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1.
The techniques of electron paramagnetic resonance (EPR) were used to measure the concentration ratio of Eu2+ to Eu3+ in quenched CaMgSi2O6, Ca3Si3O12, and CaAl2Si2O8 liquids as functions of partial pressure of oxygen and temperature. The redox equilibrium of the Eu ions was described by the reaction 4Eu3+ + 202? = 4Eu2+ + O2. The reduction of Eu3+ to Eu2+ was endothermic, and for CaMgSi2O6 and Ca3Al2Si3O12 liquids the mean value of ΔH0 and the standard deviation from that mean were 25 ± 7 kcal/mole.The magnitude of the Eu anomaly in the distribution coefficients is discussed in terms of the compositions of the solid and liquid phases.  相似文献   

2.
Partitioning of Ni2+, Co2+, Fe2+, Mn2+ and Mg2+ between olivine and silicate melts has been determined near the join (Mg0.5·-Fe0.5)2SiO4-K2O 4SiO2 and for seven different basaltic compositions. The experiments were made at 1 atm total pressure, 1500-1100°C, and under moderate to reducing oxygen fugacities. The concentration factor, defined as KMO = (MO)ol/(MO)liq (molar ratio), increases markedly for all the cations studied as the olivine component of the liquid decreases. Much of the increase in KMO is considered as due to the compositional effect of the coexisting liquid: the temperature effect on KMO is probably opposite to the compositional effect (KMO decreases as temperature decreases).The partition coefficient KMO-MgO = (MO/MgO)ol/(MO/MgO)liq for the reaction, Mol2+ + Mgliq2+ = Mliq2+ + Mgol2+. is relatively constant over a wide range of SiO2 content of the liquid, except in the case of Ni2+. The partition coefficients have similar ranges both in synthetic and natural rock systems: KNiO-MgO = 1.8–3.0, KCoO-MgO = 0.6–0.8, KFeO-MgO = 0.27–0.38, and KMnO-MgO = 0.23–0.32. There is a systematic variation in the partition coefficient KMO-MgO with the composition of liquid; KMO-MgO increases with increasing SiO2 content of melt. The partition coefficients for Co2+, Fe2+ and Mn2+ are useful to test the equilibration of olivine with magma of a wide compositional range.  相似文献   

3.
A new sapphirine-analog, Mg4Ga8Ge2O20, has been identified and characterized in the system MgO-Ga2O3-GeO2 in samples quenched from 1400° C, 1 atm. It crystallizes with a triclinic unit-cell and is isostructural with the mineral sapphirine-1Tc, Mg4Al8Si2O20. It is stable over a broad range of stoichiometry associated with the exchange reaction 2 Ga3+ = Mg2+ }+Ge4+. TEM examination shows the common occurrence of polysynthetic twinning and suggests the existence of a high-temperature triclinic→monoclinic transition. The observation of sapphirine/spinel intergrowths is consistent with the structural relationships described previously.  相似文献   

4.
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   

5.
The two most abundant network-modifying cations in magmatic liquids are Ca2+ and Mg2+. To evaluate the influence of melt structure on exchange of Ca2+ and Mg2+ with other geochemically important divalent cations (m-cations) between coexisting minerals and melts, high-temperature (1470-1650 °C), ambient-pressure (0.1 MPa) forsterite/melt partitioning experiments were carried out in the system Mg2SiO4-CaMgSi2O6-SiO2 with ?1 wt% m-cations (Mn2+, Co2+, and Ni2+) substituting for Ca2+ and Mg2+. The bulk melt NBO/Si-range (NBO/Si: nonbridging oxygen per silicon) of melt in equilibrium with forsterite was between 1.89 and 2.74. In this NBO/Si-range, the NBO/Si(Ca) (fraction of nonbridging oxygens, NBO, that form bonds with Ca2+, Ca2+-NBO) is linearly related to NBO/Si, whereas fraction of Mg2+-NBO bonds is essentially independent of NBO/Si. For individual m-cations, rate of change of KD(m−Mg) with NBO/Si(Ca) for the exchange equilibrium, mmelt + Mgolivine ? molivine + Mgmelt, is linear. KD(m−Mg) decreases as an exponential function of increasing ionic potential, Z/r2 (Z: formal electrical charge, r: ionic radius—here calculated with oxygen in sixfold coordination around the divalent cations) of the m-cation. The enthalpy change of the exchange equilibrium, ΔH, decreases linearly with increasing Z/r2 [ΔH = 261(9)-81(3)·Z/r2 (Å−2)]. From existing information on (Ca,Mg)O-SiO2 melt structure at ambient pressure, these relationships are understood by considering the exchange of divalent cations that form bonds with nonbridging oxygen in individual Qn-species in the melts. The negative ∂KD(m−Mg)/∂(Z/r2) and ∂(ΔH)/∂(Z/r2) is because increasing Z/r2 is because the cations forming bonds with nonbridging oxygen in increasingly depolymerized Qn-species where steric hindrance is decreasingly important. In other words, principles of ionic size/site mismatch commonly observed for trace and minor elements in crystals, also govern their solubility behavior in silicate melts.  相似文献   

