Function Of Melt Composition Research Articles

Sulfur oxidation state and solubility in silicate melts

We have determined the solubility of sulfur (S) as sulfide (S2–) for 13 different natural melt compositions at temperatures of 1473–1773 K under controlled conditions of oxygen and sulfur fugacities (fO2 and fS2, respectively). The S and major element contents of the quenched glasses were determined by electron microprobe. The sulfide capacity parameter (CS2–) was used to express S2– solubility as a function of the oxygen and sulfur fugacities according to the equation: log C_{{S^{2 - } }} = log S_{melt} left( {wt% } right) + 0.5log left( {frac{{fO_{2} }}{{fS_{2} }}} right). Sulfide capacities of silicate melts were found to increase with temperature and the FeO content of the melt. We combined our sulfide data at 1473–1773 K with (O’Neill and Mavrogenes, J Petrol 43:1049–1087, 2002) results at 1673 K, and obtained by stepwise linear regression the following equation for sulfide capacity log C_{{S^{2 - } }} = 0.225 + left( {25237X_{FeO} + 5214X_{CaO} + 12705X_{MnO} + 19829X_{{K_{2} O}} - 1109X_{{Si_{0.5} O}} - 8879} right)/T{ }. XMO is the mole fraction of the oxide of M on a single-oxygen basis, and T is in Kelvin. The sulfide capacity equation was combined with sulfate capacity (CS6+) data for similar compositions and at the same temperatures (Boulliung and Wood, Geochim Cosmochim Acta 336:150–164, 2022), to estimate the S redox state (S6+/S2– ratio) as a function of melt composition, temperature and oxygen fugacity. Results obtained are in good agreement with earlier measurements of S6+/S2– for basaltic and andesitic compositions. We observe a significant increase, however, relative to FMQ of the oxygen fugacity of the S2– to S6+ transition as temperature is lowered from 1773 to 1473 K. We used our results to simulate sulfur-degassing paths for basaltic compositions under various redox conditions (FMQ –2 log fO2 units to FMQ + 2). The calculations indicate that, given an initial concentration of 0.12 wt% S in an ascending melt at 250 MPa, most of the S (> 80%) will be degassed before the magma reaches 100 MPa pressure.

Partitioning of OH-F-Cl between biotite and silicate melt: Experiments and an empirical model

The partitioning of hydroxyl (OH), fluorine (F) and chlorine (Cl) between biotite and silicate melt has been experimentally investigated at 100–500 MPa and 850–975 °C. A variety of starting glasses was used, covering alkali-rich basalt, trachyandesite, trachyte, tephriphonolite and dacite, in order to investigate potential compositional effects of both melt and biotite on the partitioning behaviors. The investigated F contents in biotite and silicate melt are ranging within 0.2–8.2 wt% and 0.1–4.8 wt% respectively, yielding a Nernst-type partition coefficient DFBt-melt within 1.5–10. The investigated Cl contents in both biotite and silicate melt are much lower (≤0.15 wt% and ≤ 1.3 wt% respectively), and DClBt-melt values are < 1 (0.02–0.4). The results are used to estimate the exchange coefficients (ratio of D values) for F-OH and Cl-OH pairs expressed as KdF/OHBt-melt=XFBt/XOHBt/XFmelt/XOHmelt, and KdCl/OHBt-melt=XClBt/XOHBt/XClmelt/XOHmelt, in which XFBt, XClBt and XOHBt are mole fractions of F, Cl and OH in biotite, and XFmelt, XClmelt and XOHmelt are mole fractions of F, Cl and OH in silicate melt, respectively. KdF/OHBt-melt is in the range of 1–50, which is higher than KdCl/OHBt-melt (0.07–5) by about one order of magnitude, implying that the relative incorporation preference of F, OH and Cl between biotite and melt is highest for F, lower for OH and lowest for Cl. Biotite and melt compositions exert important effects on the OH-F-Cl partitioning between biotite and melt. Multiple linear regression of data from literature and this study allows us to propose the following empirical equations:lnKdF/OHBt-melt=0.995-1.619∗DASI+3.613∗XMgBt-17.523∗XTiBt (R2 = 0.83, sd = 0.31),lnKdCl/OHBt-melt=3.380-2.454∗CSI-0.398∗XMgBt-22.377∗XTiBt (R2 = 0.94, sd = 0.26),in which XMgBt = Mg/(Mg + Fe3++Fe2++Mn + Ti + AlVI) of biotite, XTiBt = Ti/(Mg + Fe3++Fe2++Mn + Ti + AlVI) of biotite, DASI (deviation from ASI of 1) = |1–ASI|, with ASI = molar Al2O3/(Na2O + K2O + CaO) of the melt, and CSI (chlorine solubility index) = molar (Al1/2 + Ca1/2 + Mg1/2 + Na)/Si of the melt. The empirical models confirm the so-called Mg-F preference and Mg-Cl avoidance rules in biotite, and also reveal that increasing Ti content in biotite tends to lower KdF/OHBt-melt and KdCl/OHBt-melt. Furthermore, DASI and CSI are effective parameters that reflect the incorporation abilities of F and Cl in melt as a function of melt composition. A wide prediction potential of the OH-F-Cl exchange partition model is expected, because fairly large ranges in temperature (640–975 °C), melt composition (ASI = 0.7–1.4; CSI = 0.6–1.6) and biotite composition (XMgBt = 0–0.88) are covered. Calculated KdF/OHBt-melt and KdCl/OHBt-melt from the models are in good agreement with experimentally determined values, and the deviation from the models are less than 20% relative in both cases. Applying the empirical models, we calculated melt F and Cl concentrations based on biotite compositions from several intrusive-volcanic rocks that are associated with different deposit types. The results show that the magmas associated with Mo and REE deposits are characterized by high F (up to ca. 10,000 ppm) but relatively low Cl. On the contrary, the magmas that have contributed to the formation of porphyry Cu and Au deposits contain much higher Cl (up to ca. 10,000 ppm) than F.

