首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 734 毫秒
1.
 Premelting effects in gehlenite (Ca2Al2SiO7) have been studied by Raman spectroscopy and calorimetry, and in gehlenite and pseudowollastonite (CaSiO3) by electrical conductivity. The enthalpy of premelting of gehlenite is 17.3 kJ mol−1 and represents 9% of the reported enthalpy of fusion, which is in the range of the reported fraction of other minerals. The Raman and electrical conductivity experiments at high temperatures, for gehlenite and pseudowollastonite, show that the premelting effects of both compositions are associated with enhanced dynamics of calcium atoms near the melting point. This conclusion agrees with the results obtained for other minerals like diopside, but contrasts with those found for sodium metasilicate in which the weaker bonding of sodium allows the silicate framework to distort near the melting temperature and deform in such a way to prefigure the silicate entities present in the melt. Received: 30 April 2002 / Accepted: 7 August 2002 Acknowledgements We thank Y. Linard for help with DSC measurements and two anonymous reviewers for their constructive comments. This work has been partly supported by the EU Marie-Curie fellowship contract no. HPMF-CT-1999-00329, the CNRS-Carnegie Institution of Washington program PICS no.192, and the NSF grants EAR-9614432 and EAR-9901886 to B.O.M.  相似文献   

2.
The structure of glasses and melts of Na2O· 0.5Fe2O3·3SiO2 and Na2O·FeO·3SiO2 compositions have been measured using high temperature Raman spectroscopy. For the oxidized sample it has been demonstrated that there is a close structural relationship between melt and glass. No coordination changes of Fe3+ with temperature and no new anionic species have been observed in the oxidized melt. The Raman spectra of the reduced sample clearly show a decrease in the degree of polymerization, as determined by the observation of the polarization character of the spectra and the details of the change of the Raman intensities during heating in hydrogen. Mössbauer spectra suggest that Fe3+ is tetrahedrally coordinated in the oxidized glass and part of the Fe2+ is tetrahedrally coordinated in the reduced glass.  相似文献   

3.
Raman spectra of γ-Mg2SiO4 taken to 200 kbar were used to calculate entropy and heat capacity at various P-T conditions. These new thermodynamic data on γ-MgSiO4, similar data on MgSiO3 perovskite (pv), previous data on β-MgSiO4 and MgO (mw), and previous volumetric data of all phases were used to calculate the phase boundaries in the Mg2SiO4 phase diagram. Our resulting slope for the β→γ transition (50±4 bar K-1) is in excellent agreement with recent multi-anvil studies. The slopes for the β→pv+MgO and γ→pv+MgO are-7±3 and -25±4 bar K-1, respectively, and are consistent with our CO2 laser heated diamond anvil studies. These slopes result in a β-γ-MgO+pv triple point at approximately 229 kbar and 2260 K for the iron free system.  相似文献   

4.
An extensive anomalously rapid increase of relative enthalpy H(T) ? H(298 K) of crystalline CaTiSiO5 was observed by means of high-temperature drop calorimetry when melting point is approached. X-ray diffraction analysis of the quenched products after drop in calorimeter shows that this effect is related to premelting. The determined excess enthalpy of crystals near the melting point reaches up to 115 kJ mol?1, that is about 82 % of the total enthalpy of melting, indicating that the premelting effect reflects configurational changes in the bulk of the crystals rather than a surface melting or any other type of partial melting. The obtained results support the presumption that calorimetrically measured premelting effect in titanite reflects the energy-consuming temperature-induced disordering of the framework elements, Si and Ti, which are strongly bonded to oxygen.  相似文献   

5.
Raman spectra of the two high-pressure polymorphs of SiO2 (coesite and stishovite) were investigated in the temperature range 105–875 K at atmospheric pressure. Coesite remained intact after the highest temperature run, but stishovite became amorphous at temperatures above about 842～872 K. Most Raman modes exhibit a negative frequency shift with temperature for these polymorphs, but positive trends were also observed for some modes. Except for some weak modes, nonlinear temperature variation were established for these polymorphs within the experimental uncertainty and temperature range spanned. The slopes of the variation (δvi/δT)P for these polymorphs were compared with the published values. When compared with quartz and stishovite, the four-membered rings of SiO4-tetrahedra in coesite exhibit very little change with both temperature and pressure. It is also suggested that temperature and pressure should have opposite effects on the Raman shift of each vibrational mode.  相似文献   

