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 109 毫秒
1.
Electrical conductivities of Ni2SiO4, Fe2SiO4, and MgSiO3 were measured on synthetic powders in the temperature range 340° to 1,100° C and at pressures up to 20 kbars. For ternary compounds such as olivines and pyroxenes the control of two further variables, like the chemical activities of two components are needed, besides temperature and pressure. The activities of the corresponding binary oxides were controlled by equilibrating the samples with their neighbour-phases. Control of the oxygen partial pressure was achieved by buffer techniques. From the slopes of the lg σ vs. 1/T lines the activation energies were calculated for 10 kbar: 0.56 eV and 2.7 eV for Ni2SiO4 in equilibrium with SiO2 and Ni/NiO-buffer for the temperature range 500°–800°C and 800°–1,000°C resp. 0.52 eV for Fe2SiO4 in equilibrium with SiO2 and metallic iron, and 0.38 eV in equilibrium with SiO2 and magnetite; 1.11 eV for MgSiO3 in equilibrium with SiO2, and 1.25 eV in equilibrium with Mg2SiO4.  相似文献   

2.
The decomposition of fayalite (Fe2SiO4) in oxygen potential gradients is studied at T=1,418 K. The compound will be decomposed into its component oxides wüstite, Fe1?δO, and silica, SiO2, by the simultaneous action of two different oxygen partial pressures, exceeding a critical ratio, despite the fact that fayalite is stable at both the lower and the higher oxygen potential. A quantitative analysis of the decomposition process caused by defect fluxes within the bulk Fe2SiO4 is given.  相似文献   

3.
X-ray structure refinements of Ni2SiO4 and Fe2SiO4 spinels have been made as a function of temperature and heating duration by intensity measurements at high temperatures and room pressure. The lattice parameters of Ni2SiO4 spinel linearly increased with temperature up to 1,000° C. However, Fe2SiO4 spinel exhibited a nonlinear thermal expansion and was converted to a polycrystalline mixture of spinel and olivine by heating of less than one-hour at 800° C. The ratios between the octahedral and tetrahedral bond lengths D oct/D tetr and between the shared and unshared edge distances (O-O)sh/(O-O)unsh in Fe2SiO4 spinel were both much larger than those in Ni2SiO4. These ratios increase with temperature. The Fe2SiO4 spinel more readily approached a activation state which facilitated the transition to the olivine structure than the Ni2SiO4 spinel. The lattice parameter of Ni2SiO4 spinel decreased with heating period at constant temperatures of 700° C and 800° C. The parameter of the quenched sample after heating for 52 h at 700° C was smaller than that of the nonheated sample. The refinements of the site occupancies at each heating duration indicated an increase in the cation deficiency in both tetrahedral and octahedral sites. Electron microprobe analysis, however, proved no significant difference in the chemical compositions between the quenched and nonheated samples. Si and Ni atoms displaced from normally occupied spinel lattice sites are assumed to settle in vacant sites defined by the cubic close packed oxygen sublattice in a manner which preserves the electric neutrality of the bulk crystal.  相似文献   

4.
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 (Al1.88Mn 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 Å3, resp., while α, β, γ, remain unchanged. Between 10 and 18 kb/900°C, Mn3+-substituted, strongly pleochroitic (emeraldgreen-yellow) andalusitess (viridine) was obtained. At 15 kb/900°C, the viridine compositional range is about (Al1.86Mn 0.14 3+ )SiO5-(Al1.56Mn 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 Å3, 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 Å3 (ideal compn.) and a o 9.374, c o 18.593 Å3, V o 1633.6 Å3 (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   

5.
The effects of the addition of Al2O3 on the large stable two liquid field in the SiO2-TiO2-CaO-MgO-FeO system were experimentally determined. The increase of Al2O3 content in the starting composition results in the decrease of critical temperature, phase separation and liquidus temperature of the two liquid field until it is rendered completely metastable. The shrinkage of the two liquid field indicates that Al2O3 is acting in the role of a network former and homogenizes the structure of the two melts. In this alkali-free system Al+3 utilizes the divalent cations, Ca+2 and Mg+2, for local charge balance with a preference for Ca+2 over Mg+2. Thus, AlO4 tetrahedra combine with SiO4 tetrahedra to form an aluminosilicate framework which polymerizes the SiO2-poor melt and makes it structurally more similar to the SiO2-rich melt. However, Ca+2 and Mg+2 are not as efficient in a charge balancing capacity as the monovalent K+ and Na+ cations. The lack of alkalis in this system limits the stability of AlO4 tetrahedra in the highly polymerized SiO2-rich melt and results in the preference of Al2O3 for the SiO2-poor melt. The partitioning systematics of Ti are virtually identical to those of Al. It is concluded that Ti occurs in tetrahedral coordination as a network forming species in both the high — and low — SiO immiscible melts.  相似文献   