The heat capacity of hydrous multicomponent natural melts and glasses

The thermophysical properties of silicate melts and glasses are of fundamental importance for the characterization of the dynamics and energetics of silicate melts on Earth and terrestrial planets. The heat capacity of silicate melts is of particular importance because of its implications for the temperature dependencies of melt enthalpy and entropy and for the potential relationship to melt structure and transport properties. Currently, there are reliable models for predicting the heat capacity of simple and multicomponent silicate glasses (Cpglass) as a function of composition and temperature. Recent differential scanning calorimetry (DSC) measurements of heat capacity for multicomponent silicate liquid (Cpliquid), however, have shown that published models do not accurately reproduce heat capacity measurements on some silicate melts. Here, we have compiled a database of heat capacity values for hydrous and anhydrous multicomponent natural samples. The measurements are on pairs of glasses and melts over the compositional range (wt%) of: SiO2 (44–79), Al2O3 (5–35), TiO2 (0–3), FeOtot (0−11); Na2O+K2O (0–27); CaO+MgO (0–39), H2O (0–6.3) and minor oxides. The compiled data show strong correlations between silica content (XSiO2) and the configurational heat capacity (Cpconfig) defined as Cpliquid−Cpglass measured across the glass transition temperature (Tg). This correlation is used to establish an empirical model for predicting Cpliquid as a function of melt composition (i.e. SiO2 content) and values of Cpglass measured at the onset of the glass transition:Cpliquid=52.6–55.88XSiO2+CpglassThe model reproduces values of Cpliquid to within an average relative error of ~2.4%. Published models for the heat capacities of silicate melts (e.g., Stebbins, 1984; Richet and Bottinga, 1985; Lange and Navrotsky, 1992) applied to the same dataset have average relative errors in excess of 5.5%.

Heat capacity of hydrous trachybasalt from Mt Etna: comparison with CaAl2Si2O8 (An)–CaMgSi2O6 (Di) as basaltic proxy compositions

The specific heat capacity (C p) of six variably hydrated (~3.5 wt% H2O) iron-bearing Etna trachybasaltic glasses and liquids has been measured using differential scanning calorimetry from room temperature across the glass transition region. These data are compared to heat capacity measurements on thirteen melt compositions in the iron-free anorthite (An)–diopside (Di) system over a similar range of H2O contents. These data extend considerably the published C p measurements for hydrous melts and glasses. The results for the Etna trachybasalts show nonlinear variations in, both, the heat capacity of the glass at the onset of the glass transition (i.e., C p g ) and the fully relaxed liquid (i.e., C p l ) with increasing H2O content. Similarly, the “configurational heat capacity” (i.e., C p c = C p l − C p g ) varies nonlinearly with H2O content. The An–Di hydrous compositions investigated show similar trends, with C p values varying as a function of melt composition and H2O content. The results show that values in hydrous C p g , C p l and C p c in the depolymerized glasses and liquids are substantially different from those observed for more polymerized hydrous albitic, leucogranitic, trachytic and phonolitic multicomponent compositions previously investigated. Polymerized melts have lower C p l and C p c and higher C p g with respect to more depolymerized compositions. The covariation between C p values and the degree of polymerization in glasses and melts is well described in terms of SMhydrous and NBO/T hydrous. Values of C p c increase sharply with increasing depolymerization up to SMhydrous ~ 30–35 mol% (NBO/T hydrous ~ 0.5) and then stabilize to an almost constant value. The partial molar heat capacity of H2O for both glasses ( $$ C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{g}} $$ ) and liquids ( $$ C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{l}} $$ ) appears to be independent of composition and, assuming ideal mixing, we obtain a value for $$ C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{l}} $$ of 79 J mol−1 K−1. However, we note that a range of values for $$ C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{l}} $$ (i.e., ~78–87 J mol−1 K−1) proposed by previous workers will reproduce the extended data to within experimental uncertainty. Our analysis suggests that more data are required in order to ascribe a compositional dependence (i.e., nonideal mixing) to $$ C_{{{\text{p}}\;{\text{H}}_{2} {\text{O}}}}^{\text{l}} $$ .