6.
Diopside (CaMgSi2O6) and pseudowollastonite (CaSiO3) have been studied by X-ray powder diffraction and Raman spectroscopy up to their respective melting points. In agreement with previous unit-cell parameters determinations below 1100 K, thermal expansion of diopside along the a and c axis is much smaller than along the b axis. For pseudowollastonite, the axis expansivity increases slightly in the order b>a>c. For both minerals, the change in unit-cell angles is very small and there are no anomalous variations of the other unit-cell parameters near the melting point. With increasing temperatures, the main changes observed in the Raman spectra are strong increases of the linewidths for those bands which mainly represent Si−O−Si bending (near 600 cm−1) or involve Ca−O or Mg−O stretching, in the range 270–500 cm−1 for diopside, and 240–450 cm−1 for pseudowollastonite. At temperatures near the onset of calorimetric premelting effects, this extensive band widening results in a broad Raman feature that can no longer be deconvoluted into its individual components. No significant changes affect the Si−O streching modes. For both diopside and pseudowollastonite, premelting appears to be associated with enhanced dynamics of the alkaline-earth elements. This conclusion contrasts markedly with that drawn for sodium metasilicate in which weaker bonding of sodium allows the silicate framework to distort and deform in such a way as to prefigure the silicate entities present in the melt. Received 16 July 1997 / Revised, accepted: 6 March 1998  相似文献   

7.
Raman spectra of Ni2SiO4 spinel (O h 7 Z=8) have been measured in the temperature range from 20 to 600 °C and the Raman active vibrations (A 1g +E g +3F 2g ) have been assigned. A calculation of the optically active lattice vibrations of this spinel has been made, assuming a potential function which combines general valence and short range force constants. The values of the force constants at 20 and 500 °C have been calculated from the vibrational frequencies of the observed Raman spectra and infrared (IR) spectral data. The Ni spinel at 20 °C has a prominently small Si-O bond stretching force constant of K(SiO)=2.356 ～ 2.680 md/Å and a large Ni-O bond stretching constant of K(NiO)=0.843 ～ 1.062 md/Å and these force constants at 500 °C decrease to K(SiO)=2.327 ～ 2.494 md/Å and K(NiO)=0.861 ～ 0.990 md/Å. The Si-O bond is noticeably weakened at high temperatures, despite the small thermal expantion from 1.657 Å (20 °C) to 1.660 Å (500 °C). These changes of the interatomic force constants of the spinel at high temperatures are in accord with the thermal structure changes observed by X-ray diffraction study. The weakened Si-O bond is consistent with the fact that Si atoms in the spinel lattice can diffuse at significant rates at elevated temperature.  相似文献   

8.
Single crystals of the garnet Mn2+ 3Mn3+ 2[SiO4]3 and coesite were synthesised from MnO2-SiO2 oxide mixtures at 1000°C and 9 GPa in a multianvil press. The crystal structure of the garnet [space group Ia3¯d, a=11.801(2) Å] was refined at room temperature and 100 K from single-crystal X-ray data to R1=2.36% and R1=2.71%, respectively. In contrast to tetragonal Ca3Mn3+ 2[GeO4]3 (space group I41/a), the high-pressure garnet is cubic and does not display an ordered Jahn-Teller distortion of octahedral Mn3+. A disordered Jahn-Teller distortion either dynamic or static is evidenced by unusual high anisotropic displacement parameters. The room temperature structure is characterised by following bond lengths: Si-O=1.636(4) Å (tetrahedron), Mn3+-O=1.995 (4) Å (octahedron), Mn2+-O=2.280(5) and 2.409(4) Å (dodecahedron). The cubic structure was preserved upon cooling to 100 K [a=11.788(2) Å] and upon compressing up to 11.8 GPa in a diamond-anvil cell. Pressure variation of the unit cell parameter expressed by a third-order Birch-Murnaghan equation of state led to a bulk modulus K 0=151.6(8) GPa and its pressure derivatives K′=6.38(19). The peak positions of the Raman spectrum recorded for Mn2+ 3Mn3+ 2[SiO4]3 were assigned based on a calderite Mn2+ 3Fe3+ 2[SiO4]3 model extrapolated from andradite and grossular literature data.  相似文献   