6.
Synthetic melilites on the join Ca2MgSi2O7 (åkermanite: Ak)-Ca2Fe3+AlSiO7 (ferrialuminium gehlenite: FAGeh) were studied using X-ray powder diffraction and 57Fe Mössbauer spectroscopic methods to determine the distribution of Fe3+ between two different tetrahedral sites (T1 and T2), and the relationship between ionic substitution and incommensurate (IC) structure. Melilites were synthesized from starting materials with compositions of Ak100, Ak80FAGeh20, Ak70FAGeh30 and Ak50FAGeh50 by sintering at 1,170–1,350 °C and 1 atm. The average chemical compositions and end-member components, Ak, FAGeh and Geh (Ca2Al2SiO7), of the synthetic melilites were Ca2.015Mg1.023Si1.981O7 (Ak100), Ca2.017Mg0.788Fe 0.187 3+ Al0.221Si1.791O7 (Ak78FAGeh19Geh3), Ca1.995Mg0.695Fe 0.258 3+ Al0.318Si1.723O7 (Ak69FAGeh25Geh6) and Ca1.982Mg0.495Fe 0.449 3+ Al0.519Si1.535O7 (Ak49FAGeh44Geh7), respectively. Rietveld refinements using X-ray powder diffraction data measured using CuK α -radiation at room temperature converged successfully with goodness-of-fits of 1.15–1.26. The refined Fe occupancies at the T1 and T2 sites and the Mg and Si contents determined by electron microprobe analysis gave the site populations of [0.788Mg + 0.082Fe3+ + 0.130Al]T1[0.104Fe3+ + 0.104Al + 1.792Si]T2 for Ak78FAGeh19Geh3, [0.695Mg + 0.127Fe3+ + 0.178Al]T1[0.132Fe3+ + 0.144Al + 1.724Si]T2 for Ak69FAGeh25Geh6 and [0.495Mg + 0.202Fe3+ + 0.303Al]T1[0.248Fe3+ + 0.216Al + 1.536Si]T2 for Ak49FAGeh44Geh7 (apfu: atoms per formula unit), respectively. The results indicate that Fe3+ is distributed at both the T1 and the T2 sites. The mean T1–O distance decreases with the substitution of Fe3+ + Al3+ for Mg2+ at the T1 site, whereas the mean T2–O distance increases with substitution of Fe3+ + Al3+ for Si4+ at the T2 site, causing decrease in the a dimension and increase in the c dimension. However, in spite of the successful Rietveld refinements for the X-ray powder diffraction data measured using CuK α-radiation at room temperature, each Bragg reflection measured using CuK α1-radiation at room temperature showed weak shoulders, which were not observed in those measured at 200 °C. The Mössbauer spectra of the melilites measured at room temperature consist of two doublets assigned to Fe3+ at the T1 site and two or three doublets to Fe3+ at the T2 site, implying the existence of multiple T1 and T2 sites with different site distortions. These facts can be interpreted in terms of the IC structure in all synthetic melilites at room temperature, respectively. The results of Mössbauer analysis indicate that the IC structure in melilite is caused by not only known multiple T1 site, but also multiple T2 site at room temperature.  相似文献   

