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1.
To assist in the assignment and interpretation of 23Na NMR spectra in silicate and aluminosilicate minerals and glasses we have calculated the 23Na NMR shieldings and the electric field gradients (EFG) at the Na for a number of Na-containing species. Included are Na(OH2) n +, n = 1, 2, 4, 5, 6 and 8, and Na+ complexes with SiH3OH, SiH3ONa and O(SiH3)2. We have also evaluated shieldings and EFGs for Na-centered clusters extracted from crystalline Na2SiO3 and anhydrous sodalite, Na6[AlSiO4]6. Using 6-31G* SCF optimized geometries and the GIAO method with a 6-31G* basis set [and 6-311(2d,p) bases for the smaller clusters] we find a calculated increase in shielding with coordination number (CN) for the Na(OH2) n +, n = 4, 6, 8 series that agrees reasonably well with experimental trends. Calculated changes in the Na shielding as water is replaced by bridging or nonbridging silicate O atoms are also consistent with experimental observations. The deshielding of Na (with respect to gas-phase Na+) which is produced by an O-containing ligand is a strongly decreasing function of the R(Na–O) and a weakly decreasing function of the underbonding or free valence of the O. Deshielding contributions to the isotropic shielding from different ligands are additive to good approximation for low CN species, so that the total deshielding can be calculated accurately by summing the contributions from the individual ligands. However for the larger CN species the directly calculated deshieldings are substantially smaller than those obtained using such an additivity approximation. We further test this approximation by calculating the deshieldings for Na in 12 different sites in silicate and aluminosilicate minerals which have recently been studied experimentally, using our calculated deshielding contributions for individual O-containing ligands and experimental values for the Na–O distances. Correlation coefficients between the experimental shifts and the calculated deshieldings are around 0.9 and the slope of the correlation is almost 1.0 . Calculations on large Na-centered clusters extracted from the crystal structures of Na2SiO3 and anhydrous sodalite reproduce the experimental values for both NMR shieldings and electric field gradients but at considerable computational cost. Comparison with recent 23Na NMR studies on hydrous albite glasses indicates that coordination of either H2O or OH− to the Na could give the magnitude of deshielding observed, depending upon the detailed Na–O distances within the hydrous glass. Received: 31 December 1998 / Revised, accepted: 11 May 1999  相似文献   

2.
29Si NMR studies on synthetic single crystal forsterite have shown an isotropic NMR shift of –63.2 ppm relative to tetramethyl silane and shift tensor components of –24.4, –7.9 and +32.2 ppm relative to this isotropic value. The most shielded component (+32.2 ppm) lies close to the Si-O1 vector, where Si-O1 is the shortest bond, R(Si-O1)=1.614Å (Weiden and Rager, Z. Naturforsch 40a, 126 (1985)). Ab initio self consistent field molecular orbital calculations using the Random Phase Approximation Localized-Orbital Local-Origin (RPA LORG) method and a polarized split valence basis set yield shift tensor components of –28.9, +2.3 and +26.6 ppm relative to the isotropic value, with the most shielded component 17.6° from the Si-O1 axis. This good agreement is obtained for a SiO 4 4– cluster with forsterite local geometry stabilized by four +1 point charges, PC, with the 相似文献   

3.
We have measured the isotropic chemical shifts (δiso) and the spin-lattice relaxation times (T1) for 17O and 25Mg in MgO from room temperature up to 1300° C. The 17O chemical shifts increase linearly from 47 ppm at room temperature to 57 ppm at 1300° C, and over the same temperature range the 25Mg chemical shift increases linearly from 25 to 27 ppm. These changes are not the result of changes in the bulk magnetic susceptibility of the samples, but may be due to increased orbital overlap which is the result of the increase in thermal vibration of the ions with temperature. In the case of 25Mg, the shift to lower shielding with increasing temperature is opposite to that expected from simple bond length versus chemical shift trends established for the oxides at room temperature. If this is a general phenomenon, high-temperature NMR data may be biased to lower shielding. Spin-lattice relaxation times (T1) were measured in order to study the energetics of defect motion. T1's for 17O and 25Mg exhibit similar behavior over the range of temperatures studied. Up to 800° C, T1's decrease gradually, but above 800° C, T1's drop rapidly, with slopes corresponding to apparent activation energies of 192±9 kJ/mol (2.0±0.1 eV) for 17O and 151±6 kJ/mol (1.56±0.06 eV) for 25Mg. While direct comparison of these activation energies to those derived from diffusion or conductivity measurements is complicated, the similar behavior for both nuclei suggests their relaxation phenomena are related.  相似文献   