6.
Crystal field stabilization (CFS) plays a significant role in determining equilibrium phase boundaries in olivine→spinel transformations involving transition-metal cations, including Fe2+ which is a major constituent of the upper mantle. Previous calculations for Fe2SiO4 ignored pressure and temperature dependencies of crystal field stabilization enthalpies (CFSE) and the electronic configurational entropy (S CFS). We have calculated free energy changes (ΔG CFS) due to differences of crystal field splittings between Fe2SiO4 spinel and fayalite from: ΔG CFS=?ΔCFSE?TΔS CFS, as functions of P and T, for different energy splittings of t 2g orbital levels of Fe2+ in spinel. The results indicate that ΔG CFS is always negative, suggesting that CFS always promotes the olivine→spinel transition in Fe2SiO4, and expands the stability field of spinel at the expense of olivine. Because of crystal field effects, transition pressures for olivine→spinel transformations in compositions (Mg1?x Fe x )2SiO4 are lowered by approximately 50x kbar, which is equivalent to having raised the olivine→spinel boundary in the upper mantle by about 15 km.  相似文献   

7.
Nanohybrid of graphene oxide (GO) and azide-modified Fe3O4 nanoparticles (NPs) were fabricated using click reaction. First, Fe3O4 NPs were modified by 3-azidopropionic acid. Then, click-coupling of azide-modified Fe3O4 NPs with alkyne-functionalized GO was carried out in the presence of CuSO4·5H2O and sodium l-ascorbate at room temperature. The attachment of Fe3O4 NPs onto the graphene nanosheets was confirmed by Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy, thermogravimetric analysis, energy dispersive X-ray spectrometry and X-ray diffraction spectrometry. As the FTIR spectroscopy and energy dispersive X-ray spectrometry analysis showed, the final magnetic graphene nanosheets were also reduced by sodium ascorbate which is a merit for click-coupling reactions. The specific saturation magnetization of the Fe3O4-clicked GO was 44.3 emu g?1. The synthesized hybrid was used in the adsorption of methylene blue and congo red (CR). The adsorption capacities in the studied concentration range were 109.5 and 98.8 mg g?1 for methylene blue and CR, respectively.  相似文献   

8.
Infrared absorption spectra of the high-pressure polymorphs β-Mg2SiO4 and β-Co2SiO4 have been measured between 0 and 27 GPa at room temperature. Grüneisen parameters determined for 11 modes of β-Mg2SiO4 (frequencies of 300 to 1,050 cm?1) and 5 modes of β-Co2SiO4 (490 to 1,050 cm?1) range between 0.8 and 1.9. Averaging the mid-infrared spectroscopic data for β-Mg2SiO4 yields an average Grüneisen parameter of 1.3 (±0.1), in good agreement with the high-temperature thermodynamic value of 1.35. Similarly, we find a value of 1.05 (±0.2) for the average spectroscopic Grüneisen parameter of β-Co2SiO4.  相似文献   

9.
Five different refraction formulas were applied to SiO2 polymorphs in order to determine the most suitable refractive index-density relation. 13 SiO2 polymorphs with topological different tetrahedral frameworks are used in this study including eight new low density SiO2 polymorphs — so called “guest free porosils”. These SiO2 polymorphs cover a density range from 1.76 to 2.92 g/cm3. The mean refractive indices (ovn) of the porosils have been determined by the immersion method, the densities (ρ) were calculated from the unit cell parameters. Assuming the polarizability (α) of all SiO2 polymorphs to be constant the general refractivity formula $$\{ 2\overline {11} 0\} \langle 0001\rangle $$ turned out to be the most suitable for SiO2 polymorphs. Regression analysis yields an electronic overlap parameter b=1.2(1).  相似文献   