Experimental and modeled chlorine solubilities in aluminosilicate melts at 1 to 7000 bars and 700 to 1250 C: Applications to magmas of Augustine Volcano, Alaska

Hydrothermal experiments were conducted at ca. 1 to 7000 bars and 700 to 1250 °C in 121 rhyolitic to basaltic systems to determine Cl solubility in silicate melts, i.e., the maximum Cl concentration in melts that are saturated in a hydrosaline liquid with or without an aqueous or aqueous-carbonic vapor. The Cl concentration of melts increases with the Cl contents of the fluid unless the melt coexists with vapor plus hydrosaline liquid at fixed pressure and temperature; this phase assemblage buffers the Cl content of each phase with increasing Cl in the system. The Cl content of fluid(s)-saturated melts is independent of the CO2 concentration of the saline liquid ± vapor with up to 21 wt% CO2 in the fluid(s). The experiments show that Cl dissolution in aluminosilicate melts increases with temperature and pressure. Chlorine solubility is also a function of melt composition; it increases with the molar ([Al1/2+Ca1/2+Mg1/2+Na]/Si) of the melt.

Activities and volatilities of trace components in silicate melts: a novel use of metal–silicate partitioning data

Ian Carmichael spent 45 years thinking about and working on the activities of components in silicate melts and their use to estimate physicochemical conditions at eruption and in the source regions of igneous rocks. These interests, principally in major components such as SiO2, led us to think about possible ways of determining the complementary activity coefficients of trace components in silicate melts. While investigating the conditions of accretion and differentiation of the Earth, a number of authors have determined the partitioning of trace elements such as Co, Ni, Mo and W between liquid Fe metal and liquid silicate. These data have the potential to provide activity information for a large number of trace components in silicate melts. In order to turn the partitioning measurements into activities, however, we need to know the activity coefficient of FeO, γFeO in the silicate. We obtained γFeO as a function of melt composition by fitting a simple model to 83 experimental data for which the authors had measured the FeO content of the silicate melt in equilibrium with metal (Fe-bearing alloy) at known fO2. The compositional dependence of γFeO is weak, but, when calculated in the system Diopside–Anorthite–Forsterite, it decreases towards the Forsterite apex. A similar approach for Ni, for which twice as many data are available, leads to similar composition dependence of activity coefficient and confirms the suggestion that γNiO/γFeO is almost constant over a wide range of silicate melt composition. The activity coefficients for FeO were used in conjunction with measured Mo and W partitioning between Fe-rich metal and silicate melt to estimate activity coefficients for trace MoO2 and WO3 dissolved in silicate melt. When combined with data on Mo- and W-saturated silicate melts a strong dependence of activity coefficient is observed. Calculated in the system Diopside–Anorthite–Forsterite, both MoO2 and WO3 exhibit similar behaviour to FeO and NiO in that activity coefficients decrease as Forsterite content increases. The effect is much larger for Mo and W, however, γMoO2 and γWO3 varying by factors of 20 and nearly 100, respectively, in this system. In order to illustrate the potential applications of the metal–silicate partitioning approach to determine the activity coefficients of volatile elements, we used it to determine activity coefficients of PbO, CuO0.5 and InO1.5 in a silica-saturated melt at 1,650 °C. We find values of 0.22, 3.5 and 0.02, respectively, indicating a strong dependence on cation charge. The value for CuO0.5 is in excellent agreement with experimental data of Holzheid and Lodders (Geochim Cosmochim Acta 65:1933–1951, 2001), which shows that the method is viable. When combined with thermodynamic data on the gas species, we find that Pb is the most volatile of the 3 elements under ‘normal’ terrestrial conditions of oxygen fugacity but that In should become the most volatile under strongly reducing conditions such as those of the solar nebula. The oxygen fugacity dependence of volatility has implications for the high relative abundance of In in silicate Earth. We conclude that metal–silicate partitioning experiments are a viable means for determining activities of trace components in silicate melts and are particularly useful if the metal of the element is unstable or volatile at igneous temperatures.