4.
Neutron powder diffraction data of phase A (Mg7Si2O8(OH)6) were collected at ambient pressure and 3.2?GPa (calculated from the compressibility of phase A) from the deuterated compound, and the structure was refined using the Rietveld method. The derived crystal structure implies that hydrogen atoms occupy two distinct sites in phase A, both forming hydrogen bonds of different lengths with the same oxygen atom. This picture is supported by IR spectra, which exhibit two absorption bands at 3400 and 3513?cm?1 corresponding to OH stretching vibrations, and proton NMR spectra, which display two peaks with equal intensities and isotropic chemical shifts of 3.7 and 5?ppm. The D-D distance [D(1)-D(2) distance] at ambient pressure was found to be 2.09?±?0.02?Å from the neutron diffraction data and 2.09?±?0.05?Å from the NMR spectra. At 3.2?GPa, there is no statistically significant increase in the O-D interatomic distance while the hydrogen bonding interaction D···O appears to increase for one of the hydrogen sites, D(1), which has the stronger hydrogen bonding interaction compared with the other hydrogen, D(2), at ambient pressure. The O-D bond valences, determined indirectly from the D···O distances were 0.86 and 0.91 at ambient pressure, and 0.83 and 0.90?at 3.2?GPa, for D(1) and D(2), respectively.  相似文献   

5.
A revised model for the volume and thermal expansivity of K2O-Na2O-CaO-MgO-Al2O3-SiO2 liquids, which can be applied at crustal magmatic temperatures, has been derived from new low temperature (701–1092 K) density measurements on sixteen supercooled liquids, for which high temperature (1421–1896 K) liquid density data are available. These data were combined with similar measurements previously performed by the present author on eight sodium aluminosilicate samples, for which high temperature density measurements are also available. Compositions (in mol%) range from 37 to 75% SiO2, 0 to 27% Al2O3, 0 to 38% MgO, 0 to 43% CaO, 0 to 33% Na2O and 0 to 29% K2O. The strategy employed for the low temperature density measurements is based on the assumption that the volume of a glass is equal to that of the liquid at the limiting fictive temperature, T f ′. The volume of the glass and liquid at T f ′ was obtained from the glass density at 298 K and the glass thermal expansion coefficient from 298 K to T f ′. The low temperature volume data were combined with the existing high temperature measurements to derive a constant thermal expansivity of each liquid over a wide temperature interval (767–1127 degrees) with a fitted 1 error of 0.5 to 5.7%. Calibration of a linear model equation leads to fitted values of Vˉ i ±1 (cc/mol) at 1373 K for SiO2 (26.86 ± 0.03), Al2O3 (37.42±0.09), MgO (10.71±0.08), CaO (15.41±0.06), Na2O (26.57±0.06), K2O (42.45 ± 0.09), and fitted values of dVˉ i /dT (10−3 cc/mol-K) for MgO (3.27±0.17), CaO (3.74±0.12), Na2O (7.68±0.10) and K2O (12.08±0.20). The results indicate that neither SiO2 nor Al2O3 contribute to the thermal expansivity of the liquids, and that dV/dT liq is independent of temperature between 701 and 1896 K over a wide range of composition. Between 59 and 78% of the thermal expansivity of the experimental liquids is derived from configurational (vs vibrational) contributions. Measured volumes and thermal expansivities can be recovered with this model with a standard deviation of 0.25% and 5.7%, respectively. Received: 2 August 1996 / Accepted: 12 June 1997  相似文献   