7.
Groundwater of the unconfined aquifer (1,100 sq. km) of a two-tier coastal aquifer located in the Amol–Ghaemshahr plain, Mazandaran Province, Northern Iran, is classified into fresh and brackish water types. Fresh groundwater (FGW) samples (n = 36) are characterized by Ca2+ > Na+ > Mg2+ > K+ and HCO3 ? > Cl? > SO4 2? > NO3 ?. Spearman’s rank correlation coefficient matrices, factor analysis data, values of the C-ratio (av. = 0.89) and CAI and values of the molar ratios of Ca2+/HCO3 ?, Ca2+/SO4 2?, Mg2+/HCO3 ? and Mg2+/SO4 2? indicate that the ionic load in the FGW is derived essentially from carbonic acid-aided weathering of carbonates and aluminosilicates, saline/sea water trapped in the aquifer sediments (now admixed with the groundwater) and ion exchange reactions. Values of the CAI and Na+/Cl? molar ratio suggest that the part of the Ca2+ (±Mg2+) content in 23 FGW samples is derived from clay minerals of the aquifer matrix, and part of the Na+ content in 20, 12, and 3 FGW samples is derived, respectively, from alkali feldspar weathering, clay minerals of the aquifer matrix and rain water and/or halite. Brackish groundwater (BGW) samples (n = 4) contain Cl? as the dominant anion and their average total ionic concentration (38.65 meq/L) is 1.79 times higher than that of the FGW samples (21.50 meq/L). BGW pockets were generated by non-conservative mixing of FGW with the upconed saline water from the underlying saline groundwater zone of the semi-confined aquifer along bore wells involved in excessive extraction of groundwater from the unconfined aquifer. Groundwater belongs essentially to “high salinity, low sodium” irrigation water class.  相似文献   

8.
Calcite dissolution rates were measured as a function of saturation state in NaCl–CaCl2–MgCl2 solutions at 1 bar (0.1 MPa) pCO2 and 25 °C. Rates measured in phosphate- and sulfate-free pseudo-seawater (Ca2+:Mg2+= 0.2, I= 0.7) were compared with those in synthetic brines. The brines were prepared by co-varying calcium and magnesium (Ca2+:Mg2+= 0.9; 2.0; 2.8; 3.1; 4.8; 5.8) along with ionic strength (I= 0.9; 1.1; 1.6; 2.1; 3.0; 3.7; 4.4 m) to yield solutions approximating those of subsurface formation waters. The rate data were modeled using the equation, R = k(1 ? Omega;) n , where k is the empirical rate constant, n describes the order of the reaction and ω is saturation state. For rates measured in the pseudo-seawater, n= 1.5 and k= 4.7 × 10?2 mol m?2 hr?1. In general, rates were not significantly faster in the synthetic brines (n= 1.4 ± 0.2 and k= 5.0 ± 7 × 10?2 mol m?2 hr?1). The rate coefficients agree within experimental error indicating that they are independent of ionic strength and Ca2+:Mg2+ over a broad range of brine compositions. These findings have important application to reaction-transport modeling because carbonate bearing saline reservoirs have been identified as potential repositories for CO2 sequestration.  相似文献   

9.
The occurrence of Cr-Al pairs in Mg2SiO4 has been detected by EPR spectroscopy. In the case where Cr3+ replaces Mg at the M2 position three different neighboring Si sites may be substituted by Al3+, which should yield different superhyperfine interactions. A new spectrum is presented which shows the presence of two of these possible pair configurations. An assignment of the spectral features to a specific Cr-Al pair with Cr at M2 from the experimental data alone was not possible, therefore, MSX α cluster calculations have been performed from which the differences in the superhyperfine interaction for the various pair configurations could be obtained. Best agreement with the data of the Cr3+(M2)-Al pair exhibiting the most intense group of lines in the EPR spectrum was obtained for the situation where Al3+ is at the Si position with the shortest distance to M2. The second observed Cr3+(M2)-Al pair, which is significantly weaker in intensity, could not yet be assigned.  相似文献   

