Distribution of Mg²⁺ between olivine and silicate melt,and its implications regarding melt structure     

William P. Leeman 《Geochimica et cosmochimica acta》1978,42(6):789-800

Consideration of experimental data on the distribution of Mg²⁺ between olivine and silicate liquid clearly demonstrates that the distribution coefficient (K_Mg) is dependent upon variations in temperature, pressure and melt composition, largely because these variables control the solubility of Mg²⁺ in the melt phase. Attempts to minimize composition dependence of K_Mg, utilizing various activity-composition models for silicate melts, have been partially successful. Composition-related effects do not appear to be large, however, for melts of restricted range in composition (e.g., tholeiitic or lunar basalts) as long as the contents of alkalis and the alkali/alumina ratio are relatively small (on a molar basis). For such melts, K_Mg may be used as a reliable geothermometer. By analogy, these conclusions can be extended to the distribution of other divalent cation such as Fe²⁺, Mn²⁺, Ni²⁺ and Co²⁺.  相似文献   

The distribution of Fe and Mg between olivine and lunar basaltic liquids     

John Longhi  David Walker  James F. Hays 《Geochimica et cosmochimica acta》1978,42(10):1545-1558

We have examined the Fe and Mg distribution between coexisting olivine and lunar basaltic liquids produced by equilibrium partial melting of natural lunar samples. In agreement with the findings of Roeder and Emslie (Contrib. Mineral. Petrol. 29, 275–289) on terrestrial compositions, the logarithms of the conventional distribution coefficients, K_ol-L^Fe and K_ol-L^Fe-Mg, are nearly linear functions of inverse temperature; and the exchange coefficient, K_D = K_ol-L^Fe-Mg, is nearly independent of temperature and composition within a given magma group. There are, however, small but significant differences in conventional and exchange distribution coefficients from one magma group to another, e.g. low-Ti vs high-Ti lunar basalts. It is possible to achieve slightly greater precision for the inverse temperature functions by including terms approximating silica activity in the conventional distribution coefficients. The term ($\text{2Si}\text{O}\text{)}{}_{\mathrm{L}}$ is apparently the best simple approximation for silica activity in olivine-saturated liquids based upon data for Fe, Mg, Mn, Ca, Ti and Cr. Pressure has noticeable effects upon Fe and Mg distribution between olivine and liquid only above 5 kbar.The excellent linear correlation of the logarithms of the distribution coefficients with inverse temperature allows calculation of approximate values of $\text{Δ}\text{H}\text{?}{}^0$ for the reactions : 2MgO_L + SiO_2LaiMg₂SiO_4ol and 2FeO_L + SiO_2LaiFe₂SiO_4ol. Values obtained, approx ?26 kcal/mole, are comparable with values of the heats of fusion of forsterite and fayalite calculated by Bradley (Am. J. Sci.260, 550–554) and measured by Orr (J. Am. Chem. Soc. 75, 528–529).The exchange distribution coefficient for Fe and Mg, K_D, is sensitive to large changes in liquid chemistry. Although K_D is explicitly independent of silica activity, K_D apparently changes with silica concentration. This change is a reflection of changes in the mixing properties of Fe and Mg in liquids with different chemistry and hence structure. Regular solution theory predicts that as the mixing properties of an element in a solution change, the most radical changes in activity coefficients occur in the range of dilute concentrations. Therefore, the distribution coefficients for trace elements will also be dependent upon large changes in liquid chemistry, even if corrections for silica and other liquid component activities are applied.  相似文献   

Calorimetric measurements of fusion enthalpies for Ni₂SiO₄ and Co₂SiO₄ olivines and application to olivine-liquid partitioning     

Toru Sugawara  Masaki Akaogi 《Geochimica et cosmochimica acta》2003,67(14):2683-2693