6.
Hartree-Fock and B3LYP NMR calculations were performed at the 6-311+G(2df,p) level on cluster models representing albite glasses using B3LYP/6 to 31G* optimized geometries. Calculation results on several well-known crystalline materials, such as low albite and KHSi2O5, were used to check the accuracy of the calculation methods.Calculated 29Si-NMR results on clusters that model protonation of Al-O-Si linkages and the replacement of Na+ by H+ indicate a major increase in Si-O(H) bond length and a 5 ppm difference in δiso for 29Si compared to that for anhydrous albite glass. The calculated δiso of 27Al in such linkages agrees with the experimental data, but shows an increase in Cq that cannot be fully diminished by H-bonding to additional water molecules. This protonation model is consistent with both experimental 17O NMR data and the major peak of 1H-NMR spectra. It cannot readily explain the existence of the small peak in the experimental 1H spectra around 1.5 ppm. Production of the depolymerized units Al [Q3]-O-H upon the dissolution of water is not consistent with 27Al, 1H, or 17O NMR experimental results. Production of Si [Q3]-O-H is consistent with all of the experimental 17O and 1H-NMR data; such units can produce both the major peak at 3.5 ppm and the small peak at 1.5 ppm in 1H spectra, either with or without hydrogen bonding. This species, however, cannot produce the main features of 29Si spectra.It is concluded that although neither protonation nor the production of Si [Q3]-O-H alone is consistent with the available experimental data, the combination of these two processes is consistent with available experimental NMR data.  相似文献   

7.
Variations in the 17O nuclear quadrupole coupling constant, NQCC, and the 17O NMR shielding constant, σO, are evaluated for bridging oxygens in H3T-O-T′H3 linkages (with T, T′=Al, Si, P), and for nonbridging O in SiH3O?, SiH3OH and SiH3OMg+ and the 19F NMR shielding constant, σF, is evaluated for bridging F in H3SiFSiH 3 ? using Hartree-Fock methods with large, flexible Gaussian basis sets. Trends in 17O NQCC as a function of T and T′ identity agree with experiment but the value for the Si-O-Al case is underestimated, indicative of neglected contributions from charge compensating cations. For H3SiOSiH3 the decrease in NQCC over the range from 180° to 140° is substantial but somewhat slower than the variation of -cosSi and σO decrease as Si of the nonbridging oxygen of SiH3O? compared to the bridging oxygen of H3SiOSiH3 but the calculated σSi in H3SiOAlH 3 ? is too large and σO too small, indicative of important contributions from counter ions. By contrast, σO for PH3OAlH3 compared to SiH3OSiH3 is consistent with experiment. In H3SiFSiH 3 + (a model for bridging F in amorphous Si:H:F) the value of σSi is smaller and the NQCC at F is considerably larger than for H3SiF, suggesting distinctive 29Si and 19F NMR spectra for this species.  相似文献   

8.
17O NMR shieldings are calculated for the central O in the molecular model OM6(OH)12 –2, for crystalline alkaline earth oxides, MO, where M=Mg, Ca, and Sr, using both Hartree–Fock and hybrid Hartree–Fock density-functional theory. Agreement of calculated and experimental NMR shifts of CaO and SrO compared to MgO is good, but only if the basis set on the M atoms has sufficient tight d polarization functions. Preliminary results are also presented for nonbridging O in the silicate Si(OH)3O– anion, perturbed by alkaline earth cations, giving trends which agree qualitatively with experiment.  相似文献   

9.
We present NMR spectroscopic data, obtained by 1H MAS, 1H static spin-echo, and 29Si{1H} CP-MAS techniques, for a series of hydrous magnesium silicate samples synthesized at high pressure. This series includes chondrodite, β-Mg2SiO4, and phases A, B, superhydrous B, and E. Phases B and superhydrous B give very narrow 29Si NMR peaks and display the most de-shielded SiVI chemical shifts yet reported: ?170.4?ppm for B and ?166.6 for superhydrous B. The 1H NMR spectra of B and superhydrous B confirm the presence of paired hydroxyls, as determined from refinement of the H positions from X-ray diffraction data. The 1H MAS NMR spectra of phase B contain peaks for the two distinct hydrogen positions, with chemical shifts of +4.7 and +3.3?ppm. The static 1H spectrum contains a powder pattern characteristic of a strongly coupled hydrogen pair, from which a dipolar coupling constant of 18.6(4)?kHz and inter-hydrogen distance of d(H–H)=1.86(2)?Å were obtained. Superhydrous B appears to give two poorly resolved 1H MAS peaks, consistent with the presence of two distinct hydrogen pairs in the P21 mn crystal structure. Analysis of its spin-echo spectrum gives d(H–H)=1.83(3)?Å, slightly shorter than for phase B. β-Mg2SiO4, coexisting with phases B and superhydrous B, appears to give 29Si{1H} CP-MAS signal, indicating that it contains significant H concentration. The 29Si chemical shifts for phases B, superhydrous B, and chondrodite, together with those reported previously for other Mg-silicates, show a good correlation with structural parameters.  相似文献   