The influence of composition on the local structure around yttrium in quenched silicate melts — Insights from EXAFS

The structural environment around Y in silicate and aluminosilicate glasses containing 5000ppm Y was investigated as a function of melt composition and polymerization using Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. The used glass compositions were taken from Prowatke and Klemme (2005) varying in the aluminum saturation index (ASI, molar ratio of Al2O3/(Na2O+K2O+CaO)) from 0.115 to 0.768. Furthermore, a set of glass compositions from the system CaO–Al2O3–SiO2 (CAS) was used, for which structural data from computer simulations are available (Haigis et al., 2013--this issue). Structural parameters of the Y–O pair correlation of the first coordination shell were determined from the EXAFS based on a gamma-like distribution function that accounts for the large static disorder and non-Gaussian pair distributions. The analysis shows an increase in the coordination number from 6 to 8, along with an increase of the average Y–O distance by 0.13Å for the composition of Prowatke and Klemme (2005). For the CAS-composition an increase of the coordination number from 6 to 7, along with an increase of the average Y–O distance by 0.06Å is obtained. The change of these parameters is associated with a considerable increase in the asymmetry and width of the Y–O pair distribution. Due to its size and charge, 6-fold coordinated Y will preferentially bond to non-bridging oxygens of the polymeric melt network to form a stable configuration, as is the case for the less polymerized melts with low ASI. In highly polymerized melts with ASI values close to one, 6-fold coordination of Y is not possible because almost only bridging oxygens are available. Consequently, over-bonding of bridging oxygens around Y is counterbalanced by an increase of coordination number and Y–O distance to satisfy local charge balance requirements.

The partitioning of sulfur and chlorine between andesite melts and magmatic volatiles and the exchange coefficients of major cations

Andesite melts were equilibrated with an H–O–S-bearing volatile phase to determine the partition coefficients for S and Cl as a function of melt composition and oxygen fugacity. The experiments were conducted in rapid-quench MHC vessel assemblies at 200MPa and 1000°C, and over a range of imposed fO2 between NNO−1.2 and NNO+1.8. High fluid/melt mass ratios (∼15) were employed, allowing precise and accurate partition coefficients to be obtained by mass balance calculations. Chlorine exhibits Henrian behavior at ClO−0.5 activities typical for arc magmas, with DClvolatile/melt=1.36±0.06 (1σ) below 0.2wt.% Cl in the melt; at higher ClO−0.5 activities, DClvolatile/melt increases linearly to 2.11±0.02 at 1wt.% Cl in the melt. In the volatile phase: FeCl2∼NaCl>KCl∼HCl. The determination of cation exchange coefficients for major cations yielded: KK,Navolatile/melt=1.23±0.10 (1σ) and ∗KFe,Navolatile/melt=DFevolatile/melt/DNavolatile/melt=1.08±0.16 (1σ). Under these conditions, the concentration of HCl in the vapor is negatively correlated with the (Na+K)/(Al+Fe3+) ratio in the melt. Reduced sulfur (S2−) appears to obey Henry’s law in andesite melt–volatile system at fH2S below pyrrhotite saturation. The partition coefficient for S at fO2=NNO−0.5 correlates negatively with the FeO concentration in the melt, changing from 254±25 at 4.0wt.% FeO to 88±6 at 7.5wt.% FeO. Pyrrhotite saturation is reached when approximately 3.2mol% S is present in the volatile phase at fO2=NNO−0.5. At the sulfide/sulfate transition, the partition coefficient of S drops from 171±23 to 21±1 at a constant FeO content of ∼6wt.% in the melt. At fO2=NNO+1.8, anhydrite saturation is reached at ∼3.3mol% S present in the volatile phase. Aqueous volatiles exsolving from intermediate to mafic magmas can efficiently extract S, and effect its transfer to sites of magmatic-hydrothermal ore deposit formation.