10.
The paper reports original thermochemical data on six natural amphibole samples of different composition. The data were obtained by high-temperature melt solution calorimetry in a Tian–Calvet microcalorometer and include the enthalpies of formation from elements for actinolite Ca1.95(Mg4.4Fe 0.5 2+ Al01)[Si8.0O22](OH)2(–12024 ± 13 kJ/mol) and Ca2.0(Mg2.9Fe 1.9 2+ Fe 0.2 3+ )[Si7.8Al0.2O22](OH)2, (–11462 ± 18 kJ/mol), and Na0.1Ca2.0(Mg3.2Fe 1.6 2+ Fe 0.2 3+ )[Si7.7Al0.3O22](OH)2 (–11588 ± 14 kJ/mol); for pargasite Na0.5K0.5Ca2.0-(Mg3.4Fe 1.8 2+ Al0.8)[Si6.2Al1.8O22](OH)2 (–12316 ± 10 kJ/mol) and Na0.8K0.2Ca2.0(Mg2.8Fe 1.3 3+ Al0.9) [Si6.1Al1.9O22](OH)2 (–12 223 ± 9 kJ/mol); and for hastingsite Na0.3K0.2Ca2.0(Mg0.4Fe 1.3 2+ Fe 0.9 3+ Al0.2) [Si6.4Al1.6O22](OH)2 (?10909 ± 11 kJ/mol). The standard entropy, enthalpy, and Gibbs free energy of formation are estimated for amphiboles of theoretical composition: end members and intermediate members of the isomorphic series tremolite–ferroactinolite, edenite–ferroedenite, pargasite–ferropargasite, and hastingsite.  相似文献   

11.
The interactions of humic substances from Esthwaite Water with hydrous iron oxides (α-FeOOH, α-Fe2O3, amorphous Fe-gel) have been examined by measuring adsorption isotherms and by microelectrophoresis. In Na+-Cl?-HCO3?at I = 0.002 M (medium I) the extent of adsorption decreases with increasing pH. The results are consistent with a mechanism involving ligand exchange of humic anionic groups with H2O and OH?of surface Fe-OH2+and Fe-OH groups respectively, with an increasing degree of protonation of the adsorbed humics as the adsorption density increases at constant pH.At pH 7 in a medium containing Mg2+, Ca2+ and SO42?, at their Esthwaite Water concentrations and at I= 0.002 M (medium II) the adsorption capacity of goethite (α-FeOOH) is approximately twice that in medium I. Electrophoresis experiments show that the extra capacity is associated with coadsorption of Mg2+ and/or Ca2+ ions.When the iron oxides are added to samples of Esthwaite Water itself they become negatively charged and plots of electrophoretic mobility against pH for the natural water are identical to those in medium II plus humics.  相似文献   

12.
One-hundred fluid inclusions in Silurian marine halite were analyzed in order to determine the major-ion composition of Silurian seawater. The samples analyzed were from three formations in the Late Silurian Michigan Basin, the A-1, A-2, and B Evaporites of the Salina Group, and one formation in the Early Silurian Canning Basin (Australia), the Mallowa Salt of the Carribuddy Group. The results indicate that the major-ion composition of Silurian seawater was not the same as present-day seawater. The Silurian ocean had lower concentrations of Mg2+, Na+, and SO42−, and much higher concentrations of Ca2+ relative to the ocean’s present-day composition. Furthermore, Silurian seawater had Ca2+ in excess of SO42−. Evaporation of Silurian seawater of the composition determined in this study produces KCl-type potash minerals that lack the MgSO4-type late stage salts formed during the evaporation of present-day seawater. The relatively low Na+ concentrations in Silurian seawater support the hypothesis that oscillations in the major-ion composition of the oceans are primarily controlled by changes in the flux of mid-ocean ridge brine and riverine inputs and not global or basin-scale, seawater-driven dolomitization. The Mg2+/Ca2+ ratio of Silurian seawater was ∼1.4, and the K+/Ca2+ ratio was ∼0.3, both of which differ from the present-day counterparts of 5 and 1, respectively. Seawaters with Mg2+/Ca2+ <2 facilitate the precipitation of low-magnesian calcite (mol % Mg < 4) marine ooids and submarine carbonate cements whereas seawaters with Mg2+/Ca2+ >2 (e.g., modern seawater) facilitate the precipitation of aragonite and high-magnesian calcite. Therefore, the early Paleozoic calcite seas were likely due to the low Mg2+/Ca2+ ratio of seawater, not the pCO2 of the Silurian atmosphere.  相似文献   

