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1.
The high pressure spinel polymorph of Ni2SiO4 persists metastably at 713°C and atmospheric pressure. The enthalpy of the olivine-spinel transition was obtained by measuring the heats of solution of both polymorphs in a molten oxide solvent, 2PbO · B2O3, at that temperature. For Ni2SiO4(ol)→Ni2SiO4, ΔH9860 = +1.4 ± 0.7kcal/mol. The heat content increments, H986 ? H297, were found to be: olivine, 25.73 ± 0.42kcal/mol, and spinel, 25.39 ± 0.20kcal/mol. The measured enthalpy of the transformation is consistent with the low slope of the phase boundary, ?P/?T = ～ 12b/deg, observed by Akimoto and others. The entropy of the olivine-spinel transition in Ni2SiO4 is accordingly about a factor of three smaller in magnitude (ΔS = ～ ?1cal/deg mol) than that for Co2SiO4,Fe2SiO4,Mg2SiO4or Mg2GeO4 (ΔS = ?3to?3.5cal/deg mol).  相似文献   

2.
Ultrasonic data for the velocities of the low- and high-pressure polymorphs of germanate compounds undergoing the pyroxene-garnet (CaGeO3, CdGeO3) and pyroxene-ilmenite (MgGeO3, MnGeO3) phase transformations have been determined as a function of pressure to 7.5 kbar at room temperature for polycrystalline specimens hot-pressed at pressures up to 60 kbar. These transitions are characterized by the following velocity (υ)-density (ρ) relationships: (1) the velocity jumps are comparable in percentage magnitude to the density jumps, with the exception of shear velocity for the pyroxene-ilmenite transition; (2) the ratio (υp/υs) of the compressional to shear velocity is constant or increases slightly across the transitions; and (3) low slopes (linear or logarithmic) on υ-ρ diagrams. The observed relationships (1) and (2) are similar to those for the coesite-stishovite transition, but are in marked contrast with those from the olivine-spinel and olivine-beta phase transformations. Coordination changes are thus important factors to be considered in establishing velocity-density systematics governing polymorphic transitions. The υ-ρ changes across the pyroxene-garnet and pyroxene-ilmenite transitions are also distinctly smaller than those produced by compression or thermal expansion of a homogeneous material or by varying composition at constant mean atomic weight. Systematic trends in the elastic properties for isostructural sequences support the concept of germanates as models for the elasticity of their silicate analogues; this scheme is applied to estimate the bulk moduli of the garnet (1.80 Mbar) and ilmenite (2.11 Mbar) polymorphs of MgSiO3.  相似文献   

3.
Magnesium orthosilicate with spinel structure (γ-Mg2SiO4) was synthesized at about 250 kbar and 1000°C. Unit cell dimension was established to be 8.076 ± 0.001Å. X-ray powder diffraction pattern revealed a significant difference between γ-Mg2SiO4 and other γ-M2SiO4 spinels (M = Fe, Co, and Ni) in the intensities of (111) and (331) reflections, both of which are virtually absent in the Mg2SiO4 spinel. This feature could be thoroughly understood by the calculation of the intensities for several silicate spinels.  相似文献   

4.
The electronic spin state of iron in perovskites with the chemical composition Mg0.9375Fe0.0625SiO3, Mg0.8750Fe0.1250SiO3 and Mg0.9375Fe0.0625Si0.9375Fe0.0625O3, have been determined at 0 K and various pressures between 40–160 GPa, using ab initio methods. The results indicate that ferric iron exhibits a wide range of spin transition pressures between about 60–160 GPa, while ferrous iron has a much narrower span, between about 130–145 GPa. In general, the lowest spin transition pressures are associated with the most energetically favorable substitution configurations, where iron atoms are closest together. The effect of spin state on calculated elastic properties is found to be small.  相似文献   