Prediction of plagioclase-melt equilibria in anhydrous silicate melts at 1-atm

Many models for plagioclase-melt equilibria have been proposed over the past 30 years, but the focus is increasingly on the effects of water content and pressure. However, many geological and petrological applications concern low pressure and low water systems, such as the differentiation of large terrestrial basaltic magma chambers, and lunar and asteroidal magmatism. There is, therefore, a justified need to quantify the influence of anhydrous liquid composition on the composition of equilibrium plagioclase at 1-atm. With this in mind, a database of over 500 experimentally determined plagioclase-liquid pairs has been created. The selected low pressure, anhydrous, experiments include both natural and synthetic liquids, whose compositions range from basalt to rhyolite. Four equations are proposed, derived from this data. The first is based on a thermodynamically inspired formalism, explicitly integrating the effect of temperature. This equation uses free energies and activities of crystalline anorthite available from the literature. For the activity of anorthite in the liquid phase, it is found that current models of the activity of individual oxides are insufficient to account for the experimental results. We have therefore derived an empirical expression for the variation of anorthite activity in the liquid as a function of melt composition, based upon inversion of the experimental data. Using this expression allows the calculation of plagioclase composition with a relative error less than 10%. However, in light of the fact that temperature is not necessarily known for many petrological applications, an alternative set of T-independent equations is also proposed. For this entirely empirical approach, the database has been divided into three compositional groups, treated independently for regression purposes: mafic–ultramafic, alkali-rich mafic–ultramafic, and intermediate-felsic. This separation into distinct subgroups was found to be necessary to maintain errors below acceptable limits, but results across group boundaries were found to be comparable. Overall, 50% of plagioclase compositions are predicted to within 2% of the experimentally derived value, and 90% to within 5%, representing a significant improvement over existing models.

Kinetic and equilibrium factors affecting saturation of chromium oxide in soda-silicate melts

Factors controlling the rate at which chromium oxide reaches saturation in Na 2O– xSiO 2 liquids have been studied as a function of melt composition and oxygen fugacity. Under an oxidizing atmosphere, liquid Na 2CrO 4 or Cr 2O 3 crystals can be in equilibrium with soda-silicate melts, depending on the concentration of sodium in the studied system. Under reducing conditions, NaCrSi 2O 6 is stabilized in silica-rich melts when T is lower than 1160 °C, while Cr 2O 3 is in equilibrium with the sodium-rich melts when T is above 1160 °C. The chromium oxide (Cr 2O 3) to pyroxene (NaCrSi 2O 6) transformation is described in terms of the time required to reach chemical and textural equilibrium. Na 2CrO 4, NaCrSi 2O 6, and Cr 2O 3 phase stability domains are reported as well as the Na 2O–SiO 2–Cr 2O 3 phase diagram in the studied temperature and fO 2 range.

Amorphous materials: Properties, structure, and durability: Atomic structure and transport properties of MgO-Al2O3 melts: A molecular dynamics simulation study

Refractory oxide melts of the binary system MgO-Al 2 O 3 have been studied by molecular dynamics simulation using an advanced ionic interaction model derived from first-principles. The simulations reproduce well experimental densities, structure factors, and transport properties. Anomalous behavior of the latter was observed as a function of melt composition. The minimum in the Al self-diffusion and the respective maximum in the shear viscosity around MgAl 4 O 7 composition are explained by structural changes in the melt.

Enhanced O 2 solubility by RE 2O 3 (RE = La, Gd) additions in molten carbonate electrolytes for MCFC

The solubilization of O 2 reactant gas in mixtures of molten alkali carbonate, which are to be used as electrolytes in the Molten Carbonate Fuel Cells, has been investigated as a function of melt composition, temperature and O 2–CO 2 gas atmosphere. The baseline content of dissolved oxygen in (52 + 48) mol% Li 2CO 3 + Na 2CO 3 and (62 + 38) mol% Li 2CO 3 + K 2CO 3 is markedly increased by the additions of 0.5 mol% rare-earth (RE = La and Gd) oxides to the melts. By a curve-fitting method the mechanisms through which oxygen dissolves into the melts are predicted. At 923 K the oxygen dissolution mechanism passes from a superoxide path, which is dominant in the additive-free melts, to either a prevalent peroxide path in the Li–Na melt or a mixture of superoxide and peroxide ions in the Li–K electrolyte. At lower temperature (873 K) RE oxide additives are effective in modifying the oxygen dissolution mechanism only for the Li–Na melts.

Partitioning of calcium, magnesium, and transition metals between olivine and melt governed by the structure of the silicate melt at ambient pressure