13.
In this work, we have reviewed a large compositional dataset (571 analyses) for natural and experimental glasses to understand the physico-chemical and compositional conditions of magmatic cordierite crystallization. Cordierite crystallizes in peraluminous liquids (A/CNK ≥1) at temperatures ≥750 °C, pressures ≤700 MPa, variable H2O activity (0.1–1.0) and relatively low fO2 conditions (≤NNO ? 0.5). In addition to A/CNK ratio ≥1, a required condition for cordierite crystallization is a Si + Al cation value of the rhyolite liquid of 4 p8O (i.e. calculated on the 8 oxygen anhydrous basis), which is consistent with low Fe3+ contents and the absence or low content of non-bridging oxygens (NBO). This geochemical condition is strongly supported by the rare, if not unique, structure of cordierite where the tetrahedral framework is composed almost exclusively of Si and Al cations the sum of which is equal to 4 p8O [i.e. (Mg,Fe)8/9Al16/9Si20/9O8], indicating that aluminium (and cordierite) saturation is limited by rhyolite liquids with Al = 4 ? Si. Indeed, synthetic or natural systems with Al > 4 ? Si always show metastable glass-in-glass separation or crystallization of refractory minerals such as corundum (Al16/3O8) and aluminosilicates (Al16/5Si8/5O8). Multivariate regression analyses of literature data for experimental glasses coexisting with magmatic cordierite produced two empirical equations to independently calculate the T (±13 °C; ME, maximum error = 29 °C) and P (±16 %; ME% = 27 %) conditions of cordierite saturation. The greatest influence on the two equations is exerted by H2Omelt and Al concentrations, respectively. Testing of these equations with other thermobarometric constraints (e.g. feldspar-liquid, GASP, Grt–Bt and Grt–Crd equilibria) and thermodynamic models (NCKFMASHTO and NCKFMASH systems) was successfully performed for Crd-bearing rhyolites and residual enclaves from San Vincenzo (Tuscany, Italy), Morococala Field (Bolivia) and El Hoyazo (Spain). The reliability of each calculated P–T pair was graphically evaluated using the minimum and maximum P–T–H2O relationships for peraluminous rhyolite liquids modified after the metaluminous relationships in this work. Both P–T calculations and checking can be easily performed with the attached user-friendly spreadsheet (i.e. Crd-sat_TB).  相似文献   

14.
Calcite crystals were grown in a closed system by recrystallization of synthetic and natural aragonite crystals, in the presence of various CaCl2-MgCl2 solutions with and without NaCl.The distribution of Mg2+ between calcite and solution at the entire temperature range is heterogeneous, closely following the Doerner-Hoskins (Doerner and Hoskins, 1925) distribution law. λMg2+C is strongly dependent on temperature, being: 0·0573 ± 0·0017 at 25°C, 0·0681 ± 0·0019 at 35°C, 0·0778 ± 0.0022 at 50°C, 0·0973 ± 0·0021 at 70°C, and 0·1163 ±0 ·0034 at 90°C. λMg2+C is independent of the absolute concentration of Ca2+ in solution as well as of the presence of NaCl.Relatively high λMg2+C values are obtained during the initial reaction stages when too-highly reactive synthetic aragonites are recrystallized. SEM micrographs show that calcite crystals grown from such aragonites are imperfect and that their earlier formed Mg-rich cores redissolve later, resulting in apparently inconsistent λMg2+C values.Calculations applying the new λMg2+C value for 25°C and the solubility data for magnesian calcites (Chaveet al., 1962) demonstrate that although no calcite should be expected to precipate directly from open sea water, its direct precipitation (or recrystallization from aragonite) is possible in closed diagenetic systems which still contain marine solutions, provided a temporary increase in the dissolved calcium concentration takes place.The λMg2+C values obtained allow for a new insight into processes of calcite cementation of reefs and a variety of other carbonate sediments, and for a more precise definition of dedolomitization chemistry.  相似文献   