5.
The electrical conductivity of three polymorphs of Mn2GeO4 was measured under high pressures in the temperature range of 300–1200 K. It was found that the electrical conductivity increases discontinuously due to the transformation both from olivine structure (α) to modified spinel structure (β) and from β to strontium plumbate structure (δ). The amount of discontinuous change is about one half order of magnitude from α to β and one third order of magnitude from β to δ at 1200 K. In order to see the effect of the presence of iron ions, the electrical conductivity of the solid solution of (Mn0.9Fe0.1)2GeO4 was also measured. It was found that at low temperatures, where impurity conduction may be dominant, the solid solution is more conductive than the pure Mn2GeO4, but at high temperature no significant differences were observed between the solid solution and pure Mn2GeO4 in all polymorphs.A phase transformation from modified spinel structure to strontium plumbate structure is considered to be one of the plausible transformations occurring at a depth around 650 km in the earth's mantle. These experiments suggest that if this kind of transformation occurs in the mantle, some degrees of discontinuous change in electrical conductivity may be expected around 650 km.  相似文献   

6.
The enthalpies of formation from the oxides of Mg2SnO4 and Co2SnO4 were found by oxide melt solution calorimetry to be +1.13 ± 0.48 kcal/mol and ?2.31 ± 0.28 kcal/mol, respectively. Using these data, the slopes, ?P/?T, for disproportionation of these spinels to the component oxides at high pressure were calculated to be +30.4 ± 4.2 bar/K for Mg2SnO4 and ?10.3 ± 2.4 bar/K for Co2SnO4, in general agreement with the data of Jackson et al. (1974a,b). Using thermochemical data for the formation of olivines, for olivine-spinel transitions and for the transformation of quartz to stishovite, we calculate pressures for the disproportionation of silicate spinels to be in the range 150–200 kbar. Calculated slopes ?P/?T for the disproportionation reactions are ?10.7, ?24.9, ?11.2, and +7.6 bar/K for Mg2SiO4, Fe2SiO4, Co2SiO4, and Ni2SiO4. The large negative slope calculated for Fe2SiO4 results from a surprisingly large positive slope reported for the olivine-spinel transition in that compound (Akimoto et al., 1969). Further consideration of the systematic trends in the thermodynamics of spinel formation from the oxides suggests that the silicate spinels should have entropies of formation close to zero, resulting in values of ?P/?T which are zero or at most only slightly negative. This confirms the conclusion of Jackson, Liebermann, and Ringwood that values of ?P/?T for spinel disproportionation are unlikely to be more negative than ?10 bar/K and may well be slightly positive. Reaction of spinels to form other post-spinel phases, particularly ilmenite and perovskite, are discussed in terms of available thermochemical data.  相似文献   

7.
By using the diamond-anvil pressure cell coupled with laser heating, Ca2GeO4 in the K2NiF4-type structure has been found to decompose into the mixture Ca3Ge2O7 plus CaO at pressures greater than 200 kbar and at about 1000°C, and the same type of structure for Ca2MnO4 has been found to decompose into the mixture CaMnO3 (perovskite) plus CaO at pressures greater than 100 kbar and at about 1400°C. The decomposition product of Ca3Ge2O7 is a new compound which is isostructural with Sr3Ti2O7 and has the lattice parameters of a = 3.72 ± 0.01 and c = 19.32 ± 0.05 A? at room temperature and 1 bar pressure. The results of the study of Ca2GeO4 and Ca2MnO4 (both with the K2NiF4-type structure) strongly support the view that compounds possessing the K2NiF4-type structure are unstable relative to corresponding mixtures possessing the perovskite and rocksalt structures. It is concluded that, in the earth's mantle, the K2NiF4-type Ca2SiO4 would ultimately decompose into the mixture CaSiO3 (perovskite) + CaO or would otherwise transform to other as-yet-unknown phase(s), and that the mixture of MgSiO3 (perovskite) + MgO (the post-spinel phase of Mg2SiO4) would not adopt the K2NiF4-type structure.  相似文献   