10.
29Si NMR shieldings have been calculated by ab initio coupled Hartree-Fock perturbation theory for the orthosilicate species Si(OH)4, Si(OH)3O-, Si(OH)2O 2 2- , Si(OH)O 3 3- and SiO 4 4- using energy optimized geometries from ab initio Hartree-Fock calculations. The shielding of Si(OH)3O- is smaller than that of Si(OH)4 by 2.7 ppm and the shielding trend along the Si(OH)4 to SiO 4 4- series is decidedly nonlinear. The unprotonated O in Si(OH)3O- is more shielded and has a much smaller electric field gradient than do the protonated oxygens. Calculated anisotropies show the largest components of the shielding to lie along or near the short Si-O bonds. Calculations employing point charges in place of H reproduce the shielding trends in T d and S 4 symmetry Si(OH)4 semiquantitatively. The calculated trends in shielding with < si-o-h=" can=" also=" be=" correlated=" with=" the=" energies=" of=" the=" si3p,o2p="> bonding orbitals.  相似文献   

11.
Carbon and oxygen isotopic analyses have been performed on live-stained aragonitic and calcitic benthic foraminifera and dissolved inorganic carbon (DIC) from the Southern California Borderland to examine carbon isotopic fractionation in foraminifera. Temperature, salinity and pH data have also been collected to permit accurate determination of the δ13C of bicarbonate ion and thus aragonite-HCO3 and calcite-HCO?3 isotopic enrichment factors (?ar-b and ?cl-b, respectively). Only species which precipitate in 18O equilibrium have been considered.?ar-b values based on Hoeglundina elegans range from 1.9%. at 2.7°C to 1.1%. at 9.5°C. Only the lower temperature values agree with a tentative carbon isotope equilibrium equation for aragonite based on the data of Rubinson and Clayton (1969) and Emrich et al. (1970). The temperature dependence of ?ar-b is considerably greater than the equilibrium equation would predict and may be due to a vital effect.The calcitic foraminifera Cassidulina tortuosa, Cassidulina braziliensis, and Cassidulina limbata, Bank and Terrace dwellers, have similar δ13C values and yield an average ?cl-b value of ?0.2 ± .1%. between 8° and 10°C. Calcitic Uvigerina curticosta, Uvigerina peregrina, and megalospheric B. argentea, Slope and Basin dwellers, are ?0.7 ± .1%. enriched relative to ambient bicarbonate for 3 to 9°C. No temperature dependence for ?cl-b was observed for the species in either habitat. The ?cl-b values for Cassidulina species are close (± 0.3%.) to the values given by the tentative equilibrium curve for calcite, while Uvigerina and Bolivina species give values 0.2–0.8%. less. The ?cl-b difference between the Cassidulina species and the Uvigerina and Bolivina species is attributed to the incorporation of 13C-depleted pore water DIC by the latter group rather than to taxonomic or temperature differences.  相似文献   

12.
Partition coefficients for the rare earth elements (REE) Ce, Sm and Tm between coexisting garnets and hydrous liquids have been determined at high pressure and temperatures (30 kbar and 1300 and 1500°C). Two synthetic systems were studied, Mg3Al2Si3O12-H2O and Ca3Al2Si3O12-H2O, in addition to a natural pyrope-bearing system.Deviations from Henry's Law behaviour occur at geologically relevant REE concentrations. At concentrations < 3 ppm Ce, < 12 ppm Sm, < 80 ppm Tm in pyrope and < 100 ppm Ce, < 250 ppm Sm, < 1000 ppm Tm in grossular (at 30 kbar and 1300°C), Dgarnet liquidREE increases as the REE concentration in the garnet decreases. At higher concentrations, DREE is constant. Dgrossular liquidREE also constant when the garnet contains less than about 2 ppm Sm or Tm. The REE concentration at which DREE becomes constant increases with increasing temperature, decreasing REE ionic radius and increasing Ca content of the garnet.Partitioning behaviour of Ce, Sm and Tm between a natural pyrope-rich garnet and hydrous liquid is analogous to that in the synthetic systems and substantiates the substitution model proposed by Harrison and Wood (1980).Values of DREEgarnet/liquid for which Henry's Law is obeyed are systematically higher for grossular than for pyrope (Dpyrope/liquid = 0.067(Ce), 0.108(Sm), 0.155(Tm) and Dgrossular/Liquid = 0.65(Ce), 0.75(Sm), 4.55(Tm).The implications of non-Henry's Law partitioning of REE for models of basalt petrogenesis involving garnet are far-ranging. Deviations from Henry's Law permit refinements to be made to calculated REE abundances once basic model parameters have been defined.  相似文献   