Olivine/melt partitioning of the transition metal cations, Fe2+, Mn2+, Co2+, and Ni2+, together with Mg2+ and Ca2+, has been examined experimentally as a function of melt composition at ambient pressure. Melt structure was inferred from bulk-chemical composition, existing structural data, and 57Fe resonant absorption Mossbauer spectroscopy. Under isothermal conditions, K D(i-Mg)olivine/melt = ( C i/ C Mg)olivine/( C i/ C Mg)melt, is an exponential function of melt NBO/T for i = Ca2+, Mn2+, Co2+, and Ni2+. For i = Fe2+, the relationship is parabolic with maximum K D(Fe2+-Mg)olivine/melt -values at NBO/T near 1. At constant melt NBO/T, K D(i-Mg)olivine/melt increases systematically with decreasing cation radius, an effect that is more pronounced the more polymerized the melt. The K D(i-Mg)olivine/melt is also a positive and linear function of Na/(Na + Ca) of Al-free melts. This latter effect results from changes in Qn-species abundance governed by Na/(Na + Ca) of the melts. The enthalpy of the exchange equilibrium, iolivine + Mgmelt = imelt + Mgolivine, derived from the temperature-dependence of K D(i-Mg)olivine/melt, is also a positive function of the ionic radius of the cation. The relationship of enthalpy to melt polymerization also depends on cation radius. The K D(Fe2+-Mg)olivine/melt does not, however, follow this trend possibly because the bond distance, d Fe2+-O, in the melts depends on melt composition. The cations examined in this study are network-modifiers in silicate melts at ambient pressure. The solution behavior of network-modifying cations in melts is governed by the extent of steric hindrance near nonbridging oxygen, which in turn affects the energetics of metal-nonbridging oxygen bonds. Those structure effects, in turn, are related to the type of Qn-species, their abundance, and on the Al-distribution between the Qn-species. It is suggested, therefore, that the observed variations of mineral/melt partition coefficients with melt composition can be understood by considering bulk polymerization (NBO/T), the distribution of Al3+ among coexisting Qn-species, and the distribution of network-modifying cations among nonbridging in these Qn-species.

The effect of chlorine on the viscosity of Na2O-Fe2O3-Al2O3-SiO2 melts

The shear viscosities of Cl-bearing melts in the system Na 2 O-Fe 2 O 3 -Al 2 O 3 -SiO 2 were determined at temperatures of 550-950 °C in the range of 10 8.5 to 10 12.0 Pa s using the micropenetration technique. The compositions are based on addition of Fe 2 O 3 or FeCl 3 to aluminosilicate glasses with a fixed amount of SiO 2 (67 mol%). Although there was loss of Cl - during the glass syntheses, no loss occurred during the viscometry experiments. It is to be expected that Cl - takes the structural position of O 2- , and thus reduces the polymerization of the melt structure, and therefore the viscosity of the melt; as F - does. Our measurements show that the presence of Cl - increases or decreases the viscosity of the melts as a function of melt composition. In the present melts, at least 10% of the Fe exists as network-modifying or charge-balancing Fe 2+ ; whereas the rest exists as network-forming Fe 3+ . It is proposed here that the different effects of Cl on viscosity are due to the preferred Cl - -Fe 2+ NBO bonding together with the different structure of peralkaline and peraluminous melts. In peralkaline aluminosilicate melts, the addition of Cl 2 O -1 will destroy 2 NBOs and create one BO if Cl - bonds primarily to the Fe 2+ creating non-bridging O atoms. This would result in an increase in viscosity. In peraluminous melts, the addition of Cl 2 O -1 may result in Cl - bonds to the charge-balancing Fe 2+ , creating 2 new tri-clusters [assuming (Al 3+ ,Fe 3+ )Si 2 O 5 tri-clusters exist]. The preference of Cl - to form bonds to the NBO-forming Fe 2+ is indicated by the small amount of Cl - soluble in the peraluminous melt structure in comparison to that soluble in the peralkaline structure.

Equilibrium electrochemical synthesis diagrams of systems, forming homogeneous alloys and compounds

In the present paper thermodynamic limitations will be derived and summarized in the form of Equilibrium Electrochemical Synthesis (EES) diagrams, in order to predict the composition of the equilibrium phase, synthesized by galvanostatic co-deposition of components on inert electrodes. As a thermodynamic parameter, a difference of deposition potentials of pure components ( ?E) on inert cathodes is used (this parameter is a function of melt composition and temperature). Generally, the EES diagram predicts the equilibrium composition of the alloy as function temperature and ?E. However, for systems with homogeneous alloy formation the composition- ?E diagrams, drawn at a fixed temperature are more informative. As examples EES diagrams are constructed for the liquid Mg-Nd alloy, for some A(III)-B(V) (where A = Al, Ga, In and B = As, Sb), Si-C and for the Al-Ti system. For the Al-rich part of the Al-Ti system, also a semi-schematic non-equilibrium ES diagram is constructed. Based on these diagrams, the synthesis conditions of various phases has been discussed.