8.
The elasticity of ferropericlase with a potential mantle composition of (Mg0.83,Fe0.17)O is determined using ultrasonic interferometry in conjunction with in situ X-radiation techniques (X-ray diffraction and X-radiography) in a DIA-type cubic anvil high-pressure apparatus to pressures of 9 GPa (NaCl pressure scale) at room temperature. In this study, we demonstrate that it is possible to directly monitor the specimen length using an X-ray image technique and show that these lengths are consistent with those derived from X-ray diffraction data when no plastic deformation of the specimen occurs during the experiment. By combining the ultrasonic and X-ray diffraction data, the adiabatic elastic bulk (KS) and shear (G) moduli and specimen volume can be measured simultaneously. This enables pressure scale-free measurements of the equation of state of the specimen using a parameterization such as the Birch-Murnaghan equation of state. The elastic moduli determined for (Mg0.83,Fe0.17)O are KS0=165.5(12) GPa, G0=112.4(4) GPa, and their pressure derivatives are KS0′=4.17(20) and G0′=1.89(6). If these results are compared with those for MgO, they demonstrate that KS0 and KS0′ are insensitive to the addition of 17 mol% FeO, but G0 and G0′ are reduced by 14% and 24%, respectively. We calculate that the P and S wave velocities of a perovskite plus ferropericlase phase assemblage with a pyrolite composition at the top of the lower mantle (660 km depth) are lowered by 0.8 and 2.3%, respectively, when compared with those calculated using the elastic properties of end-member MgO. Consequently, the magnitudes of the calculated wave velocity jumps across the 660 km discontinuity are reduced by about 11% for P wave and 20% for S wave, if this discontinuity is considered as a phase transformation boundary only (ringwoodite→perovskite+ferropericlase).  相似文献   

9.
In this study, the oxidative decolorization of C.I. reactive yellow 145 (RY 145) from synthetic textile wastewater including RY 145 and polyvinyl alcohol by Fenton and sono‐Fenton processes which are the combination of Fenton process with ultrasound has been carried out. The effects of some operating parameters which are the initial pH of the solution, the initial concentration of Fe2+, H2O2, and the dye, temperature, and agitation speed on the color and chemical oxygen demand (COD) removals have been investigated. The optimum conditions have been found as [Fe2+] = 20 mg/L, [H2O2] = 20 mg/L, pH 3 for Fenton process and [Fe2+] = 20 mg/L, [H2O2] = 15 mg/L, pH 3 for sono‐Fenton process by indirectly sonication at 35 kHz ultrasonic frequency and 80 W ultrasonic power. The color and COD removal efficiencies have been obtained as 91 and 47% by Fenton process, and 95 and 51% by sono‐Fenton processes, respectively. Kinetic studies have been performed for the decolorization of RY 145 under optimum conditions at room temperature. It has been determined that the decolorization has occurred rapidly by sono‐Fenton process, compared to Fenton process.  相似文献   

10.
The melting curves of the fluorides ZnF2 and NiF2 (rutile structure), CaF2, SrF2 and BaF2 (fluorite structure), and of the fluoroberyllates Na2BeF4 and Li2BeF4 have been studied at pressures ? 40 kbar by differential thermal analysis in a piston-cylinder high-pressure device. The initial slopes (dTm/dP)0 of these melting curves are respectively 7.2, 5.8, 16.7, 15.2, 15.7, 15.1 and <0°C/kbar. A new Li2BeF4 polymorph, apparently of the olivine structure type, is stable at pressures > 10 kbar and its melting curve has an average slope of ～6.7°C/kbar. These new data and those for SiO2, BeF2, GeO2, LiF and MgF2, recently studied by Jackson, are combined with existing data for elements, ionic compounds and silicates to assess the influence of crystal structure, molar volume and the nature of interatomic bonding on the initial slopes of melting curves. It is found that the entropy of fusion (ΔSm) is primarily a function of crystal structure while the volume change on fusion (ΔVm) is controlled by crystal molar volume within each isostructural series. Such systematics have recently facilitated estimation of the initial slopes of the melting curves of periclase and stishovite. New and existing melting data for silicates and their analogues have been analysed and a systematic dependence of (dTm/dP)0 on SiO2 concentration has been demonstrated. Possible implications of this trend for partial melting of upper-mantle garnet lherzolite are illustrated. Finally, the use of the traditional silicate-germanate and oxide-fluoride modelling schemes is reviewed in the light of information from this present study.  相似文献   