13.
High-resolution aluminum-27 and silicon-29 nuclear magnetic resonance spectra of natural and synthetic imogolites and allophanes obtained using high-field“magic-angle” sample-spinning (MASS) techniques indicate that the imogolite and protoimogolite components of allophanes are characterized by sharp (≈3 ppm) silicon-29 resonances at ?78±1 ppm from tetramethylsilane (in accord with Barron et al. 1982), and quite narrow (≈10 ppm at 11.7 Tesla) aluminum-27 resonances, at 5.2±1 ppm from Al(H2O) 6 3+ (in accord with Wilson et al. 1984). However, the spectra of natural allophanes usually contain significant intensity arising from a less well defined material, characterized by a broad (≈20 ppm) silicon-29 resonance centered at ?90±2 ppm from tetramethylsilane, and a second relatively narrow (≈15 ppm at 11.7 Tesla) aluminum-27 resonance at 58.5±2 ppm from Al(H2O) 6 3+ . Similar characteristic spectral features are exhibited by a synthetic amorphous Si:Al (1:1) gel, and presumably indicate the presence of framework aluminosilicate materials in the gel, and in most allophanes.  相似文献   

14.
The Let?eng-la-Terae kimberlite (Lesotho), famous for its large high-value diamonds, has five distinct phases that are mined in a Main and a Satellite pipe. These diatreme phases are heavily altered but parts of a directly adjacent kimberlite blow are exceptionally fresh. The blow groundmass consists of preserved primary olivine with Fo86?88, chromite, magnesio-ulvöspinel and magnetite, perovskite, monticellite, occasional Sr-rich carbonate, phlogopite, apatite, calcite and serpentine. The bulk composition of the groundmass, extracted by micro-drilling, yields 24–26 wt% SiO2, 20–21 wt% MgO, 16–19 wt% CaO and 1.9–2.1 wt% K2O, the latter being retained in phlogopite. Without a proper mineral host, groundmass Na2O is only 0.09–0.16 wt%. However, Na-rich K-richterite observed in orthopyroxene coronae allows to reconstruct a parent melt Na2O content of 3.5–5 wt%, an amount similar to that of highly undersaturated primitive ocean island basanites. The groundmass contains 10–12 wt% CO2, H2O is estimated to 4–5 wt%, but volatiles and alkalis were considerably reduced by degassing. Mg# of 77.9 and 530 ppm Ni are in equilibrium with olivine phenocrysts, characterize the parent melt and are not due to olivine fractionation. 87Sr/86Sr(i)?=?0.703602–0.703656, 143Nd/144Nd(i)?=?0.512660 and 176Hf/177Hf(i)?=?0.282677–0.282679 indicate that the Let?eng kimberlite originates from the convective upper mantle. U–Pb dating of groundmass perovskite reveals an emplacement age of 85.5?±?0.3 (2σ) Ma, which is significantly younger than previously proposed for the Let?eng kimberlite.  相似文献   

