共查询到20条相似文献，搜索用时 46 毫秒
1.
S. Endo T. Akai Y. Akahama M. Wakatsuki T. Nakamura Y. Tomii K. Koto Y. Ito M. Tokonami 《Physics and Chemistry of Minerals》1986,13(3):146-151
Single crystal stishovite with a square prismatic habit and maximum length 0.8 mm was grown from α-quartz at 120 kbar and ～1,300° C. Li2WO4, chosen as a result of a previous experiment in growing coesite, was also successful as flux for stishovite. Single crystal X-ray structure analysis of the crystals thus obtained has been carried out at high temperatures under ambient pressure. Lattice constant measurements give a larger thermal expansion coefficient along the a-axis than along the c-axis. The bond distances and bond angles show a decreasing distortion of the SiO6 octahedron with increasing temperature. The increasing amplitude of thermal vibrations of oxygen atoms with increasing temperature results in increasing O-O repulsion in the basal plane, which explains the observed crystallographic changes. 相似文献
2.
Pascal Richet 《Physics and Chemistry of Minerals》1990,17(1):79-88
Relative-enthalpy measurements have been made on the hexagonal, tetragonal, glass and liquid phases of GeO2. The glass transition is very sensitive to the impurity content, with a T g ranging from 980 K for a pure product to 780 K for a Li-doped sample with 0.06 mol % Li. The relative C p change at T g of about 5% increases with the impurity content as a result of lower glass transition temperatures. Above 298 K the derived heat capacities are similar for all forms, with slightly higher values for the amorphous phases and two C p cross-overs at 400 and 1000 K between the hexagonal and tetragonal modifications. For both GeO2 and SiO2 the coordination state markedly affects C p and the entropy below 300 K, where the properties are much lower for the tetragonal than for the hexagonal modifications, i.e., S 298 = 39.7 vs 55.3 J/mole K and 27.8 vs 41.4 J/ mole K for GeO2 and SiO2, respectively. The high-temperature C p's of coesite and stishovite are likely similar to those of the low-pressure SiO2 forms. Finally, these results, low-temperature C p data and enthalpy-of-solution measurements have been used to derive a consistent set of thermodynamic properties for the GeO2 modifications. 相似文献
3.
MENG Dawei WU Xiuling FAN Xiaoyu ZHANG Zhengjie CHEN Hong MENG Xin ZHENG Jianping 《《地质学报》英文版》2008,82(2):371-376
α-PbO2-type TiO2 （TiO2-Ⅱ） is an important index mineral for ultrahigh-pressure metamorphism. After the discovery of a natural high-pressure phase of titanium oxide with α-PbO2- structure in omphacite from coesite-bearing eclogite at Shima in the Dabie Mountains, China, a nanoscale （〈2 nm） α-PbO2-type TiO2 has been identified through electron diffraction and high-resolution transmission electron microscopy in coesite-bearing jadeite quartzite at Shuanghe in the Dabie Mountains. The crystal structure is orthorhombic with lattice parameters a = 4.58×10-1 nm, b = 5.42×10-1 nm, c = 4.96×10-1 nm and space group Pbcn. The analysis results reveal that ruffle {011}R twin interface is a basic structural unit of α-PbO2-type TiO2. Nucleation of α-PbO2-type TiO2 lamellae is caused by the displacement of one half of the titanium cations within the {011}R twin slab. This displacement reduces the Ti-O-Ti distance and is favored by high pressure. The identification of α- PbO2-type TiO2 in coesite-bearing jadeite quartzite from Shuanghe, Dabie Mountains, provides a new and powerful evidence of ultrahigh-pressure metamorphism at 4--7 GPa, 850℃-900℃, and implies a burial of continental crustal rocks to 130-200 kilometers depth or deeper. The α-PbO2-type TiO2 may be a useful indicator of the pressure and temperature in the diamond stability field. 相似文献
4.
We present a covalent potential model of tetrahedrally coordinated SiO2. The interactions include covalent effects in the form of a Si-O bond-stretching potential, O-Si-O and Si-O-Si angle-bending potentials, and oxygen-oxygen repulsion. Calculated equations of state of α-quartz and coesite agree well with experiment (calculated densities within 1 percent of experiment up to 6 GPa). The calculated α-quartz-coesite transition pressure agrees with the experimental value of ≈2 GPa. Furthermore, the compression mechanisms predicted by the model (i.e. pressure induced changes in Si-O bond lengths and O-Si-O and Si-O-Si angles) are accurate. 相似文献
5.