11.
High temperature calorimetric measurements of the enthalpies of solution in molten if2 PbO · B2O3 of α- and γ-Fe2SiO4 and α-, β-, and γ-Co2SiO4 permit the calculation of phase relations at high pressure and temperature. The reported triple point involving α-, β-, and γ-Co2SiO4 is confirmed to represent stable equilibrium. The curvature in the α?β phase boundary in Co2SiO4 and of an α?γ boundary in Fe2SiO4 at high temperature is explained in part by the effects of compressibility and thermal expansion, but better agreement with the observed phase diagram is obtained when one considers the effect of small amounts of cation disorder in the spinel and/or modified spinel phases. The calculated ΔH0 and ΔS0 values for the α?β, α?γ, and β?γ transitions show that enthalpy and en changes both vary strongly in the series Mg, Fe, Co, and Ni, and are of equal importance in determining the stability relations. The disproportionation of Fe2SiO4 and Co2SiO4 spinel to rocksalt plus stishovite is calculated to occur in the 170–190 kbar region; cation disorder and/or changes in wüstite stoichiometry can affect the P?T slope. The calorimetric data for CoSiO3 and FeSiO3 are in good agreement with the observed phase boundary for pyroxene formation from olivine and quartz. The decomposition of pyroxene to spinel and stishovite at pressures near the coesite-stishovite transition is predicted in both iron and cobalt systems. The use of calorimetric data, obtained from small samples of high pressure phases, is very useful in predicting equilibrium phase diagrams in the 50–300 kbar range.  相似文献   

12.
Phase equilibria in a natural garnet lherzolite nodule (PHN 1611) from Lesotho kimberlite and its chemical analogue have been studied in the pressure range 45–205 kbar and in the temperature range 1050–1200°C. Partition of elements, particularly Mg2+Fe2+, among coexisting minerals at varying pressures has also been examined. High-pressure transformations of olivine(α) to spinel(γ) through modified spinel(β) were confirmed in the garnet lherzolite. The transformation behavior is quite consistent with the information previously accumulated for the simple system Mg2SiO4Fe2SiO4. At pressures of 50–150 kbar, a continuous increase in the solid solubility of the pyroxene component in garnet was demonstrated in the lherzolite system by means of microprobe analyses. At 45–75 kbar and 1200°C, the Fe2+/(Mg + Fe2+) value becomes greater in the ascending order orthopyroxene, Ca-rich clinopyroxene, olivine and garnet. At 144–146 kbar and 1200°C, garnet exhibits the highest Fe2+/(Mg + Fe2+) value; modified spinel(β) and Ca-poor clinopyroxene follow it. When the modified spinel(β)-spinel(γ) transformation occurred, a higher concentration of Fe2+ was found in spinel(γ) rather than in garnet. As a result of the change in the Mg2+Fe2+ partition relation among coexisting minerals, an increase of about 1% in the Fe2SiO4 component in (Mg,Fe)2SiO4 modified spinel and spinel was observed compared with olivine.These experimental results strongly suggest that the olivine(α)-modified spinel(β) transformation is responsible for the seismic discontinuity at depths of 380–410 km in the mantle. They also support the idea that the minor seismic discontinuity around 520 km is due to the superposition effect of two types of phase transformation, i.e. the modified spinel(β)-spinel(γ) transformation and the pyroxene-garnet transformation. Mineral assemblages in the upper mantle and the upper half of the transition zone are given as a function of depth for the following regions: 100–150, 150–380, 380–410, 410–500, 500–600 and 600–650 km.  相似文献   