15.
Quartz from sandstone‐type uranium deposits in the east part of the Ordos Basin contains abundant secondary fluid inclusions hosted along sealed fractures or in overgrowths. These inclusions consist mainly of water with NaCl, KCl, CO2 (135–913 ppm) and trace amounts of CO (0.22–16.8 ppm), CH4 (0.10–1.38 ppm) and [SO4]2? (0.35–111 ppm). Homogenization temperatures of the studied fluid inclusions range from 90 to 210°C, with salinities varying from 0.35 to 12.6 wt‐% (converted to NaCl wt%), implying multiple stages of thermal alteration. Although high U is associated with a high homogenization temperature in one case, overall U mineralization is not correlated with homogenization temperature nor with salinity. The H and O isotopic compositions of fluid inclusions show typical characteristics of formation water, with δ18O ranging from 9.8 to 12.3‰ and δD from 26.9 to ?48.6‰, indicating that these fluid inclusions are mixtures of magmatic and meteoric waters. The oxygen isotope ratios of carbonates in cement are systematically higher than those of the fluid inclusions. Limited fluid inclusion‐cement pairs show that the oxygen closely approaches equilibrium between water and aragonite at 150°C. Highly varied and overall negative δ13C in calcite from cement implies different degrees of biogenetic carbon involvement. Correlations between U in bulk rocks and trace components in fluid inclusions are lacking; however, high U contents are typically coupled with high [SO4]2?, implying pre‐enrichment of oxidized materials in the U mineralization layer. All these relationships can be plausibly interpreted to indicate that U (IV), [SO4]2? as well as Na, K were washed out from the overlying thick sandstone by oxidizing meteoric water, and then were reduced by reducing agents, such as CH4 and petroleum, likely from underlying coal and petroleum deposits, and possibly also in situ microbes at low temperatures.  相似文献   

16.
We have conducted detailed studies of the behavior of carbon on the surfaces of MgO and olivine single crystals using various surface analytical techniques: viz. secondary ion mass spectrometry (SIMS), Auger electron spectrometry (AES) and X-ray photoelectron spectrometry (XPS). In order to distinguish without ambiguity the effect of diffusion of carbon from the bulk to the surface and the effect of surface contamination by carbon-containing species, the experiments were conducted in ultrahigh vacuum, i.e. 10?11–10?9 torr. In addition to MgO and olivine single crystals, we have conducted the same studies on TiO2, MnO, SiO2 and Ta2O5 which serve as blank samples. The MgO and olivine samples were also intentionally implanted with known doses of carbon and the mobility of this particular carbon was investigated in detail. Our results show that the bulk carbon content in MgO is around 40 wt. ppm, considerably lower than the quantities quoted by Freund and co-workers in the past. We also show that the carbon in both MgO and olivine does not display any rapid diffusion behaviour leading to surface segreation in the temperature range 78–723 K, in contrast to the previous findings of Freund and co-workers.  相似文献   

17.
Theory is derived from the work of Urey (Urey H. C. [1947] The thermodynamic properties of isotopic substances. J. Chem. Soc. 562-581) to calculate equilibrium constants commonly used in geochemical equilibrium and reaction-transport models for reactions of individual isotopic species. Urey showed that equilibrium constants of isotope exchange reactions for molecules that contain two or more atoms of the same element in equivalent positions are related to isotope fractionation factors by α = (Kex)1/n, where n is the number of atoms exchanged. This relation is extended to include species containing multiple isotopes, for example 13C16O18O and 1H2H18O. The equilibrium constants of the isotope exchange reactions can be expressed as ratios of individual isotope equilibrium constants for geochemical reactions. Knowledge of the equilibrium constant for the dominant isotopic species can then be used to calculate the individual isotope equilibrium constants.Individual isotope equilibrium constants are calculated for the reaction CO2g = CO2aq for all species that can be formed from 12C, 13C, 16O, and 18O; for the reaction between 12C18O2aq and 1H218Ol; and among the various 1H, 2H, 16O, and 18O species of H2O. This is a subset of a larger number of equilibrium constants calculated elsewhere (Thorstenson D. C. and Parkhurst D. L. [2002] Calculation of individual isotope equilibrium constants for implementation in geochemical models. Water-Resources Investigation Report 02-4172. U.S. Geological Survey). Activity coefficients, activity-concentration conventions for the isotopic variants of H2O in the solvent 1H216Ol, and salt effects on isotope fractionation have been included in the derivations. The effects of nonideality are small because of the chemical similarity of different isotopic species of the same molecule or ion. The temperature dependence of the individual isotope equilibrium constants can be calculated from the temperature dependence of the fractionation factors.The derivations can be extended to calculation of individual isotope equilibrium constants for ion pairs and equilibrium constants for isotopic species of other chemical elements. The individual isotope approach calculates the same phase isotopic compositions as existing methods, but also provides concentrations of individual species, which are needed in calculations of mass-dependent effects in transport processes. The equilibrium constants derived in this paper are used to calculate the example of gas-water equilibrium for CO2 in an acidic aqueous solution.  相似文献   