Calorimetric measurements of fusion enthalpies for Ni₂SiO₄ and Co₂SiO₄ olivines were carried out using a high-temperature calorimeter, and Ni and Co partitioning between olivine and silicate liquid was analyzed using the measured heats of fusion. The fusion enthalpy of Co₂SiO₄ olivine measured by transposed-temperature drop calorimetry was 103 ± 15 kJ/mol at melting point (1688 K). The fusion enthalpy of Ni₂SiO₄ olivine was calculated based on the enthalpies of liquids in the system An₅₀Di₅₀-Ni₂SiO₄ measured by transposed-temperature drop calorimetry at 1773 K, and was 221 ± 26 kJ/mol at its metastable melting point (1923 K). The fusion enthalpy of Ni₂SiO₄ is the largest among those of olivine group, this is caused by the large crystal field stabilization energy of six-coordinated Ni²⁺ in olivine. The larger fusion enthalpy of Ni₂SiO₄ can account for the large and variable partition coefficient of Ni between olivine and silicate liquid. Based on the comparison between partition coefficients calculated from thermodynamic data and those observed in partition experiments, it is considered that the magnitude of partition coefficients is primarily dependent on the heats of fusion of the components. Furthermore, the activity coefficients for Ni-, Co- and Mn-bearing components in magmatic liquid are nearly of the same magnitude.  相似文献   

Partitioning of Ni²⁺ between basaltic and synthetic melts and olivines—an experimental study     

William P. Leeman  David J. Lindstrom 《Geochimica et cosmochimica acta》1978,42(6):801-816

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An electrochemical study of Ni²⁺, Co²⁺, and Zn²⁺ ions in melts of composition CaMgSi₂O₆     

Krystyna W. Semkow  Ronald A. Rizzo  Larry A. Haskin  David J. Lindstrom 《Geochimica et cosmochimica acta》1982,46(10):1879-1889

Cyclic voltammetry has been done for Ni²⁺, Co²⁺, and Zn²⁺ in melts of diopside composition in the temperature range 1425 to 1575°C. Voltammetric curves for all three ions excellently match theoretical curves for uncomplicated, reversible charge transfer at the Pt electrode. This implies that the neutral metal atoms remain dissolved in the melt. The reference electrode is a form of oxygen electrode. Relative to that reference assigned a reduction potential of 0.00 volt, the values of standard reduction potential for the ions are $\text{E}{}^1\text{(}\text{Ni}{}^{\mathrm{2+}}\text{Ni}{}^0\text{,}\text{diopside}\text{, 1500°C) = ?0.32 ± .01}\text{V}$, $\text{E}{}^1\text{(}\text{Co}{}^{\mathrm{2+}}\text{Co}{}^0\text{,}\text{diopside}\text{, 1500°C) = ?0.45 ± .02}\text{V}$, and $\text{E}{}^1\text{(}\text{Zn}{}^{\mathrm{2+}}\text{Zn}{}^0\text{,}\text{diopside}\text{, 1500°C) = ?0.53 ± .01}\text{V}$. The electrode reactions are rapid, with first order rate constants of the order of 10^?2 cm/sec. Diffusion coefficients were found to be 2.6 × 10^?6 cm²/sec for Ni²⁺, 3.4 × 10^?6 cm²/sec for Co²⁺, and 3.8 × 10^?6 cm²/sec for Zn²⁺ at 1500°C. The value of $\text{E1}$ ($\text{Ni}{}^{\mathrm{2+}}\text{Ni}{}^0$, diopside) is a linear function of temperature over the range studied, with values of ?0.35 V at 1425°C and ?0.29 V at 1575°C. At constant temperature the value of $\text{E}{}^1\text{(}\text{Ni}{}^{\mathrm{2+}}\text{Ni}{}^0\text{, 1525°C}$) was not observed to vary with composition over the range CaO · MgO · 2SiO₂ to CaO·MgO·3SiO₂ or from 1.67 CaO·0.33MgO·2SiO₂ to 0.5 CaO·1.5MgO·2SiO₂. The value for the diffusion coefficient for Ni²⁺ decreased by an order of magnitude at 1525°C over the compositional range CaO · MgO · 1.25SiO₂ to CaO · MgO · 3SiO₂. This is consistent with a mechanism by which Ni²⁺ ions diffuse by moving from one octahedral coordination site to another in the melt, with the same Ni²⁺ species discharging at the cathode regardless of the SiO₂ concentration in the melt.  相似文献   

Experimental determination of activity-composition relations in Ni2SiO4− Mg2SiO4 and Co2SiO4-Mg2SiO4 olivine solid solutions at 1200 K and 0.1 MPa and 1573 K and 0.5 GPa     