K. R. Rao S. L. Chaplot Narayani Choudhury Subrata Ghose J. M. Hastings L. M. Corliss D. L. Price 《Physics and Chemistry of Minerals》1988,16(1):83-97
Magnesium-rich olivine (Mg0.9Fe0.1)2SiO4 is considered to be a major constituent of the Earth's upper mantle. Because of its major geophysical importance, the temperature and pressure dependence of its crystal structure, elastic and dielectric constants, long-wavelength phonon modes and specific heat have been measured using a variety of experimental techniques. Theoretical study of lattice dynamics provides a means of analyzing and understanding a host of such experimental data in a unified manner. A detailed study of the lattice dynamics of forsterite, Mg2SiO4, has been made using a crystal potential function consisting of Coulombic and short-range terms. Quasiharmonic lattice dynamical calculations based on a rigid molecular-ion model have provided theoretical estimates of elastic constants, long-wavelength modes, phonon dispersion relation for external modes along the three high symmetry directions in the Brillouin zone, total and partial density of states and inelastic neutron scattering cross-sections. The neutron cross-sections were used as guides for the coherent inelastic neutron scattering experiment on a large single crystal using a triple axis spectrometer in the constant Q mode. The observed and predicted phonon dispersion relation show excellent agreement. The inelastically scattered neutron spectra from a powder sample have been analyzed on the basis of a phonon density of states calculated from a rigid-ion model, which includes both external and internal modes. The experimental data from a powder sample show good agreement with the calculated spectra, which include a multiphonon contribution in the incoherent approximation. The computed phonon densities of states are used to calculate the specific heat as a function of temperature using both the rigid molecular-ion and rigid ion models. These results are in very good agreement with the calorimetric measurement of the specific heat. The interatomic potential developed here can be used with some confidence to study physical properties of forsterite as a function of pressure and temperature. 相似文献
6.
Exsolution systems in synthetic pyroxenes were studied by transmission electron microscopy. An iron free sample En80Wo20 was prepared by devitrifying glass at 1300°C. Samples with bulk composition En50Fs30Wo20 and En35Fs38Wo27 were given various but well-defined heat treatments. The exsolution systems observed cannot unambiguously be related to the heat treatment. Periodic lamellar exsolution was observed parallel to (001) and (100) with sharp satellite reflections in the diffraction diagram. In more complex exsolution systems coarse (100) lamellae were found together with fine lamellae parallel to (001) and (100). An unusual phenomenon occurs at a (100) twin boundary where both individuals display exsolution lamellae parallel to (001). Pigeonite lamellae in one twin meet augite lamellae of the other individual at the twin boundary and vice-versa. The precise matching is achieved by a change in width near the boundary. Smoothly curved phase boundaries are developed in the obtuse angle of crosshatched (100) and (001) pigeonite lamellae in augite, whereas the boundaries in the acute angle are straight with sharp edges. This is consistent with elastic energy constraints. 相似文献
7.
8.
Ab initio STO-3G molecular orbital theory has been used to calculate energy-optimized Si-O bond lengths and angles for molecular orthosilicic and pyrosilicic acids. The resulting bond length for orthosilicic acid and the nonbridging bonds for pyrosilicic acid compare well with Si-OH bonds observed for a number of hydrated silicate minerals. Minimum energy Si-O bond lengths to the bridging oxygen of the pyrosilicic molecule show a close correspondence with bridging bond length data observed for the silica polymorphs and for gas phase and molecular crystal siloxanes when plotted against the SiOSi angle. In addition, the calculations show that the mean Si-O bond length of a silicate tetrahedron increases slightly as the SiOSi angle narrows. The close correspondence between the Si-O bond length and angle variations calculated for pyrosilicic acid and those observed for the silica polymorphs and siloxanes substantiates the suggestion that local bonding forces in solids are not very different from those in molecules and clusters consisting of the same atoms with the same coordination numbers. An extended basis calculation for H4SiO4 implies that there are about 0.6 electrons in the 3d-orbitals on Si. An analysis of bond overlap populations obtained from STO-3G* calculations for H6Si2O7 indicates that Si-O bond length and SiOSi angle correlations may be ascribed to changes in the hybridization state of the bridging oxygen and (d – p) π-bonding involving all five of the 3d AO's of Si and the lone-pair AO's of the oxygen. Theoretical density difference maps calculated for H6Si2O7 show a build-up of charge density between Si and O, with the peak-height charge densities of the nonbridging bonds exceeding those of the bridging bonds by about 0.05 e Å?3. In addition, atomic charges (+1.3 and ?0.65) calculated for Si and O in a SiO2 moiety of the low quartz structure conform reasonably well with the electroneutrality postulate and with experimental charges obtained from monopole and radial refinements of diffraction data recorded for low quartz and coesite. 相似文献
9.