15.
Using trioctahedral smectites synthesized at low temperature (25 and 75°C). partition coefficients have been determined for M2+ transition metals (Mn, Fe, Co, Ni, Cu, Zn) between octahedral sheets of smectites and water. These coefficients D(M2+?Mg) = (M2+)/(Mg) solid/(M2+)/(Mg) liquid have high values near 104 for Cu, 1000 for Ni, Co, Zn, 300 for Fe and 30 for Mn. All transition metals are strongly stabilized in the magnesian solid phase, even Mn which leads to noncrystallized products. Within the range of experimental uncertainties, it is found that tetrahedral substitution of Si by Al and differences in temperature (from 25 to 75°C) have no influence on partition coefficients. Experimental data are closely related to thermodynamic properties of the cations and on this basis other partition coefficients can be calculated, for the (M2+ ? Fe2+) pair for instance. The behaviour of transition metals is explained using crystal field theory.  相似文献   

16.
《Chemical Geology》2004,203(1-2):139-151
Aragonite is precipitated by a new CO2-diffusion technique from a Ca2+–Mg2+–Cl− solution between 10 and 50 °C. Crystallisation of aragonite instead of calcite occurs by maintaining a [Mg2+]/[Ca2+] ratio of 2 in the fluid. The dissolved inorganic carbon (DIC) is received by diffusion of CO2 through a polyethylene membrane (diffusion coefficient: DCO2=10−6.4 cm2 s−1 at 19 °C). It is suggested that significant amounts of DIC may be transferred by diffusion of CO2 in natural systems if the CO2 gradient is high. The CO2-diffusion technique is used as a kind of simple mixed flow reactor for the co-precipitation of barium and strontium with aragonite. The distribution coefficients of Ba2+ and Sr2+ decrease from 10 to 50 °C according to DBa,a*=2.42−0.03595T (°C) and DSr,a*=1.32−0.005091T (°C). At 25 °C, the distribution coefficients are DBa,a*=1.5±0.1 and DSr,a*=1.19±0.03. The effect of temperature on DBa,a* is about one order of magnitude higher versus that on DSr,a*. Thus, Ba2+ may be a potential paleotemperature indicator if the composition of the solution is known.  相似文献   

17.
The dissolution behavior of natural, ordered kutnahorite (Mn1.14Ca0.82Mg0.04Fe0.012(CO3)2) and a disordered, calcian rhodochrosite (Mn1.16Ca0.78Mg0.06(CO3)2) precipitated in the laboratory was investigated in deionized distilled water and artificial seawater in both open and closed systems at 25 °C, one atmosphere total pressure, and various pCO2s. Both solids dissolved congruently in distilled water in an open system and yielded identical long-term equilibration or extrapolated ion activity products, IAPpkt = aCa 2+aMn 2+(aCO 3 2?)2 = 1.7 (±0.12)× 10?21 or pIAPpkt = 20.77 (±0.03). This value is believed to be the thermodynamic solubility product of pseudokutnahorite. In contrast, the steady state ion concentration products, ICPpkt = [Ca2+][Mn2+][CO3 2?]2, measured following the dissolution of both minerals in artificial seawater increase as the CO2 partial pressure decreases and the [Mn2+]:[Ca2+] ratio increases. These observations are interpreted as resulting from the formation of phases of different stoichiometry in response to large variations of the [Mn2+]:[Ca2+] ratio in solution. These data and results of calcite-seawater equilibration experiments in the presence of various dissolved Mn(II) concentrations define the fields of stability of manganoan calcites and calcian rhodochrosites in seawater within Lippmann phase diagrams for the CaCO3–MnCO3–H2O system. Results of this study reveal that the nature (i.e., mineralogy) and composition of manganese-rich carbonate phases that may form under suboxic/anoxic conditions in marine sediments are dictated by the porewater [Mn2+]:[Ca2+] ratio, the abundance of calcite surfaces and reaction kinetics.  相似文献   

18.
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 DEu approaches the value of DSr.  相似文献   