《Geochimica et cosmochimica acta》1987,51(1):97-104

Activity-composition relations in the olivine solid solutions Ni₂SiO₄⁻ -Mg₂SiO₄ and Co₂SiO₄-Mg₂SiO₄ have been determined at 1200 K and 0.1 MPa and at 1573 K and 0.5 GPa by equilibration with the corresponding oxide solutions. Both olivine solutions show small positive deviations from ideal (two site) mixing, which, within the limits of accuracy of the method, may be described by the simple regular solution model with parameters W_Ni+Mg^ol= 0.35 ± 1.0 kJ/g-atom and W_Co-Mg^ol = 1.37 ± 0.9 kJ/g-atom. The requirements of internal consistency between the two systems also show that the recent determination by Brousse et al. (1984) of the enthalpy of formation of Mg₂SiO₄is to be preferred over earlier work, and that their value is also probably more accurate than the uncertainty in their own measurements indicates; activities in the NiO-MgO system are close to ideal.  相似文献   

Mg²⁺ removal in the system Mg²⁺—amorphous SiO₂—H₂O by adsorption and Mg-hydroxysilicate precipitation     

Douglas B. Kent  Miriam Kastner 《Geochimica et cosmochimica acta》1985,49(5):1123-1136

Two chemical processes can remove Mg²⁺ from suspensions containing amorphous silica (am-SiO₂) at low temperatures: adsorption and precipitation of a Mg-hydroxysilicate resembling sepiolite. Mg²⁺ removal from am-SiO₂ suspensions was investigated, and the relative role of the two removal processes evaluated, as a function of: pH, ionic strength, Mg²⁺ concentration, and temperature.The extent of Mg²⁺ adsorption onto am-SiO₂ decreases with increasing NaCl concentration due to displacement of Mg²⁺ by Na⁺. At NaCl concentrations of 0.05 M and above, adsorption occurs only at pH values above 8.5, where rapid dissolution of am-SiO₂ gives rise to high concentrations of dissolved silica, resulting in supersaturation with respect to sepiolite. Removal of Mg²⁺, at concentrations of 40 to 650 μM, from am-SiO₂ suspensions in 0.70 M NaCl at 25 °C occurs at pH 9.0 and above. Experiments show that under these conditions adsorption and Mg-hydroxysilicate precipitation remove Mg²⁺ at similar rates. For 0.05 M Mg²⁺, at 0.70 M ionic strength and 25 °C, measurable Mg²⁺ removal occurs down to ca. pH 7.5 but is primarily due to Mg-hydroxysilicate precipitation. For the same solution conditions at 5°C, Mg²⁺ removal occurs above pH 8.0 and is primarily due to adsorption.Assuming that increasing pressure does not greatly enhance adsorption, Mg²⁺ adsorption onto am-SiO₂ is an insignificant process in sea water. The surface charge of pristine am-SiO₂ in sea water is primarily controlled by interactions with Na⁺. The principal reaction between Mg²⁺ and am-SiO₂ in marine sediments is sepiolite precipitation.The age distribution of sepiolite in siliceous pelagic sediments is influenced by temperatures of bottom waters and by geothermal gradients.  相似文献   

The partitioning of Fe and Mg between olivine and carbonate and the stability of carbonate under mantle conditions   总被引：4，自引：1，他引：4  

John A. Dalton  Bernard J. Wood 《Contributions to Mineralogy and Petrology》1993,114(4):501-509

We have investigated the effect of Fe on the stabilities of carbonate (carb) in lherzolite assemblages by determining the partitioning of Fe and Mg between silicate (olivine; ol) and carbonates (magnesite, dolomite, magnesian calcite) at high pressures and temperatures. Fe enters olivine preferentially relative to magnesite and ordered dolomite, but Fe and Mg partition almost equally between disordered calcic carbonate and olivine. Measurement of K _d (X _Fe ^carb X _Mg ^ol /X _Fe ^ol X _Mg ^carb ) as a function of Fe/ Mg ratio indicates that Fe–Mg carbonates deviate only slightly from ideality. Using the regular solution parameter for olivine W _FeMg ^ol of 3.7±0.8 kJ/mol (Wiser and Wood 1991) we obtain for (FeMg)CO₃ a W _FeMg ^carb of 3.05±1.50 kJ/mol. The effect of Ca–Mg–Fe disordering is to raise K _d substantially enabling us to calculate W _CaMg ^carb -W _CaFe ^carb of 5.3±2.2 kJ/mol. The activity-composition relationships and partitioning data have been used to calculate the effect of Fe/Mg ratio on mantle decarbonation and exchange reactions. We find that carbonate (dolomite and magnesian calcite) is stable to slightly lower pressures (by 1 kbar) in mantle lherzolitic assemblages than in the CaO–MgO–SiO₂(CMS)–CO₂ system. The high pressure breakdown of dolomite + orthopyroxene to magnesite + clinopyroxene is displaced to higher pressures (by 2 kbar) in natural compositions relative to CMS. CO₂. We also find a stability field of magnesian calcite in lherzolite at 15–25 kbar and 750–1000°C.  相似文献   