The possibilities of exoelectron spectroscopy to investigate defects in dielectrics are demonstrated for phenakite Be2SiO4, its structural analogs Zn2 SiO4, Be2GeO4, solid solutions Be2Si1?x Ge x O4 (x=0÷1) and α-quartz. Emission maxima at 330 and 670 K in phenakite have been found to be due to [GeO4]5? andE' centers, respectively. Structural disturbances in the silicon and oxygen positions have been shown to control the exoemission activity of the crystals. Radiation induced decrease of exoemission activity connected with generation ofE' centers by neutron irradiation has been discovered. The energy level scheme of active centers in the subsurface region of Be2SiO4 has been established. 相似文献
10.
P. Thierry C. Chatillon-Colinet J. C. Mathieu J. R. Regnard J. Amossé 《Physics and Chemistry of Minerals》1981,7(1):43-46
A high temperature calorimetric solution method involving the use of a sodium and lithium metaborate (NaBO2+LiBO2) molten bath at 1,180 K, has been developed in order to measure the enthalpy of formation of the forsterite-fayalite (Mg2SiO4-Fe2SiO4) system. The samples used for these measurements have been obtained by synthesis. They have been carefully controlled by chemical and X-ray analysis and Mössbauer spectroscopy. The results obtained for forsterite and fayalite agree closely with the data commonly found in the literature. Owing to the uncertainty of the experimental results, it is not possible to determine conclusively whether an enthalpy of mixing exists in this system. 相似文献
11.
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
12.
High pressure Ca2SiO4, the silicate K2NiF4-isotype with crystalchemical and geophysical implications
Lin-gun Liu 《Physics and Chemistry of Minerals》1978,3(3):291-299
The first silicate possessing a K2NiF4-type structure (Ca2SiO4) has been synthesized at loading pressures between 220 and 260 kbar and a temperature of about 1000° C in a diamond-anvil press coupled with a YAG laser heater. The lattice parameters for Ca2SiO4 (K2NiF4-type) area=3.564±0.002 andc=11.66±0.01 Å at room temperature and 1 bar pressure, and the molar volume is 44.57±0.05 cm3. The lattice parameter for the non-quenchable high-pressure perovskite modification of CaSiO3 is estimated to be 3.56±0.03 Å at STP conditions. To date, A2BX4 compounds possessing the K2NiF4-type structure arein all cases less dense than their corresponding mixtures of ABX3 and AX compounds possessing, respectively, the perovskite (or related structures) and rocksalt structures. Hence the K2NiF4 structure is unstable relative to the mixture perovskite plus rocksalt at high pressures. For example, in a preliminary experimental study Ca2GeO4 in the K2NiF4-type structure has been found to transform to an as-yet-undetermined phase or assemblage at pressures between 200 and 250 kbar and at about 1000° C. It is concluded that a similar phase transformation might also occur in Ca2SiO4 (K2NiF4 type) but that the K2NiF4-type structure would not be adopted by Mg2SiO4 in the earth's mantle. 相似文献
13.
The generation of both Brazil and Dauphiné twins is triggered by strain fields associated with the fluctuation of growth parameters when solid inclusions of goethite (?) are precipitated on to a growth surface. Brazil twins are generated either directly from solid inclusions of smaller size or dislocations originating therefrom, whereas Dauphiné twins are generated only directly from solid inclusions of larger size. Precipitation of polymerized embryonic particles of SiO2 on the surface of solid inclusions is considered to be responsible for the generation of Brazil twins, whereas for Dauphiné twins it is necessary that embryonic particles precipitated in twin orientation on the surface of solid inclusions grow beyond a critical size. 相似文献
14.