10.
In the lattice energy expression of forsterite, based on a Born-Mayer (electrostatic+repulsive+dispersive) potential, the oxygen charge z o, the hardness parameter ρ and the repulsive radii r Mg and r Si appear as unknown parameters. These were determined by calculating the first and second partial derivatives of the energy with respect to the cell edges, and equalizing them to quantities related to the crystal elastic constants; the overdetermined system of equations was solved numerically, minimizing the root-mean-square deviation. To test the results obtained, the SiO 4 4? ion was assumed to move in the unit-cell, and the least-energy configuration was sought and compared with the experimental one. By combining the two methods, the optimum set of parameters was: z o=?1.34, ρ=0.27 Å, r Mg=0.72 Å, r Si=0.64 Å. The values ?8565.12 and ?8927.28 kJ mol?1 were obtained, respectively, for the lattice energy E Land for its ionic component E L 0 ,which accounts for interactions between Mg2+ and SiO 4 4? ions only. The charge distribution calculated on the SiO 4 4? ion was discussed and compared with other results. Using appropriate thermochemical cycles, the formation enthalpy and the binding energy of SiO 4 4? were estimated to be: ΔH f(SiO 4 4? )=2117.6 and E(SiO 4 4? )=708.6 kJ mol?1, respectively.  相似文献   

11.
We give a brief review of ion dynamics studies of liquid and glassy states of SiO2 and silicate colutions which have been carried out in recent years in this laboratory. We summarize studies on SiO2, Na+ migration in Na2O·SiO2 in the “glassy state”, and ionic coordination in multicomponent framework silicates. We present new results on the coordination of Al3+ in albite as a function of pressure and show that it is consistent with results of laboratory studies on albite glasses formed at high pressure. We compare calculated PVT data for jadeite, albite and diopside and relate the behavior of the low pressure compressibility to the spinodal limit at negative pressures. Some preliminary studies of inert gas solution in jadeite and of CO2 solution in a glass having a composition of approximately Na2O·3SiO2 are described.  相似文献   

12.
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   

13.
  相似文献   

14.
Enthalpies of solution in molten 2 PbO · B2O3 at 974 K were measured for four spinelloids, phases I (0.75 NiAl2O4 · 0.25 Ni2SiO4), II (0.60 NiAl2O4 · 0.40 Ni2SiO4), III and IV (0.50 NiAl2O4 · 0.50 Ni2SiO4) in the system NiAl2O4 · Ni2SiO4. The enthalpies (in cal per 4-oxygen mol) of formation from NiAl2O4 and Ni2SiO4 spinels are: phase I, 945±366; phase II, 1072±360; phase III, 2253±390; phase IV, 3565±544. Using these enthalpy data in combination with phase relations at high pressure at 1373 K, positive entropies of formation of the spinelloids from NiAl2O4 and Ni2SiO4 spinels were estimated (in cal mol?1 K?1): phase I, 1.2; phase II, 1.5; phase III, 2.0–2.3; phase IV, 3.0–3.1. The thermochemical data obtained above suggest that the spinelloids are “entropy-stabilized” phases with partially disordered cation distributions. The configurational entropies of the spinelloids were calculated based on the observed cation distribution in each spinelloid phase. The positive entropies of formation of the spinelloids from the spinel endmembers are due primarily to the configurational entropies although small positive vibrational entropy changes may also exist.  相似文献   

15.
The products of the transformation of natural (Mg0.83Fe0.17)2SiO4 olivine have been prepared at various high pressures (between 25 GPa and 90 GPa), and high temperature in a laser-heated diamond-anvil cell (DAC). Studies of the high-pressure phases have been made by transmission electron microscopy (TEM), and X-ray microanalysis. The olivine/spinel boundaries exhibit all the characteristics of a diffusionless shear transition, having a finely sheared structure and a constant orientation relationship between the close-packed planes of the two structures ((100)ol∥(111)sp). The TEM observations of zones where olivine (or spinel) transforms into post-spinel phases show that the transformation possesses the features of an eutectoïdal decomposition, leading to a lamellar intergrowth of magnesiowüstite (Mg,Fe)O and perovskite (Mg,Fe)SiO3. With increasing temperature and/or decreasing pressure, the grain size of the high-pressure phases increases and obeys an Arrhenius law with an activation volume equal to zero. (Mg,Fe)O grains exhibit a very high density of dislocations (higher than 1011cm?2), whereas (Mg,Fe)SiO3 grains exhibit no dislocations but systematic twinning. The composition plane of the twins is (112) of the GdFeO3-type perovskite, corresponding to the {110} plane of the cubic lattice of ideal perovskite.  相似文献   