Electron paramagnetic resonance study of the site preferences of Gd³⁺ and Eu²⁺ in polycrystalline silicate and aluminate minerals     

Richard V. Morris 《Geochimica et cosmochimica acta》1975,39(5):621-634

Electron paramagnetic resonance (EPR) measurements were made on Gd³⁺ and Eu²⁺ ions in polycrystalline samples to determine the nature of the sites occupied by those ions in mineral structures. Both Gd³⁺ and Eu²⁺ ions were incorporated at Ca²⁺ structural sites in β-Ca₂SiO₄, pseudo-CaSiO₃, CaMgSiO₄, CaMgSi₂O₆, hex-CaAl₂Si₂O₈, CaAl₂O₄, and Ca₃Al₂O₆. For tri-CaAl₂Si₂O₈, Eu²⁺ was incorporated at a Ca²⁺ site and Gd³⁺ was incorporated at a site where the crystalline electric field was disordered. That difference in behavior may contribute to the anomalous behavior of Eu in plagioclase feldspar. Both Gd³⁺ and Eu²⁺ were incorporated as aggregates or clusters of those ions in Mg₂SiO₄ and clino-MgSiO₃.  相似文献   

The solubility of olivine in basaltic liquids: an ionic model     

Claude T. Herzberg 《Geochimica et cosmochimica acta》1979,43(8):1241-1251

A simple ionic model which describes the solution of the forsterite component of olivine in silicate liquids is reported. The melting relation is represented: (Mg₂SiO₄)_ol = 2(Mg²⁺)_L + (SiO₄^4?_L and is extended to all silicate liquids by normalizing their compositions to 4 oxygens. At 1 bar, the temperature at which olivine is in equilibrium with any alkali-depleted basaltic composition can be calculated to within ±30°C. This error is increased considerably when applied to terrestrial basalts which contain several weight percent alkalis. Alkalis interfere with the equilibrium by generating strongly repulsive interionic forces which can be crudely modelled in a manner consistent with constraints imposed by regular solution theory.The model quantifies the reduced activity of SiO₄^4? monomers due to increasing SiO₂ concentrations in the melt. This is a consequence of polymerization which does not appear to operate gradually over the entire spectrum of mafic and ultramafic compositions. The coordination of alumina in melts which precipitate olivine only appears to be dominantly octahedral. Titanium acts as a polymerizing agent by interconnecting previously isolated SiO₄^4? monomers. Calcium associated with normative diopside tends to exhibit small but perceptible repulsive forces involving Mg²⁺.  相似文献   

Effects of fO₂, fS₂, temperature, and melt composition on Fe-Ni exchange between olivine and sulfide liquid: implications for natural olivine-sulfide assemblages     