Melt composition control of Zr/Hf fractionation in magmatic processes

Zircon (ZrSiO 4) and hafnon (HfSiO 4) solubilities in water-saturated granitic melts have been determined as a function of melt composition at 800° and 1035°C at 200 MPa. The solubilities of zircon and hafnon in metaluminous or peraluminous melts are orders of magnitude lower than in strongly peralkaline melt. Moreover, the molar ratio of zircon and hafnon solubility is a function of melt composition. Although the solubilities are nearly identical in peralkaline melts, zircon on a molar basis is up to five times more soluble than hafnon in peraluminous melts. Accordingly, calculated partition coefficients of Zr and Hf between zircon and melt are nearly equal for the peralkaline melts, whereas for metaluminous and peraluminous melts D Hf/D Zr for zircon is 0.5 to 0.2. Consequently, zircon fractionation will strongly decrease Zr/Hf in some granites, whereas it has little effect on the Zr/Hf ratio in alkaline melts or similar depolymerized melt compositions. The ratio of the molar solubilities of zircon and hafnon for a given melt composition, temperature, and pressure is proportional to the Hf/Zr activity coefficient ratio in the melt. The data imply that this ratio is nearly constant and probably close to unity for a wide range of peralkaline and similar depolymerized melts. However, it changes by a factor of two to five over a relatively small interval of melt compositions when a nearly fully polymerized melt structure is approached. For most ferromagnesian minerals in equilibrium with a depolymerized melt, D Hf > D Zr. Typical values of D Hf/D Zr range from 1.5 to 2.5 for clinopyroxene, amphibole, and titanite. Because of the change in the Hf/Zr activity ratio in the melt, the relative fractionation of Zr and Hf by these minerals will disappear or even be reversed when the melt composition approaches that of a metaluminous or peraluminous granite. It is thus not surprising that fractional crystallization of such granitic magmas leads to a decrease in Zr/Hf, whereas fractional crystallization of depolymerized melts tends to increase Zr/Hf. There is no need to invoke fluid metasomatism to explain these effects. Results demonstrate that for ions with identical charge and nearly identical radius, crystal chemistry does not alone determine relative compatibilities. Rather, the effect of changing activity coefficients in the melt may be comparable to or even larger than elastic strain effects in the crystal lattice.

The Sulfide Capacity and the Sulfur Content at Sulfide Saturation of Silicate Melts at 1400degreesC and 1 bar

The solubility of sulfur as S2– has been experimentally determined for 19 silicate melt compositions in the system CaO–MgO–Al2O3–SiO2(CMAS) ± TiO2 ± FeO, at 1400°C and 1 bar, using CO–CO2–SO2 gas mixtures to vary oxygen fugacity (fO2) and sulfur fugacity (fS2). For all compositions, the S solubility is confirmed to be proportional to (fS2/fO2)1/2, allowing the definition of the sulfide capacity (CS) of a silicate melt as CS = [S](fO2/fS2)1/2. Additional experiments covering over 150 melt compositions, including some with Na and K, were then used to determine CS as a function of melt composition at 1400°C. The results were fitted to the equation \(\hbox{C}_{S} = \hbox{A}_{0}+\displaystyle{\sum_{M}}\hbox{A}_{M}\hbox{X}_{M}+\hbox{B}_{Fe-Ti}\hbox{X}_{Fe}\hbox{X}_{Ti}\) ⁠, where AFe ≫ ACa > AMg,ANa/K,ATi. The FeO content of natural basalts is the dominant control on CS. The equation for CS was then combined with the equation for the thermodynamic equilibrium between silicate melt and immiscible FeS-rich sulfide melt, to develop an expression for the sulfur content at sulfide saturation (SCSS) of the silicate melt. The value of SCSS is independent of fO2 and fS2, but shows an asymmetric U-shaped dependence on the FeO content of the silicate melt. Many of the experiments on Fe-containing melt compositions were saturated with immiscible FeS melt, and these experiments were used to calibrate quantitatively SCSS at 1400°C as a function of melt composition.

Equilibrium Pressures over BeF2/LiF(Flibe) Molten Mixtures

The pressures of the vapor species in equilibrium with Flibe at ~600°C are determined from work by Buchler and Stauffer and by Baes and coworkers. The former authors show that the principal vapor species are BeF2(g) and LiBeF3(g). The measurements and the theoretical model of Baes provide accurate values of the activity coefficient of BeF2 in Flibe. When combined with the vapor pressure of pure BeF2, the equilibrium pressure of BeF2 is determined as a function of melt composition and temperature. The activity coefficient of LiF is not measured, but it is obtained by application of the Gibbs-Duhem equation to the measured activity coefficient of BeF2. Thus, the partial pressure of LiF(g) is also known. The pressure of the mixed dimer LiBeF3 is calculated from the gas phase equilibrium for the formation of the dimer from the two monomers, with the equilibrium constant given by Buchler and Stauffer. The vapor pressure at 600°C extrapolated from high-temperature Oak Ridge National Laboratory data is ~60% higher than the predicted values.