18.
The iron-magnesium distribution coefficient, $$K'_D = (X_{\Sigma {\text{FeO}}} /X_{{\text{MgO}}} )^{{\text{olivine}}} (X_{{\text{MgO}}} /X_{\Sigma {\text{FeO}}} )^{{\text{liquid}}} ,$$ has frequently been used as a means of testing whether experimental and natural silicate liquids could have been in equilibrium with olivine of mantle composition. It is shown here that this K′ D decreases with increasing oxygen fugacity (xxx) for a hydrous partial melt in equilibrium with a natural spinel peridotite assemblage under pressure and temperature conditions corresponding to those of the upper mantle (from 0.52 at the xxx of the iron-wüstite buffer to 0.04 at the xxx of the magnetite-hematite buffer). K′ D also increases with increasing pressure, with decreasing temperature, and probably with increasing Mg/(Mg+∑ Fe) of the parental peridotite, suggesting that $$K_D = (X_{{\text{FeO}}} /X_{{\text{MgO}}} )^{{\text{olivine}}} (X_{{\text{MgO}}} /X_{{\text{FeO}}} )^{{\text{liquid}}}$$ also increases with increasing pressure and decreasing temperature. Thus, unless these four variables (P, T, xxx, silicate composition) are known for a natural magma, K′ D and probably K D are variables, and the Mg/(Mg+∑ Fe) of such a magma cannot be correlated to that of the parent. The K D determined at 1 atm pressure by Roeder and Emslie has frequently been used to test whether the Mg/(Mg+∑ Fe) ratios of experimentally formed liquids at high pressure in equilibrium with olivine of known Fo content represent the equilibrium Mg/(Mg+Fe2+) of this liquid, assuming that ∑Fe=Fe2+ and that K′ D does not vary with P, T, and composition of the system. Published data demonstrate that the oxygen fugacities of the experimental designs employed by different laboratories vary between those of the magnetite-hematite and magnetite-wüstite buffers (6 orders of magnitude), resulting in K′ D between 0.04 and 0.31 at 1050° C and 15 kbar, for example. Thus, published arguments as to whether the quenched liquids represent equilibrium compositions based on iron-magnesium partitioning are inadequate. The effects of P, T, xxx, and the composition of the starting material must also be considered.  相似文献   

19.
The existence and structures of various polyanion species, A k n? ,occurring in inorganic compounds and minerals are rationalized using qualitative molecular orbital (MO) theory. Polyanions which may be stabilized by high pressure or lattice defects are discussed and compounds likely to show such species are identified. Previously observed high pressure properties of Fe riebeckite and MgO are interpreted in terms of O - O bond formation at high pressure.  相似文献   

20.
After its initial synthesis as the new compound Mg2Al3B2O9(OH) (Daniels et al. 1997) pseudosinhalite has now been discovered as a new mineral. It occurs, together with hydrotalcite, as a replacement product of sinhalite, MgAlBO4, in an impure marble of the contact metasomatic iron boron deposit of Tayozhnoye in the Aldan Shield of Siberia. Its chemical composition determined by electron microprobe is (wt%): Al2O3 46.88; MgO 25.12; FeO 1.99; B2O3 (calculated) 21.75; H2O (calculated) 2.81 giving a total of 98.55 and leading to the empirical formula (Mg2.00 Fe2+ 0.09)Σ=2.09 Al2.94 B2O9(OH). The small deviation from the ideal stoichiometry with (Mg?+?Fe2+):Al?≠?2:3 may be caused by either solid solution towards, or submicroscopic interlayering with lamellae of, the structurally similar mineral sinhalite. The underlying substitution involving also B and H would be (Mg?+?Fe)+?B=Al+2H. Pseudosinhalite is monoclinic, space group P21/c, with a=7.49(1), b=4.33(1), c=9.85(2) Å; β=110.7(1)°; V?=?299(1) Å3; Z?=?2. Calculated density is 3.508?g/cm3. Pseudosinhalite is colourless with white streak and has a vitreous lustre. It is transparent; no fluorescence was detected. There is no cleavage and parting; fractures are concoidal. Optical constants could not be measured properly due to polysynthetic microtwinning, but α<1.72<γ. For synthetic pseudosinhalite α=1.691(1); β=1.713(1); γ=1.730(1); Δ=0.039; 2?V=80°. The temperature of pseudosinhalite formation was below about 400?°C at low pressures and with a hydrous, CO2-bearing fluid participating in the reaction.  相似文献   

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