James M. Brenan 《Geochimica et cosmochimica acta》2003,67(14):2663-2681

The apparent equilibrium constant for the exchange of Fe and Ni between coexisting olivine and sulfide liquid (K_D = (X_NiS/X_FeS)_liquid/(X_NiSi12O2/X_FeSi12O2)_olivine; X_i = mole fraction) has been measured at controlled oxygen and sulfur fugacities (fO₂ = 10^−8.1 to 10⁻¹⁰ and fS₂ = 10^−0.9 to 10^−1.7) over the temperature range 1200 to 1385°C, with 5 to 37 wt% Ni and 7 to 18 wt% Cu in the sulfide liquid. At log fO₂ of −8.7 ± 0.1, and log fS₂ of −0.9 to −1.7, K_D is relatively insensitive to sulfur fugacity, but comparison with previous results shows that K_D increases at very low sulfur fugacities. K_D values show an increase with the nickel content of the sulfide liquid, but this effect is more complex than found previously, and is greatest at log fO₂ of −8.1, lessens with decreasing fO₂, and K_D becomes independent of melt Ni content at log fO₂ ≤ −9.5. The origin of this variation in K_D with fO₂ and fS₂ is most likely the result of nonideal mixing of Fe and Ni species in the sulfide liquid. Such behavior causes activity coefficients to change with either melt oxygen content or metal/sulfur ratio, effects that are well documented for metal-rich sulfide melts.Application of these experimental results to natural samples shows that the relatively large dispersion that exists in K_D values from different olivine + sulfide-saturated rock suites can be interpreted as arising from variations in fO₂, fS₂, and the nickel content of the sulfide liquid. Estimates of fO₂ based on K_D and sulfide melt composition in natural samples yields a range from fayalite-magnetite-quartz (FMQ)-1 to FMQ-2 or lower, which is in good agreement with previous values determined for oceanic basalts that use glass ferric/ferrous ratios. Anomalously high K_D values recorded in some suites, such as Disko Island, probably reflect low fS₂ during sulfide saturation, which is consistent with indications of low fO₂ for those samples. It is concluded that the variation in K_D values from natural samples reflects olivine-sulfide melt equilibrium at conditions within the T-fO₂-fS₂ range of terrestrial mafic magmas.  相似文献   

Strong site preference of Co2+ in olivine,Co1.10Mg0.90SiO4     

Subrata Ghose  Che'ng Wan 《Contributions to Mineralogy and Petrology》1974,47(2):131-140

The crystal structure and site preference of Co²⁺ in a synthetic Co_1.10Mg_0.90SiO₄ olivine have been determined from single crystal X-ray diffraction data collected on an automatic diffractometer. The R factor is 0.044 for 612 reflections. The site occupancies are: Ml site: Co 0.730±0.006; Mg 0.270; M2 site: Co 0.370, Mg 0.630. The Gibbs free energy change, ΔG° for the ion-exchange reaction between M1 and M2 sites is ?4.06 kcals/mole, assuming ideal mixing at each set of sites. This energy may be called ‘site preference energy’ of Co²⁺ in olivine. The strong preference of Co²⁺ for the M1 site can be quantitatively explained by two competing forces: preference of ions larger than Mg²⁺ for the M2 site and stronger covalent bonding of transition metal ions at the M1 site. For Fe²⁺, Mg²⁺, these two effects nearly neutralize each other, explaining the lack of considerable cation-ordering in Fe-Mg olivines.  相似文献   

Partition of Sr,Ba, Ca,Y, Eu²⁺, Eu³⁺, and other REE between plagioclase feldspar and magmatic liquid: an experimental study   总被引：1，自引：0，他引：1  

Michael J Drake  Daniel F Weill 《Geochimica et cosmochimica acta》1975,39(5):689-712

Plagioclase feldspar/magmatic liquid partition coefficients for Sr, Ba, Ca, Y, Eu²⁺, Eu³⁺ and other REE have been determined experimentally at 1 atm total pressure in the temperature range 1150–1400°C. Natural and synthetic melts representative of basaltic and andesitic bulk compositions were used, crystallizing plagioclase feldspar in the composition range An₃₅–An₈₅. Partition coefficients for Sr are greater than unity at all geologically reasonable temperatures, and for Ba are less than unity above approximately 1060°C. Both are strongly dependent upon temperature. Partition coefficients for the trivalent REE are relatively insensitive to temperature. At fixed temperature they decrease monotonically from La to Lu. The partition of Eu is a strong function of oxygen fugacity. Under extreme reducing conditions D_Eu approaches the value of D_Sr.  相似文献   

Microscope-photometric methods for non-destructive Fe²⁺ – Fe³⁺ determination in chloritoids (Fe²⁺, Mn²⁺, Mg)₂ (Al,Fe³⁺)₄ Si₂O₁₀(OH)₄     