16.
A thermochemical data base for phases in the system Fe-Mg-Si-O at high pressures up to 300 kbar is established by supplementing the available calorimetric data with data calculated from experimental high pressure synthesis studies. Phases included in the data base are the SiO2 polymorphs, rock salt solid solutions (Fe-Mg-O), Fe2O3, Fe3O4, (Mg, Fe)2SiO4 olivine, spinel, modified spinel and (Mg, Fe)SiO3 perovskite and pyroxene. Phases not included are the MgSiO3-ilmenite and -garnet. Fe-Mg solution properties of olivine, spinel, perovskite and wustite (rock salt) are estimated. The wüstite solid solution has been modeled as a nonideal solution of three end members; FeO, FeO1.5 and MgO. The new data base is made consistent with most of the available information on high pressure phase studies. The data base is useful in generating phase diagrams of various different compositions for the purpose of planning new experiments and analysing existing phase synthesis data.  相似文献   

17.
As a basis for eventual control of the vicious anthropophilic and ornithophilic simuliid females of the Marquesas Islands, a taxonomic revision of Polynesian Simuliidae is in progress. Cytotaxonomic studies reveal two simuliid species in Rarotonga, Cook Islands; nine species in Tahiti, Society Islands, and two in the Marquesas Islands. A cytophylogeny is now available. Polynesian Simuliidae are unique in possessing heterogametic females. A reconstructed phylogeny, based mainly on morphological character states of larvae and pupae, agrees well with the cytophylogeny. One of the new species which is closely related to Simulium oviceps Edwards has larvae with highly reduced, non-functional cephalic fans. Ages of the Polynesian islands are discussed in relation to possible dispersal of Simuliidae into Polynesia.  相似文献   

18.
The Central American isthmus hosts a highly variable Moho structure due to the diverse origin and composition of the crustal basement and the influence of large-scale neotectonic processes. Gravity data from the combined geopotential model EGM2008 were interpreted via forward modelling to outline the three-dimensional lithospheric density structure along the Middle American Trench, as well as the segmentation of the oceanic Cocos and Nazca plates and the overriding Caribbean plate. In this work, results for the depth of the Moho obtained from the density model are presented. The Quaternary volcanic arc correlates with a maximum Moho depth of 44 km in western Guatemala. To the south-east of the continental shelf, the Caribbean plate shows Moho depths between 20 and 12 km whereas to the north, values as shallow as 8 km are observed at the Cayman trough. For the oceanic Cocos plate, depths between 16 and 21 km are obtained for the Moho along the Cocos ridge, contrasting with values between 15 and 12 km for the seamount segment and 8 and 11 km for the segments of the crust that are not affected by the Galapagos hot-spot track.  相似文献   

19.
The vibrational frequencies of a series of splatquenched, olivine glasses spanning the compositional range from Mg2SiO4 to Mn2SiO4 have been determined using both infrared and Raman spectroscopies. The spectra of all glasses show evidence of tetrahedral coordination of silicon (possibly with some slight distortions), and largely octahedral coordination of magnesium. Spectra of Mn-rich glasses indicate that there is some manganese in 4 or 5-fold coordination. The frequencies observed for the fundamental vibrations of the silica tetrahedra are similar to those previously observed for SiO4 groups in both crystalline and glassy orthosilicates. Additionally, there is evidence for a small amount of silicate polymerization in all glasses characterized: vibrations attributable to Si2O7 groups are visible in both infrared and Raman spectra.  相似文献   

20.
This work is one of the stages of study of the deep C-O-H fluid and investigates the behavior of polycyclic aromatic hydrocarbons (PAHs) under conditions of the Earth’s mantle. The composition of the C-O-H fluid in the upper mantle is estimated as a mixture of H2O and CH4 with a minor amount of H2 and heavier hydrocarbons. Some theoretical calculations show that the stability of heavy hydrocarbons (alkanes, alkenes, and PAHs) increases with an increase in temperature. This paper presents the results of an XRD study of PAHs stability in multianvil presses on a Spring-8 accelerator (Japan). The primary compositions were chosen according to the abundance of PAHs in nature. In situ diffraction spectrums were recorded to determine the PAHs stability field. It was established that the PAHs become unstable at a pressure of 6–9 GPa and a temperature of 873–1073 K.  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