9.
A study of Ca self-diffusion along the b axis in synthetic (iron free) diopside single crystal was performed at temperatures ranging from 1273 K to 1653 K. Diffusion profiles of 44Ca were measured using α-particles Rutherford Backscattering (α-RBS) micro analysis. We unambiguously find two distinct diffusional regimes, characterized by activation enthalpies H = 280 ± 26 kJ/mol and H = 951 ± 87 kJ/mol at temperatures lower and upper than 1515 K, respectively. This change of diffusion regime takes place near the onset of premelting as detected in calorimetric measurements and can be interpreted in terms of enhanced formation of Frenkel point defects with an activation enthalpy of formation of 1524 ± 266 kJ/mol (H f/2 = 762 kJ/mol), in accordance with our high-temperature diffusion data. If premelting of diopside is actually related to Ca-Frenkel point defect concentration, this concentration could reach up to few mole percents close to the melting temperature.  相似文献   

10.
The vibrational properties of metastable Ca-GeO3 perovskite are investigated as a function of temperature using Raman scattering. Mode-assignments are derived from polarized spectra of optically oriented single crystals. Neither symmetry-breaking structural transformations nor soft-mode behavior is revealed in the Raman spectra between room temperature and 650° C. Only a small decrease in the local static octahedral tilt angles can be inferred from the Raman data over this temperature range. A Landau extrapolation of the lowest frequency Ag modes suggests that these modes become critical near 2000° C, a temperature that is above the extrapolated zero pressure melting point.  相似文献   

11.
 One well-defined OH Raman band at 3651 ± 1 cm−1 and one weak feature near 3700 ± 5 cm−1 are recognized for the hydrous γ-phase of Mg2SiO4. Like the hydrous β-phase, the H2O content in the γ-phase shifts most of the corresponding silicate modes towards lower frequencies. Variations in Raman spectra of the hydrous γ-phase were investigated up to about 200 kbar at room temperature and in the range 81–873 K at atmospheric pressure. Unlike the anhydrous γ-phase, which remains intact up to at least 873 K, the hydrous γ-phase sometimes converts to a defective forsterite structure above 800 K. Although the hydrous γ-phase remains intact up to at least 800 K, Raman signals of the OH bands disappear completely above 423 K. The Raman frequency of the well-defined OH band decreases linearly with increasing temperature between 81 and 423 K. In the region of the silicate vibrations, the Raman frequencies of the two most intense bands increase nonlinearly with increasing pressure, and decrease with increasing temperature. The frequencies for all other weak bands, however, decreased linearly with increasing temperature. The latter most likely reflects the larger scatter of the data for the weak bands. Received: 27 April 2001 / Accepted: 12 September 2001  相似文献   

12.
A drop calorimetric study, between 900 and 1800 K, of amorphous SiO2, NaAlSi3O8, NaAlSi2O6, NaAlSiO4 and KAlSi3O8 shows the increase in heat capacity which results from glass transitions. For these glasses, the fictive temperature has a negligible effect on the heat capacity above room temperature, but it has an important influence on the enthalpy of formation as obtained from solution calorimetry. From these results and published Cp and enthalpy of solution data, several properties have been calculated: the enthalpies of fusion of high albite, nepheline, Jadeite and high sanidine, the thermodynamic functions of amorphous NaAlSi3O8 and KAlSi3O8 between 0 and 2000 K, and some mixing properties of liquids along the join SiO2-NaAlSi3O8. The latter data suggest that these liquids behave more closely as athermal solutions than as regular solutions.  相似文献   

13.
The heat capacities of 29 glasses and supercooled liquids in the Na2O-SiO2, Na2O-Al2O3-SiO2, Na2O-(FeO)-Fe2O3-SiO2, and Na2O-TiO2-SiO2 systems were measured in air from 328 to 998 K with a differential scanning calorimeter. The reproducibility of the data determined from multiple heat capacity runs on a single crystal MgO standard is within ± 1% of the accepted values at temperatures ≤ 800 K and within ± 1.5% between 800 and 1000 K. Within the resolution of the data, the heat capacities of sodium silicate and sodium aluminosilicate liquids are temperature independent. Heat capacity data in the supercooled liquid region for the sodium silicates and sodium aluminosilicates were combined and modelled assuming a linear compositional dependence. The derived values for the partial molar heat capacities of Na2O, Al2O3, and SiO2 are 112.35 ± 0.42, 153.16 ± 0.82, and 76.38 ± 0.20 J/gfw · K respectively. The partial molar heat capacities of Fe2O3 and TiO2 could not be determined in the same manner because the heat capacities of the Fe2O3- and TiO2-bearing sodium silicate melts showed varying degrees of negative temperature dependence. The negative temperature dependence to the configurational C P may be related to the occurrence of sub-microscopic domains (relatively polymerized and depolymerized) that break down to a more homogeneous melt structure with increasing temperature. Such an interpretation is consistent with data from in situ Raman, Mössbauer, and X-ray absorption fine structure (XAFS) spectroscopic studies on similar melts.  相似文献   