Effects of aluminum on Cr dimerization in silicate melts and implications for Cr partitioning and redox equilibria

The electrochemical measurements reported here provide insight into the activities and structure of individual Cr and Sn species in the melt. This represents an improvement over activity coefficients averaged over multiple species such as can be derived from simple melt/mineral equilibrium experiments. Knowledge of the presence and activities of individual species of Cr in a melt can lead to better understanding and prediction of the complex variations in partitioning of Cr between minerals and melt, which have thus far defied accurate description. Electrochemical experiments in four different silicate melts indicate that, under important magmatic conditions, Cr can be present in the melt as any of three species: Cr2+, Cr3+, and some polymeric form which we represent in this paper as the dimer Cr26+. The presence of two trivalent Cr species, and the oxidation state change between Cr2+ and Cr3+, makes it easy to understand why the partitioning behavior of Cr is complex. The presence of two species (a dimer and a monomer), each containing Cr3+, means that the activity coefficient for a single composite Cr3+ component will vary in a complex way with melt composition, temperature, fO2, and Cr concentration even when mixing in the melt is ideal. For example, if a fixed Cr3+ component is used to model the melt, its activity coefficient will vary by an order of magnitude over the region in which the Cr3+ goes from being present as mostly monomer to mostly dimer. This occurs even if there were no nonideal mixing. Trivalent Cr dimers are particularly sensitive to Al concentration in the melt, consistent with the interpretation that dimers exist as spinel-like structures in the melt. The preceding discussion bears on melts with ideal mixing. But the experiments reported here indicate that mixing in the compositions studied is not ideal. Activity coefficients for the Cr3+ dimer vary by over a factor of 3 as a function of composition for the silicate melts reported here. In addition, we report variations of a factor of 3 in the activity coefficient for divalent Cr as a function of melt composition, meaning that Cr partition coefficients will be significantly sensitive to melt composition even where little trivalent Cr is present. Thus, the effects of nonideal mixing will be added to the effects of the presence of a dimer. The transition from mostly dimer to mostly monomer occurs at concentrations of Cr and Al typical of natural systems, with the dimer becoming less important as Cr3+ concentration decreases. At concentrations of Al of about 8 wt.%, the dimer is an important component down to about 0.25 wt.% Cr3+. At Al concentrations of 11–12 wt.%, the dimer was important to the limit of our experimental resolution, about 0.1 wt.% Cr3+. This means that in the range of natural melts, not only is the Cr not present as a single species, but the proportion that each of the species comprises of the total Cr3+ is not fixed. Because the trivalent Cr exists as more than a single species in the melt, and the proportion of those species is not fixed, use of a single species model to predict variations in activity will work only poorly and require complex variations in activity coefficients. Trivalent Cr activity coefficients, relative to typically-used fixed component models, can be expected to vary as a function not only of melt composition, but of fO2 and total Cr concentration as well. Based on the results presented here, variations in activity of trivalent Cr of more than 100% can be expected over natural ranges of conditions. These results have broader implications for how we view thermodynamic components in magmatic systems. In a thermodynamic treatment of the behavior of an element in a melt, any form of that element can serve as the species of description. However, choosing the principle form of the element actually present in the melt usually minimizes variations of the activity coefficient and improves intuitive understanding of the behavior of the element in the system. What’s more, if an element exists in multiple species, no single species will provide a simple or intuitive model for the variation of activity of the element as a whole. Use of a simple ionic model for melts, although successful in some general applications, is not correct in detail as shown by the dimerization of Cr in silicate melts. Other elements may exhibit similar speciation in melts. We demonstrate in this report that Sn also exists as a dimer in silicate melts. At all concentrations of Sn which we studied, the Sn dimer (which we represent as Sn24+) dominated over any monomer present. However, the concentrations we studied were far above natural Sn concentrations, and Sn may exhibit similar behavior to Cr at more natural concentrations of Sn where monomerization would be more likely to occur.

Mg‐Fe partitioning between olivine and mafic‐ultramafic melts

The partitioning of Mg and Fe between olivine and mafic‐ultramafic melts has been examined based on previous and new experimental data. We find that the exchange partition coefficient, KD, changes primarily as a function of melt composition, expressed either as MgO+FeO content or degree of polymerization, NBO/T, rather than temperature or pressure. KD increases from about 0.23±0.02 at an NBO/T of ∼0 to a maximum of 0.35±0.02 at an NBO/T of ∼ 2, and then decreases to 0.23±0.02 for the pure olivine system (NBO/T=4). We suggest that the following empirical relation yields a reliable estimate of KD for mantle derived melts: KD = 0.235 + 0.120·(NBO/T) ‐ 0.031·(NBO/T)². The change of KD shown here should be taken into account when calculating the composition of olivine coexisting with basaltic to ultramafic melts.