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1.
This study illustrates the degradation of an azo dye, Reactive Yellow 81 (RY81), by the combined irradiation of UV‐C and ultrasound in the presence of homogeneous (Fe2+) and heterogeneous (TiO2, ZnO) catalysts. The efficiency of homogeneous and heterogeneous oxidation systems was evaluated in regard of the decolorization and mineralization of RY81. Decolorization followed pseudo‐first‐order kinetics with homogeneous and heterogeneous catalysts. Complete color removal was accomplished by homogeneous sonocatalytic and sonophotocatalytic oxidation processes with apparent rate constants of 0.96 × 10?3 and 46.77 × 10?3 s?1, respectively, in the presence of Fe2+. However, partial color removal was obtained by heterogeneous sonocatalytic, photocatalytic, and sonophotocatalytic oxidation processes with apparent rate constants of 2.32 × 10?3, 3.60 × 10?3, and 3.67 × 10?3 s?1, respectively, in the presence of ZnO. TiO2 had the worst catalytic effect of all of the oxidation processes. The addition of hydrogen peroxide increased the rate constants of the heterogeneous oxidation processes and decreased the rate constants of the homogeneous oxidation processes. RY81 mineralization was 62.8% for the US/UV/Fe2+ homogeneous oxidation process, which was the best oxidation process, whereas it was 43.5% for the US/UV/ZnO/H2O2 heterogeneous oxidation process within 2 h reaction time.  相似文献   

2.
Natural ilmenite (Fe,Mg)TiO3 has been found to transform to the perovskite structure and then to disproportionate into its component oxides, (Fe,Mg)O plus a cubic phase of TiO2, at loading pressures of 140 and 250 kbar respectively, and at temperatures of 1,400 to 1,800°C. Samples were compressed in a diamond-anvil press and heated by irradiation with a YAG laser. The lattice parameters of the perovskite phase of (Fe,Mg)TiO3 at room temperature and 1 bar are a0 = 4.471 ± 0.004, b0 = 5.753 ± 0.005, and c0 = 7.429 ± 0.006 A? with 4 molecules per cell. The zero-pressure volume change is 8.0% for the ilmenite-perovskite transition, 13.3% for the perovskite-mixed-oxides transition, and 20.2% for the ilmenite-mixed-oxides transition. The cubic phase of TiO2 can be indexed on the basis of space group Fm3m with Z = 4 and a0 = 4.455 ± 0.008 A? at room temperature and 1 bar, which corresponds to a decrease in zero-pressure volume of 29.2% for the rutile-cubic-phase transition. An isentropic bulk modulus at zero pressure of 5.75 ± 0.30 Mbar and a pressure derivative greater than 8 were calculated for the high-pressure cubic phase. The calculated bulk modulus for the mixture of (Fe,Mg)O and cubic TiO2 is 2.48 ± 0.25 Mbar. All the phase transformations, the calculated lattice parameters, and the bulk moduli observed in this study are in good agreement with published shock-Hugoniot data for ilmenite and rutile.  相似文献   

3.
Interdiffusion experiments were performed between Fe3O4 (single crystal) and Fe2.8Ti0.2O4 (powder), under self-buffering conditions (temperature range 600–1034°C), and for various oxygen potentials at 1400°C. Profiles of Fe and Ti were obtained by electronprobe microanalysis, and the interdiffusion coefficient D was calculated by the Boltzmann-Matano method. Low-temperature data at 3 mole% Ti could be described by D = (3.85?1.11+1.68) × 10?3exp(2.23 ± 0.04 eV/kT) cm2/s. An estimate is given for the time to interdiffuse 2μm at various temperatures, and the results compared with recent experiments.  相似文献   

