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1.
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   

2.
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   

3.
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   

4.
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   

5.
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   

6.
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   

7.
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   

8.
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   

9.
The amorphous phase of SiO2 produced upon recovery of shock-compressed quartz demonstrates a wide range of refractive indices which can be correlated to the shock state. Both infra-red absorption spectra and X-ray diffraction patterns indicate that the shock-produced amorphous SiO2 has a statistically more-random atomic distribution and longer Si-O and shorter Si-Si separation than does fused silica. By accounting for phase transformation, the calculated values of the shock and residual temperature are much higher than those obtained by Wackerle (1962) to a pressure of 700 kbar. Our results are consistent with experimental ones.  相似文献   

10.
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   

11.
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   

12.
Seven clinopyroxenes in the system CaScAlSiO6- CaAl2SiO6 synthesized at 1 atm and under high pressure have been studied by Raman spectroscopy. The T-O-T stretching band of CaScAlSiO6 pyroxene can be deconvoluted into three bands corresponding to Al-O-Al, Al-O-Si, and Si-O-Si stretching vibrations, although that of CaAl2SiO6 can be deconvoluted into the two bands (Al-O-Al+Al-O-Si) and Si-O-Si. The Al-O-Si Raman shifts of CaScAlSiO6 and CaAl2SiO6 pyroxenes are found to fall on the linear plot of the relationship between T-T distance and Raman shifts in ABSi2O6-type pyroxenes, suggesting that the Al-O-Si chains are relatively long. Variation of areal fractions of the Raman bands demonstrates that the partial disordering of Al/Si depends on the ionic radius and electronegativity of the octahedral ion.  相似文献   

13.
Molar elastic strain energy arising from dislocations in andalusite and sillimanite were calculated using equations derived from a non-core, linear elasticity model. For perfect (unit) c screw dislocations in these polymorphs, minimum dislocation densities of about 1010/cm2 are necessary to significantly perturb the andalusite=sillimanite equilibrium boundary in P-T space. Compared to unit c dislocations, smaller energy perturbations arise from dissociated c screw dislocations, which are commonly observed in kyanite and sillimanite. A low computed value of stacking fault energy (～30 ergs/cm2) in these polymorphs is compatible with the large separations of dissociated dislocations in these phases. Dislocation densities in naturally occurring Al2SiO5 polymorphs are typically <108/cm2. Assuming that these densities are representative of those existing during metamorphism, as is supported by the lack of microtextures indicative of strong recovery, it is concluded that molar strain energies corresponding to observed dislocation densities (<108/cm2) result in insignificant perturbation of P-T phase equilibrium boundaries of the Al2SiO5 polymorphs.  相似文献   

14.
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   

15.
Polarised Raman and infrared spectra of (ir) andalusite (Al2SiO5) single crystals have been measured and interpreted on the basis of a rigid-ion model calculation. The Al-O bond strength is found to be about 70% ionic in character whereas the mainly covalently bound SiO4 tetrahedra show ca. 40% ionicity. The interatomic short range forces are strongest between silicon and oxygen and rather weak around the fivefold coordinated aluminium. Thermal soft modes appear above 200°C and are correlated with a weakening of the Al-O bonds.  相似文献   

16.
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   

17.
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18.
In this paper we present a theoretical investigation of the structures and relative stability of the olivine and spinel phases of Mg2SiO4. We use both a purely ionic model, based on the Modified Electron Gas (MEG) model of intermolecular forces, and a bond polarization model, developed for low pressure silica phases, to investigate the role of covalency in these compounds. The standard MEG ionic model gives adequate structural results for the two phases but incorrectly predicts the spinel phase to be more stable at zero pressure. This is mainly because the ionic modeling of Mg2SiO4 only accounts for 95 percent of the lattice energy. The remainder can be attributed to covalency and many-body effects. An extension of the MEG ionic model using “many-body” pair potentials corrects the phase stability error, but predicts structures which are in poorer agreement with experiment than the standard ionic approach. In addition, calculations using these many-body pair potentials can only account for 10 percent of the missing lattice energy. This model predicts an olivine-spinel phase transition of 8 GPa, below the experimental value of 20 GPa. Therefore, in order to understand more fully the stability of these structures we must consider polarization. A two-shell bond polarization model enhances the stability of both structures, with the olivine structure being stabilized more. This model predicts a phase transition at about 80 GPa, well above the observed value. Also, the olivine and spinel structures calculated with this approach are in poorer agreement with experiment than the ionic model. Therefore, based on our investigations, to properly model covalency in Mg2SiO4, a treatment more sophisticated than the two-shell model is needed.  相似文献   

19.
We use an approach based upon the Born model of solids, in which potential functions represent the interactions between atoms in a structure, to calculate the phonon dispersion of forsterite and the lattice dynamical behaviour of the beta-phase and spinel polymorphs of Mg2SiO4. The potential used (THB1) was derived largely empirically using data from simple binary oxides, and has previously been successfully used to model the infrared and Raman behaviour of forsterite. It includes ‘bond bending’ terms, that model the directionality of the Si-O bond, in addition to the pair-wise additive Coulombic and short range terms. The phonon dispersion relationships of the Mg2SiO4 polymorphs predicted by THB1 were used to calculate the heat capacities, entropies, thermal expansion coefficients and Gruneisen parameters of these phases. The predicted heat capacities and entropies are in outstandingly good agreement with those determined experimentally. The predicted thermodynamic data of these phases were used to construct a phase diagram for this system, which has Clausius-Clapeyron slopes in very close agreement with those found by experiment, but which has predicted transformation pressures that show less close agreement with those inferred from experiment. The overall success, however, that we have in predicting the lattice dynamical and thermodynamic properties of the Mg2SiO4 polymorphs shows that our potential THB1 represents a significant step towards finding the elusive quantitative link between the microscopic or atomistic behaviour of minerals and their macroscopic properties.  相似文献   

20.
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   

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