Ulf Hålenius  Klaus Langer 《Lithos》1980,13(3):291-294

Six natural chloritoid crystals with Fe²⁺ and Fe³⁺ contents ranging from 4.15 to 12.81 and from 0.411 to 0.849g-atoms/l, respectively, as determined by means of microprobe and Mössbauer techniques, served as reference material to develop non-destructive microscope-spectrophotometric methods for quantitative Fe²⁺ – Fe³⁺ determinations in chloritoids from unpolarized spectra of (001) platelets. Fe²⁺ concentrations in g-atom/l can be obtained from [ [Fe³⁺]=C₁xD₁/t where D₁ = log₁₀(I_0/I at 28,000 cm^-1 and t=crystal thickness in cm; C₁ is a conttant that may be influenced somewhat by experimental conditions and is found to be 0.002289 with the experimental set-up used in this study. Fe²⁺ concentrations in g-atom/l can be obtained from [Fe²⁺]=C₁xD₁/D₁-C₃ with D₂=log₁₀(I₀/I) at 16,300 cm^?1 and constants C₄ = 45.36 and C₅ = 3.540. Due to the uncertainties in absorbance measurements, D₁ and D₂ and the thickness measurements, the accuracies are ±0.05 and ±0.15 g-atom/l for [Fe³⁺] and [Fe²⁺], respectively. The determinations may be carried out on chloritoid grains in normal thin sections with an areal resolution of ～10 μm.  相似文献   

Synthesis and optical absorption spectra of Cr2+-containing orthosilicates     

Barry E. Scheetz  William B. White 《Contributions to Mineralogy and Petrology》1972,37(3):221-227

By reacting chromium metal with appropriate oxides in a neutral atmosphere at high temperature it is possible to prepare Mg₂SiO₄ with the olivine structure containing Cr⁺⁺ and also Cr₂SiO₄. X-ray powder diffraction data for chromous orthosilicate may be indexed on an orthorhombic cell with a=5.690, b=11.262, and c=9.584 Å. The compound is not isotructural with olivine. Optical absorption spectra of Cr²⁺ in both hosts have been obtained and also indicate large differences between the two structures.  相似文献   

Polymerization of silicate and aluminate tetrahedra in glasses,melts and aqueous solutions—II. The network modifying effects of Mg²⁺ , K⁺, Na⁺, Li⁺, H⁺, OH⁻, F⁻, Cl⁻, H₂O,CO₂ and H₃O⁺ on silicate polymers     

B.H.W.S. de Jong  Gordon E. Brown 《Geochimica et cosmochimica acta》1980,44(11):1627-1642

The effect of the group IA and VIIA ions, as well as Mg²⁺, and the molecules H₂O, CO₂, H₃O⁺ and OH^? on the energy of the Si-O bond in a H₆Si₂O₇ cluster has been calculated using semiempirical molecular orbital calculations (CNDO/2). Three types of elementary processes, i.e. substitution, addition, and polymerization reactions have been used to interpret data on the dynamic viscosity, surface tension and surface charge, hydrolytic weakening, diffusivity, conductivity, freezing point depression, and degree of polymerization of silicates in melts, glasses, and aqueous solutions. As a test of our calculational procedure, observed X-ray emission spectra of binary alkali silicate glasses were compared with calculated electronic spectra. The well known bondlength variations between the bridging bond [Si-O(br)] and the non-bridging bond [Si-O(nbr)] in alkali silicates are shown to be due to the propagation of oscillating bond-energy patterns through the silica framework. A kinetic interpretation of some results of our calculations is given in terms of the Bell-Evans-Polanyi reaction principle.  相似文献   

Evidence from olivine/melt element partitioning that nonbridging oxygen in silicate melts are not equivalent     

Bjorn O. Mysen  Jessica Shang 《Geochimica et cosmochimica acta》2005,69(11):2861-2875