13.
Meteoric smoke forms in the mesosphere from the recondensation of the metallic species and silica produced by meteoric ablation. A photochemical flow reactor was used to generate meteoric smoke mimics using appropriate photolytic precursors of Fe and Si atoms in an excess of oxidant. The following systems were studied: (i) Fe+O3/O2, (ii) Fe+O3/O2+H2O, (iii) Fe+Si/SiO+O3/O2 and (iv) Si/SiO+O3/O2. The resulting nano-particles were captured for imaging by transmission electron microscopy, combined with elemental analysis using X-ray (EDX) and electron energy loss (EELS) techniques. These systems generated particle compositions consistent with: (i) Fe2O3 (hematite), (ii) FeOOH (goethite), (iii) Fe2SiO4 (fayalite) and (iv) SiO2 (silica). Electron diffraction revealed that the Fe-containing particles were entirely amorphous, while the SiO2 particles displayed some degree of crystallinity. The Fe-containing particles formed fractal aggregates with chain-like morphologies, whereas the SiO2 particles were predominantly spherical and compact in appearance. The optical extinction spectra of the Fe-containing particles were measured from 300 nm<λ<650 nm. Excellent agreement was found with the extinction calculated from Mie theory using the refractive indices for the bulk compounds, and assuming that the fractal aggregates are composed of poly-disperse distributions of constituent particles with radii ranging from 5 to 100 nm. These sizes were confirmed from measurements of the particle size distributions and microscopic imaging. Finally, the particle growth kinetics of the Fe-containing systems exhibit unexpectedly rapid agglomerative coagulation. This was modelled by assuming an initial period of coalescent particle growth resulting from diffusional (Brownian) coagulation to form primary particles; further growth of these particles is then dominated by long-range magnetic dipole–dipole interactions, leading to the fractal aggregates observed. The atmospheric implications of this work are then discussed.  相似文献   

14.
Three synthetic Fe3+ bearing λ-Fe2SiO4 were analyzed using electron probe method, and the M?ssbauer spectra of the samples at 298 K, 150 K, and 95 K were measured. Each spectrum at three temperatures is composed of two doublets. These two doublets are assigned to Fe2+ in the octahedral sites and Fe3+ in the tetrahedral sites, respectively. Site occupancies were determined. The results show that Fe3+ and a small amount of Si4+ are in the tetrahedral and octahedral sites, respectively. The average bond lengths of the octahedral and tetrahedral sites were calculated according to the equations primarily given by Hill et al., O’Neill and Navrotsky and modified by the authors. Furthermore, the octahedral and tetrahedral bond lengths were used to calculate cell parameters and oxygen parameters. In addition, Fe3+ line broadening in the M?ssbauer spectra of Fe3+ bearing λ-Fe2SiO4 were interpreted by using the next nearest neighbor effects  相似文献   

15.
Two synthetic pyroxenes (FeSiO3, MgSiO3) and five natural pyroxenes with compositions of about Fs80En20, Fs60En40, Fs50En50, Fs40En60, and Fs20En80 have been subjected to pressures up to250 ± 50kbars at a temperature of about1500 ± 200°C in a diamond anvil cell heated by an infrared laser beam. After quenching and unloading X-ray data analysis indicates that (1) those with Mg less than 50% undergo the following reactions: 2(Mg,Fe)SiO3 (pyroxene) → (Mg,Fe)2SiO4 (spinel) + SiO2 (stishovite) → 2(Mg,Fe)O (magnesiowu¨stite) + SiO2 (stishovite) with increase of pressure, and (2) those with Mg higher than 60%, undergo the following reactions: 2(Mg,Fe)SiO3 (pyroxene) → (Mg,Fe)2SiO4 (spinel) + SiO2 (stishovite) → 2(Mg,Fe)SiO3 (hexagonal phase) → 2(Mg,Fe)O (magnesiowu¨stite) + SiO2 (stishovite) with increase of pressure.  相似文献   

16.
Melting point of germanate forsterite, Mg2GeO4, was raised by compression at the rate of 30°C/GPa. The triple point, at which three phases of olivine- and spinel-type solids and liquid coexisted, was fixed at 1950°C and 3.5GPa. Wen these results are combined with the thermodynamical data of forsterite, Mg2SiO4, it is estimated that the triple point of forsterite lies in a region ranging from 2700° to 3000°C in temperature and from 20 to 30GPa in pressure.  相似文献   