14.
Thermal expansion properties of synthetic orthopyroxenes (Fe0.20Mg0.80)SiO3, (Fe0.40Mg0.60)SiO3, (Fe0.50Mg0.50)SiO3, (Fe0.75Mg0.25)SiO3 and (Fe0.83Mg0.17)SiO3 were systematically studied by means of single-crystal x-ray diffraction in the temperature range from 296 to 1300 K. The measurements of unit cell dimensions as a function of temperature reveal that the a and c dimensions and the unit cell volume V increase nonlinearly with a positive curvature with rising temperature, whereas the b dimension behaves differently, depending on the total Fe content. For Mg-rich orthopyroxenes (Fe/(Fe+Mg)<30%), the b dimension expands similarly as the a and c dimensions, but it exhibits a nonlinear increase with a negative curvature for orthopyroxenes with Fe/(Fe+Mg)>30%. Together with the high temperature neutron diffraction data on enstatite (MgSiO3) (McMullan, Haga and Ghose, unpublished) and x-ray diffraction data on ferrosilite (FeSiO3) (Sueno et al. 1976), the measured unit cell dimensions were analyzed in terms of the Grüneisen theory of thermal expansion. The linear thermal expansion coefficients α a and α c both increase as temperature is elevated, with α c increasing faster, while α b changes gradually from increasing for Mg-rich orthopyroxenes to decreasing for Fe-rich orthopyroxenes. The relative magnitudes of linear thermal expansion coefficients are always in the order α b >α c >α a between 300 and 500 K, but at higher temperatures, the order changes to α c >α b >α a for Mg-rich orthopyroxenes and α c >α a >α b for Fe-rich ones. The linear thermal expansion behavior is interpreted on the basis of the structural mechanical model of Weidner and Vaughan (1982). The anomalous behavior of α b is mainly attributed to the changes in the Fe2+ population at the M2 site and the relative stiffness of the M2(Fe2+)-O bonds compared to the M2(Mg2+)-O bonds. The volume thermal expansion coefficients are nonlinear functions of temperature and lie between 23 and 49×10?6/K. The previously reported results of mean volume thermal expansion coefficients appear to represent the α V values characteristic of higher temperatures compared to our results. The thermal Debye temperatures are composition-dependent, decreasing linearly from 812 (MgSiO3) to 561 K (FeSiO3), and are systematically higher than the corresponding acoustic Debye temperatures. The Grüneisen parameters range from 0.85 to 0.89 and do not seem to vary with composition. The linear compressibilities derived from thermal expansion and elastic moduli data agree very well. The pressure derivatives of the isothermal bulk modulus (dK0/dP) are also composition-dependent and decrease from 11.2 (MgSiO3) to 8.77 (FeSiO3). Such large values indicate possible anomalous elastic behavior of orthopyroxenes at high pressures in the Earth's upper mantle.  相似文献   

15.
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   

16.
Cr2SiO4 has been prepared both as quench crystals and as an apparently stable subsolidus phase, at T=1600 and 1650 °C, P=37 kbar on Cr2SiO4 and Cr-SiO3 bulk compositions. Crystal structure determination by Rietveld analysis of X-ray powder data from 3 samples show the structure, in space group Fddd, to be related to that of Cd2SiO4 (thenardite Na2SO4-V structure) but with approximately “square planar” coordination of the Cr2+ (d 4) atom. In one of the axial ligand positions, normal to the strongly bonded CrO4 equatorial plane, is found a Cr atom (Cr-Cr=2.75 Å). Stereochemistry permits this contact to be a weak metal-metal bond, though it cannot be ruled out to be a non-bonded result of polyhedral connectivity.  相似文献   