Partitioning of Ca, Mn, Mg, and Fe²⁺ between olivine and melt has been used to examine the influence of energetically nonequivalent nonbridging oxygen in silicate melts. Partitioning experiments were conducted at ambient pressure in air and 1400°C with melts in equilibrium with forsterite-rich olivine (Fo >95 mol%). The main compositional variables of the melts were NBO/T and Na/(Na+Ca). In all melts, the main structural units were of Q⁴, Q³, and Q² type with nonbridging oxygen, therefore, in the Q³ and Q² units.For melts with high Q³/Q²-abundance ratio (corresponding to NBO/T near 1), increasing Na/(Na+Ca) [and Na/(Na+Ca+Mn+Mg+Fe²⁺)] results in a systematic decrease of the partition coefficients, K_Ca^ol/melt, K_Mn^ol/melt, K_Mg^ol/melt, and K_Fe2+^ol/melt, because of ordering of the network-modifying Ca, Mn, Mg, and Fe²⁺ among nonbridging oxygen in Q³ and Q² structural units. This decrease is more pronounced the smaller the ionic radius of the cation. With decreasing Q³/Q² abundance ratio (less-polymerized melts) this effect becomes less pronounced.Activity-composition relations among network-modifying cations in silicate melts are, therefore, governed by availability of energetically nonequivalent nonbridging oxygen in individual Qⁿ-species in the melt. As a result, any composition change that enhances abundance of highly depolymerized Qⁿ-species will cause partition coefficients to decrease.  相似文献   

A possible effect of melt structure on the Mg-Fe partitioning between olivine and melt     

Ikuo Kushiro  Bjorn O Mysen 《Geochimica et cosmochimica acta》2002,66(12):2267-2272

Partitioning of Mg and Fe²⁺ between olivine and mafic melts has been determined experimentally for eight different synthetic compositions in the temperature range between 1335 and 1425°C at 0.1 MPa pressure and at fo₂ ∼1 log unit below the quartz-fayalite-magnetite buffer. The partition coefficient [K_D = (Fe²⁺/Mg)^ol/(Fe²⁺/Mg)^melt] increases from 0.25 to 0.34 with increasing depolymerization of melt (NBO/T of melt from 0.25-1.2), and then decreases with further depolymerization of melt (NBO/T from 1.2-2.8). These variations are similar to those observed in natural basalt-peridotite systems. In particular, the variation in NBO/T ranges for basaltic-picritic melts (0.4-1.5) is nearly identical to that obtained in the present experiments. Because the present experiments were carried out at constant pressure (0.1 MPa) and in a relatively small temperature range (90°C), the observed variations of Mg and Fe²⁺ partitioning between olivine and melt must depend primarily on the composition or structure of melt. Such variations of K_D may depend on the relative proportions of four-, five-, and six-coordinated Mg²⁺ and Fe²⁺ in melt as a function of degree of NBO/T.  相似文献   

Synthetic Mn3+-kyanite and viridine, (Al2-xMn x 3+ ) SiO5, in the system Al2O3-MnO-MnO2-SiO2     

I. Abs-Wurmbach  Prof. Dr. K. Langer 《Contributions to Mineralogy and Petrology》1975,49(1):21-38

Experiments on the join Al₂SiO₅-“Mn₂SiO₅” of the system Al₂O₃-SiO₂-MnO-MnO₂ in the pressure/temperature range 10–20 kb/900–1050° C with gem quality andalusite, Mn₂O₃, and high purity SiO₂ as starting materials and using /_O2-buffer techniques to preserve the Mn³⁺ oxidation state had following results: At 20 kb/1000°C orange-yellow kyanite mixed crystals are formed. The kyanite solid solubility is limited at about (Al_1.88Mn _0.12 ³⁺ )SiO₅ and, thus, equals approximately that on the join Al₂SiO₅-“Fe₂SiO₅” (Langer and Frentrup, 1973) indicating that there is no Jahn-Teller stabilisation of Mn³⁺ in the kyanite matrix. 5 mole % substitution causes the kyanite lattice constants a _o, b _o, c _o, and V _o to increase by 0.015, 0.009, 0.014 Å, and 1.6 ų, resp., while α, β, γ, remain unchanged. Between 10 and 18 kb/900°C, Mn³⁺-substituted, strongly pleochroitic (emeraldgreen-yellow) andalusite_ss (viridine) was obtained. At 15 kb/900°C, the viridine compositional range is about (Al_1.86Mn _0.14 ³⁺ )SiO₅-(Al_1.56Mn _0,44 ³⁺ )SiO₅. Thus, Al→Mn³⁺ substitutional degrees are appreciably higher in andalusite than in kyanite, proving a strong Jahn-Teller effect of Mn³⁺ in the andalusite structure, which stabilises this structure type at the expense of kyanite and sillimanite and, thus, enlarges its PT-stability range extremely. 17 mole % substitution cause the andalusite constants a _o, b _o, c _o, and V _o to increase by 0.118, 0.029, 0.047 Å and 9.4 ų, resp. At “Mn₂SiO₅”-contents smaller than about 7 mole %, viridine coexists with Mn-poor kyanite. At “Mn₂SiO₅”-concentrations higher than the maximum kyanite or viridine miscibility, braunite (tetragonal, ideal formula Mn²⁺Mn³⁺[O₈/Si0₄]), pyrolusite and SiO₂ were found to coexist with the Mn³⁺-saturated ky _ss or and _ss, respectively. In both cases, braunites were Al-substituted (about 1 Al for 1 Mn³⁺). Pure synthetic braunites had the lattice constants a _o 9.425, c _o, 18.700 Å, V _o 1661.1 ų (ideal compn.) and a _o 9.374, c _o 18.593 ų, V _o 1633.6 ų (1 Al for 1 Mn³⁺). Stable coexistence of the Mn²⁺-bearing phase braunite with the Mn⁴⁺-bearing phase pyrolusite was proved by runs in the limiting system MnO-MnO₂-SiO₂.  相似文献   