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
15.
Diopside twins mechanically on two planes, (100) and (001), and the associated macroscopic twinning strains are identical (Raleigh and Talbot, 1967). An analysis based on crystal structural arguments predicts that both twin mechanisms involve shearing of the (100) octahedral layers (containing Ca2+, Mg2+ and Fe2+ ions) by a magnitude of c/2. Small adjustments or shuffles occur in the adjacent layers containing the [SiO4]4? tetrahedral chains. While the (100) twins are conventional with shear parallel to the composition plane, this analysis predicts that (001) twins form by a mechanism closely related to kinking. A polycrystalline diopside specimen was compressed 8% at a temperature of 400° C, a pressure of 16 kilobars, and a compressive strain rate of about 10?4/s. Transmission electron microscopy on this specimen has revealed four basic lamellar features:
- (100) mechanical twin lamellae;
- (100) glide bands containing unit dislocations;
- (001) twin lamellae;
- (101) lamellar features, not as yet identified.
16.
Hiroshi Sawamoto 《Physics and Chemistry of Minerals》1986,13(1):1-10
Single crystals of ferromagnesian orthosilicates with modified spinel (β) and spinel (γ) structure as large as 500 μm have been grown by solid state crystallization at high temperature and high pressure using an MA8-type apparatus driven in a 2,000-ton uniaxial press. This system is capable of generating pressures of 24.0 (±0.3) GPa at 2,400 (±50)°C for one hour in a sample assembly volume of 0.14 cm3. Crystals larger than 100 μm were observed to grow only at pressures within 5 percent of the phase boundary between the stability fields of the β and γ phases. Experimental determination of the phase boundaries between β or β+γ and γ phases for (Mg,Fe)2SiO4 has been extended to 22 GPa and 2,400°C. The effect of configurational entropy due to disordering is evaluated to be minimal on the basis of the cationic distribution in the synthesized samples; thus, we conclude that the phase boundary between β or β+γ and γ phases remains essentially linear to 2,400°C. In (Mg,Fe)2SiO4 solid solutions, the stability field of the γ phase shifts towards the lower pressures with increasing iron content at a rate of a 1 GPa for each 10 mole percent Fe. Assignment of the β→β+γ→γ transition to the seismic 550 km discontinuity is rejected by the present phase diagram results for (Mg0.9Fe0.1)2SiO4 and measurement of acoustic velocities for β and γ Mg2SiO4, but the discontinuity may be caused by a phase transition of pyroxene to a garnet-like structure. 相似文献
17.
Boninites from Cape Vogel, PNG, are dominantly pyroxene-glass rocks, but many contain olivine, sometimes as refractory as Fo94. We derive a parental magma for this suite (in equilibrium with Fo94) which contains 20 wt.% MgO and is quartz-normative. This liquid is hydrous, and from petrographie evidence and whole rock H2O+ values, we estimate it to contain 2–3 wt.% H2O. These data suggest olivine fractionation and primary magmatic water are important in boninite genesis, but both are often obscured by later alteration. The derived parental magma has probably formed at 1,250–1,300° C and low pressures (< ?10kB) and is similar to those which gave rise to olivine-clinoenstatite phyric boninites from New Caledonia and from Howqua, Australia, and possibly to a proposed parental magma for the Bushveld Complex. 相似文献
18.
19.
B. Marler 《Physics and Chemistry of Minerals》1988,16(3):286-290
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
20.
The P-T path of magma associated with the 1944 Vesuvius eruption has been outlined on the basis of probe mineralogy and the relationships between the crystallising phases. Equilibrium P-T values, obtained from the reactions:
- CaMgSi2O6(liq) = CaMgSi2O6(cpx)
- NaAlSi3O8 (liq) = NaAlSi3O8 (plag)
- CaAl2Si2O8 (plag)=CaAl2SiO6(cpx)+SiO2(liq) have been established for three intracrustal crystallisation stages: I) 8.0 kbar and 1255 °C; II) 4.0 kbar and 1178 °C; III) 0.5 kbar and 1105 °C.