19.
Growing recognition of triple-chain silicates in nature has prompted experimental research into the conditions under which they can form and the extent of solid solution that is feasible for some key chemical substitutions. Experiments were done primarily in the range of 0.1–0.5 GPa and 200–850 °C for durations of 18–1,034 h. A wide range of bulk compositions were explored in this study that can be classified broadly into two groups: those that are Na free and involve various possible chemical substitutions into jimthompsonite (Mg10Si12O32(OH)4), and those that are Na bearing and involve chemical substitutions into the ideal end-member Na4Mg8Si12O32(OH)4. Numerous attempts to synthesize jimthompsonite or clinojimthompsonite were unsuccessful despite the type of starting material used (reagent oxides, magnesite + SiO2, talc + enstatite, or anthophyllite). Similarly, the chemical substitutions of F− for OH−, Mn2+, Ca2+, or Fe2+ for Mg2+, and 2Li+ for Mg2+ and a vacancy were unsuccessful at nucleating triple-chain silicates. Conversely, nearly pure yields of monoclinic triple-chain silicate could be made at temperatures of 440–630 °C and 0.2 GPa from the composition Na4Mg8Si12O32(OH)4, as found in previous studies, though its composition is most likely depleted in Na as evidenced by electron microprobe and FTIR analysis. Pure yields of triple-chain silicate were also obtained for the F-analog composition Na4Mg8Si12O32F4 at 550–750 °C and 0.2–0.5 GPa if a flux consisting of Na-halide salt and water in a 2:1 ratio by weight was used. In addition, limited chemical substitution could be documented for the substitutions of 2 Na+ for Na+ + H+ and of Mg2+ + vacancy for 2Na+. For the former, the Na content appears to be limited to 2.5 cations giving the ideal composition of Na2.5Mg8Si12O30.5(OH)5.5, while for the latter substitution the Na content may go as low as 1.1 cations giving the composition Na1.1Mg9.4Si12O31.9(OH)4.1 based on a fixed number of Si cations. Further investigation involving Mg for Na cation exchange may provide a pathway for the synthesis of Na-free clinojimthompsonite. Fairly extensive solid solution was also observed for triple-chain silicates made along the compositional join Na4Mg8Si12O32(OH)4–Ca2Mg8Si12O32(OH)4 where the limit of Ca substitution at 450 °C and 0.2 GPa corresponds to Na0.7Ca1.8Mg7.8Si12O31.9(OH)4.1 (with the OH content adjusted to achieve charge balance). Aside from the Na content, this composition is similar to that observed as wide-chain lamellae in host actinolite. The relative ease with which Na-rich triple chains can be made experimentally suggests that these phases might exist in nature; this study provides additional insights into the range of compositions and formation conditions at which they might occur.  相似文献   

20.
The short term (2–40 days) dissolution of enstatite, diopside, and tremolite in aqueous solution at low temperatures (20–60°C) and pH 1–6 has been studied in the laboratory by means of chemical analyses of reacting solutions for Ca2+, Mg2+, and Si(OH)4 and by the use of X-ray photoelectron spectroscopy (XPS) for detecting changes in surface chemistry of the minerals. All three minerals were found to release silica at a constant rate (linear kinetics) providing that ultrafine particles, produced by grinding, were removed initially by HF treatment. All three also underwent incongruent dissolution with preferential release of Ca and/or Mg relative to Si from their outermost surfaces. The preferential release of Ca, but not Mg for diopside at pH 6 was found by both XPS and solution chemistry verifying the theoretical prediction of greater mobility of cations located in M2 structural sites. Loss mainly from M2 sites also explains the degree of preferential loss of Mg from enstatite at pH 6; similar structural arguments apply to the loss of Ca and Mg from the surface of tremolite. In the case of diopside and tremolite initial incongruency was followed by essentially congruent cation-plus-silica dissolution indicating rapid formation of a constant-thickness, cation-depleted surface layer. Cation depletion at elevated temperature and low pH (~ 1) for enstatite and diopside was much greater than at low temperature and neutral pH, and continued reaction resulted in the formation of a surface precipitate of pure silica as indicated by solubility calculations, XPS analyses, and scanning electron microscopy.From XPS results at pH 6, model calculations indicate a cation-depleted altered surface layer of only a few atoms thickness in all three minerals. Also, lack of shifts in XPS peak energies for Si, Ca, and Mg, along with undersaturation of solutions with respect to all known Mg and Ca silicate minerals, suggest that cation depletion results from the substitution of hydrogen ion for Ca2+ and/or Mg2+ in a modified silicate structure and not from the precipitation of a new, radically different surface phase. These results, combined with findings of high activation energies for dissolution, a non-linear dependence on aH+ for silica release from enstatite and diopside, and the occurrence of etch pitting, all point to surface chemical reaction and not bulk diffusion (either in solution or through altered surface layers) as the rate controlling mechanism of iron-free pyroxene and amphibole dissolution at earth surface temperatures.  相似文献   

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