen silikatischen und synthetischen karbonatitischen Schmelzen entlang dieser Verbindungslinien, es wäre aber möglich, daß Fraktionierungspfade, die von diesen Verbindungslinien ausgehen, sie verbinden. Komplexere Beziehungen, die die Kristallisation von Monticellit und Åkermanit beinhalten, werden ebenfalls aufgezeigt. Magnetit-Magnesioferrit könntean die Stelle von Periklas in nnatürlichenmagmatischen Systemen treten. Die Ergebnisse weisen darauf bin, daß die Vergesellschaftung Calcit-Dolomit-Magnetit-Forsterit das Endstadium der Karbonatitkristallisation repräsentiert, während die Vergesellsschaftungen von Calcit-Magnetit-Forsterit bzw. Dolomit-Magnetit-Forsterit die gesamte Spannweite der Karbonatitevolution hinsichtlich Temperatur und Zusammensetzung umfassen und demnach ein Verbindungsglied zwischen silikat- und karbonatausscheidenden Schmelzen darstellen.

---

---

With 8 Figures     

Experimental studies to 10 kb of the bulk composition tremolite50-tschermakite50+excess H2O

Phase relations for the magnesio-hornblende bulk composition, 2 CaO·4 MgO·Al₂O₃·7 SiO₂+ excess H₂O, have been investigated to 10 kb employing hydrothermal and piston-cylinder techniques. The low-temperature limit of amphibole in this system lies at 519° C, 1,000 bars, 541° C, 2,000 bars, and 718° C, 10 kb. The low-T assemblage consists of an+chl+di+tc(+f), and is related to the adjacent high-T equilibrium assemblage, amph+an+chl+f, by the solid-solid reaction (A): 2 di+tc=tr. Small amounts of aluminum, hypothesized to be preferentially dissolved in the cpx (and in the tc) relative to amph, may account for the broad P-T stability range of the di+tc assemblage in the synthetic work relative to systems involving stoichiometric tr, Ca₂Mg₅Si₈O₂₂(OH)₂, such as are common in natural, Al-poor calc-silicate parageneses. Alternatively, the low-temperature assemblage produced in the experiments may be metastable. For the investigated bulk composition, synthetic tremolitic-cummingtonitic amphibole contains relatively modest amounts of ts, Ca₂Mg₃Al₂ ^IVSi₆-Al₂ ^IVO₂₂(OH)₂; at pressures of 1,000–3,000 bars, solid solution extends from near tremolite only to about cu₁₁tr₆₉ts₂₀, analogous to most analyzed natural magnesio-hornblendic specimens. At 10 kb fluid pressure, the solid solution reaches approximately cu₀₆tr₅₃ts₄₁ for the investigated bulk composition, and appears to be virtually independent of temperature. Amphibole and 14 Å chl react within the amphibole stability field, along curve (B), at about 704° C and 2,000 bars, to produce an, en, fo and f (H=40.9 kcal/ mole); at pressures greater than approximately 7kb, due to the incompatibility of an and fo, the higher temperature assemblage consists of amph, an, en, sp and f. Above P _fluid– T curve (B), the amphibole coexists with an+en+fo+f at low pressures; at higher pressures, the amphibole, which is in equilibrium with an+en+sp+f, is relatively more aluminous. The high-T stability limit of aluminous tr+fo lies approximately 20–25° C below the dehydration curve for stoichiometric tremolite on its own bulk composition. Reaction (C), tr+fo=2 di+5 en+f (H = 39.4 kcal/mole), produces an+di+en+f, the highest temperature subsolidus assemblage investigated for the tr₅₀ts₅₀ bulk composition. Hydrous melt is encountered at temperatures at least as low as 900° C at 10 kb, and at that fluid pressure coexists with amphibole over an interval of more than 60° C. Limited solid solution observed between tr and ts in nature (tr_100-70) is accounted for by the restricted range of amphibole compositions produced in the present study. Such amphiboles, moreover, appear to have both high- and low-temperature stability limits, as demonstrated by the experimental results.Institute of Geophysics and Planetary Physics Publication No. 2811