Role of basicity and tetrahedral speciation in controlling the thermodynamic properties of silicate liquids, part 1: the system CaO-MgO-Al₂O₃-SiO₂     

John R Beckett 《Geochimica et cosmochimica acta》2002,66(1):93-107

Activity coefficients of oxide components in the system CaO-MgO-Al2O3-SiO2 (CMAS) were calculated with the model of Berman (Berman R. G., “A thermodynamic model for multicomponent melts with application to the system CaO-MgO-Al2O3-SiO2,” Ph.D. dissertation, University of British Columbia, 1983) and used to explore large-scale relationships among these variables and between them and the liquid composition. On the basis of Berman’s model, the natural logarithm of the activity coefficient of MgO, ln(γMgOLiq), and ln(γMgOLiq/γSiO2Liq) are nearly linear functions of ln(γCaOLiq). All three of these variables are simple functions of the optical basicity Λ with which they display minima near Λ ∼ 0.54 that are generated by liquids with low ratios of nonbridging to tetrahedral oxygens (NBO/T) (<0.3) and a mole fraction ratio, XSiO2Liq/XAl2O3Liq, in the range 4 to 20. Variations in ln(γCaOLiq) at constant Λ near the minimum are due mostly to liquids with (XCaOLiq + XMgOLiq)/XAl2O3Liq < 1. The correlations with optical basicity imply that the electron donor power is an important factor in determining the thermodynamic properties of aluminosilicate liquids.For a constant NBO/T, ln(γCaOLiq/γAl2O3Liq) and ln(γMgOLiqγAl2O3Liq) form curves in terms of XSiO2Liq/XAl2O3Liq. The same liquids that generate minima in the Λ plots are also associated with minima in ln(γCaOLiqγAl2O3Liq) and ln(γMgOLiqγAl2O3Liq) as a function of XSiO2Liq/XAl2O3Liq. In addition, there are maxima or sharp changes in slope for NBO/T > 0.3, which occur for XSiO2Liq/XAl2O3Liq ranging from ∼0 to ∼6 and increase with increasing NBO/T. The systematic variations in activity coefficients as a function of composition and optical basicity reflect underlying shifts in speciation as the composition of the liquid is changed. On the basis of correlations among the activity coefficients, it is likely that the use of CaO, an exchange component such as SiMg−1 and two of MgO, CaAl2O4, or MgAl2O4 would yield significant savings in the number of parameters required to model the excess free energy surface of liquids over large portions of CMAS relative to the use of oxide end members.Systematic behavior of thermodynamic properties extends to small amounts of other elements dissolved in otherwise CMAS liquids. For example, ln(XFe2+Liq/XFe3+Liq) at constant oxygen fugacity is linearly correlated with ln(γCaOLiq). Similarly, ln(CS), where CS is the sulfide capacity is linearly correlated at constant temperature with each of the optical basicity, ln(aCaOLiq) and ln(γCaOLiq), although the correlation for the latter breaks down for low values of Λ. The well-known systematic behavior of sulfide capacity as a function of optical basicity for systems inside as well as outside CMAS suggests that ln(γCaSLiq) is also a simple function of optical basicity and that the relationships observed among the activity coefficients in CMAS may hold for more complex systems.  相似文献