17.
The most abundant mineral on Earth has a perovskite crystal structure and a chemistry that is dominated by MgSiO3 with the next most abundant cations probably being aluminum and ferric iron. The dearth of experimental elasticity data for this chemically complex mineral limits our ability to calculate model seismic velocities for the lower mantle. We have calculated the single crystal elastic moduli (cij) for (Mg, Fe3 +)(Si, Al)O3 perovskite using density functional theory in order to investigate the effect of chemical variations and spin state transitions of the Fe3+ ions. Considering the favored coupled substitution of Mg2+-Si4 + by Fe3+-Al3+, we find that the effect of ferric iron on seismic properties is comparable with the same amount of ferrous iron. Ferric iron lowers the elastic moduli relative to the Al charge-coupled substitution. Substitution of Fe3+ for Al3+, giving rise to an Fe/Mg ratio of 6%, causes 1.8% lower longitudinal velocity and 2.5% lower shear velocity at ambient pressure and 1.1% lower longitudinal velocity and 1.8% lower shear velocity at 142 GPa. The spin state of the iron for this composition has a relatively small effect (< 0.5% variation) on both bulk modulus and shear modulus.  相似文献   

18.
A simple, low cost, highly effective, and useful Fenton oxidation treatment of synthetic dye bath waste with pickling liquor as a source of iron (Fe2+ catalyst) is reported. Optimizations of contact time, Fe2+ and H2O2 doses are carried out. Oxidative de‐colorization and degradation of Reactive Blue 4 and Reactive Orange 16 was measured in terms of decrease in absorbance at their wavelength of maximum absorption (RB4, 599 nm; and RO16, 493 nm) and also as reduction in chemical oxygen demand (COD). Approximately, 62% COD was removed in 2 h at optimized doses of Fe2+ (8.95 mM) and H2O2 (61.8 mM) by using pickling waste as a source of Fe2+ catalyst. Similar performance efficiency was observed when neat FeSO4 was used as a source of Fe2+, indicating that pickling liquor can be a low cost source of Fe2+ to treat synthetic dye bath waste by Fenton method.  相似文献   

19.
In a diamond-anvil press coupled with YAG laser heating, the spinels of Co2GeO4 and Ni2GeO4 have been found to disproportionate into their isochemical oxide mixtures at about 250 kbar and 1400–1800°C in the same manner as their silicate analogues. At about the same P-T conditions MnGeO3 transforms to the orthorhombic perovskite structure (space group Pbnm); the lattice parameters at room temperature and 1 bar are a0 = 5.084 ± 0.002, b0 = 5.214 ± 0.002, and c0 = 7.323 ± 0.003Å with Z = 4 for the perovskite phase. The zero-pressure volume change associated with the ilmenite-perovskite phase transition in MnGeO3 is ?6.6%. Mn2GeO4 disproportionates into a mixture of the perovskite phase of MnGeO3 plus the rocksalt phase of MnO at P = 250kbar and T = 1400–1800°C. The concept of utilizing germanates as high-pressure models for silicates is valid in general. The results of this study support the previous conclusion that the lower mantle comprises predominantly the orthorhombic perovskite phase of ferromagnesian silicate.  相似文献   

20.
Phase transformations in six fluorite-type dioxides (“TbO2”, PbO2, “PrO2”, CeO2, UO2 and ThO2 in the order of increasing cation size, where the quotation marks indicate non-stoichiometric materials) have been investigated in the diamond-anvil press coupled with laser heating. Together with earlier work, the results show that the post-fluorite phase transformations of these dioxides fall into two groups. The smaller cation group (HfO2, ZrO2 and “TbO2”) transforms to a cotunnite or a distorted cotunnite-type structure at pressures in the vicinity of 100 kbar and at about 1000°C. The larger cation group (from PbO2 to ThO2) is believed to transform to a different type of orthorhombic modification at high pressures. It is plausible that this high-pressure phase may possess a Ni2Si-related structure, as was observed in ThO2 and “PrO2” at pressures greater than 150 and 200 kbar, respectively. The dioxides with Ni2Si-related structure are the most closely packed dioxides known to exist. It is suggested that all the polymorphic structures of ZrO2 (baddeleyite, tetragonal fluorite, cubic fluorite and cotunnite) plus the new phase found in the large cation group dioxides are possible high-pressure models for the post-rutile and/or post-Fe2N phases of SiO2. The polyhedral coordination in these model structures varies from 7 to 10, compared with 6 for the rutile-and Fe2N-type SiO2. The fact that the larger cation group adopts a very closely packed structure at high pressures substantiates the earlier hypothesis that the radioactive elements U and Th in the earth's deep mantle are most likely to occur as dioxide rather than as silicate.  相似文献   

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