17.
High-temperature Raman spectra and thermal expansion of tuite, γ-Ca3(PO4)2, have been investigated. The effect of temperature on the Raman spectra of synthetic tuite was studied in the range from 80 to 973 K at atmospheric pressure. The Raman frequencies of all observed bands for tuite continuously decrease with increasing temperature. The quantitative analysis of temperature dependence of Raman bands indicates that the changes in Raman frequencies for stretching modes (ν3 and ν1) are faster than those for bending modes (ν4 and ν2) of PO4 in the present temperature range, which may be attributed to the structural evolution of PO4 tetrahedron in tuite at high temperature. The thermal expansion of tuite was examined by means of in situ X-ray diffraction measurements in the temperature range from 298 to 923 K. Unit cell parameters and volume were analyzed, and the thermal expansion coefficients were obtained as 3.67 (3), 1.18 (1), and 1.32 (3) × 10?5 K?1 for V, a, and c, respectively. Thermal expansion of tuite shows an axial anisotropy with a larger expansion coefficient along the c-axis. The isothermal and isobaric mode Grüneisen parameters and intrinsic anharmonicity of tuite have been calculated by using present high-temperature Raman spectra and thermal expansion coefficient combined with previous results of the isothermal bulk modulus and high-pressure Raman spectra.  相似文献   

18.
The effect of composition and temperature on the relaxed adiabatic bulk modulus of melts in the P2O5-Al2O 3-Na2SiO3 system have been investigated in the temperature range of 1140 to 1450 °C using ultrasonic interferometric methods at frequencies of 3, 5 and 7 MHz. The density of these melts was determined using Pt-double-bob Archimedean densitometry techiques. P2O5 is known to dramatically affect the structure and the chemical and physical properties of granitic and pegmatitic melts as a function of the peralkalinity of the melt. The physical results of the structural changes occurring in Na2O-Al2O3-SiO2 melt upon the addition of P2O5 are observed by variations in the properties such as density and compressibility. For the present peralkaline melts, the bulk modulus and density decrease with addition of 15 mol% P2O5, and increase with the addition of 15 mol% Al2O3. The addition of P2O5 to the present melts results in a larger increase in melt compressibility than that observed with increasing polymerization between Na2SiO3 and Na2Si2O5 melts. This would suggest that not only is the polymerization of the melt increasing with the addition of P2O5 (Mysen et al. 1981; Nelson and Tallant 1984; Gan and Hess 1992), but that the tetrahedrally co-ordinated phosphorus complexes are influencing the bond lengths and energies within the melt structure; resulting in the structure becoming more compressible than expected, although incompressible (Vaughan and Weidner 1987) tetrahedral P2O5 polyhedra (Mysen et al. 1981; Gan and Hess 1992; Toplis et al. 1994) are being added to the melt structure.  相似文献   

19.
Raman spectra of the three Al2SiO5 polymorphs; andalusite, sillimanite and kyanite were recorded as a function of pressure at room temperature. All the Raman active bands which could be observed from the high-pressure cell showed a linear pressure dependence for each of the three Al2SiO5 polymorphs and no phase changes were observed over the pressure ranges used in this study. In andalusite and to a lesser extent in sillimanite, vibrations which could be correlated with internal motions of the SiO4 tetrahedra were generally well separated from the lattice modes and showed a greater pressure dependence than that observed for other modes. The distinct pressure dependence of the internal SiO4 modes is less evident in kyanite, probably due to the lack of continuous tetrahedral chains and the fact that the rigid SiO4 tetrahedra now form an integral part of the structural network. At ambient pressure, kyanite also exhibits two fluorescence bands at 705 and 706.2 nm which are due to small amounts of Cr3+ in the kyanite crystals. These fluorescence bands showed a non-linear frequency shift as pressure was increased.  相似文献   

20.
The solubility mechanism of fluorine in quenched SiO2-NaF and SiO2-AlF3 melts has been determined with Raman spectroscopy. In the fluorine abundance range of F/(F+Si) from 0.15 to 0.5, a portion of the fluorine is exchanged with bridging oxygen in the silicate network to form Si-F bonds. In individual SiO4-tetrahedra, one oxygen per silicon is replaced in this manner to form fluorine-bearing silicate complexes in the melt. The proportion of these complexes is nearly linearly correlated with bulk melt F/(F+Si) in the system SiO2-AlF3, but its abundance increases at a lower rate and nonlinearly with increasing F/(F+Si) in the system SiO2-NaF. The process results in the formation ofnonbridging oxygen (NBO), resulting in stabilization of Si2O 5 2? units as well as metal (Na+ or Al3+) fluoride complexes in the melts. Sodium fluoride complexes are significantly more stable than those of aluminum fluoride.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号