4.
The concentration of rock-forming elements, the static magnetic susceptibility κ, spectra of electron paramagnetic resonance, and their relative intensities I are studied in samples from a borehole drilled in Cenozoic sedimentary deposits of southern Western Siberia. All measured values experience appreciable irregular variations with depth. A linear dependence exists between κ and I within the range of their medium and large values; κ and I have maximum values in the same sample, and κmax = 1920 × 10?6SI, κmin = 210 × 10?6 SI, and κav = 630 × 10?6 SI. The magnetic properties of the samples are controlled by Fe2+ ions present in clastic material and by microphases (clusters) with Fe3+ ions of the goethite and lepidocrocite type present in the cement. The theoretically possible magnetic susceptibility of the Fe2+ ion system (provided that all iron exists in this form) is quite comparable with κmin but, even with very high concentrations of Fe2+, does not reach half of κav: (154 < κ(Fe2+) < 254) × 10?6 SI. Anomalously high values of κ are due to a large number of clusters with Fe3+ ions if structural units FeOOH do not dissociate and the interaction of the clusters with hydroxides of aluminum and precipitation medium impedes the process of their coagulation. Otherwise, the cluster sizes gradually increase, an antiferromagnetic structure develops in clusters, and the magnetic susceptibility decreases.  相似文献   

5.
The high-speed correction factor to the O+-O collision frequency, resulting from drift velocities between ions and neutrals, is calculated by solving the integral expression in this factor both numerically and analytically. Although the analytic solution is valid for either small or large drift velocities between ions and neutrals, for temperatures of interest and all drift velocities considered, agreement is found between analytic and detailed numerical integration results within less than 1% error. Let T r designate the average of the ion and neutral temperatures in K, and u=v d /<alpha>, where v d is the relative drift velocity in cm s?1, and <alpha>=4.56×103\sqrtT r cm s?1 is the thermal velocity of the O+-O system. Then, as u ranges from 0 to 2, the correction factor multiplying the collision frequency increases monotonically from 1 to about 1.5. An interesting result emerging from this calculation is that the correction factor for temperatures of aeronomical interest is to a good approximation independent of the temperature, depending only on the scaled velocity u.  相似文献   

6.
The influence of atmospheric solar radiation absorption on the photodetachment, dissociative photodetachment, and photodissociation rate coefficients (photodestruction rate coefficients) of O?, Cl?, O2 ?, O3 ?, OH?, NO2 ?, NO3 ?, O4 ?, OH?(H2O), CO3 ?, CO4 ?, ONOO?, HCO3 ?, CO3 ?(H2O), NO3 ?(H2O), O2 +(H2O), O4 +, N4 +, NO+(H2O), NO+(H2O)2, H+(H2O) n for n = 2–4, NO+(N2), and NO+(CO2) at D-region altitudes of the ionosphere is studied. A numerical one-dimensional time-dependent neutral atmospheric composition model has been developed to estimate this influence. The model simulations are carried out for the geomagnetically quiet time period of 15 October 1998 at moderate solar activity over the Boulder ozonesonde. If the solar zenith angle is not more than 90° then the strongest influence of atmospheric solar radiation absorption on photodestruction of ions is found for photodissociation of CO4 ? ions when CO3 ? ions are formed. It follows from the calculations that decreases in the photodestruction rate coefficients of ions under consideration caused by this influence are less than 2 % at 70 km altitude and above this altitude if the solar zenith angle does not exceed 90°.  相似文献   

7.
Photochemistry of Ions at D-region Altitudes of the Ionosphere: A Review   总被引：2，自引：2，他引：0  
The current state of knowledge of the D-region ion photochemistry is reviewed. Equations determining production rates of electrons and positive ions by photoionization of atmospheric neutral species are presented and briefly discussed. Considerable attention is given to the progress in the chemistry of O+(4S), O+(2D), O+(2P), N+, N2 +, O2 +, NO+, N4 +, O4 +, NO+(N2), NO+(CO2), NO+(CO2)2, NO+(H2O) n for n = 1–3, NO+(H2O)(N2), NO+(H2O)2(N2), NO+(H2O)(CO2), NO+(H2O)2(CO2), O2 +(H2O), H3O+(OH), H+(H2O) n for n = 1–8, O?, O2 ?, O3 ?, O4 ?, OH?, CO3 ?, CO4 ?, NO2 ?, NO3 ?, ONOO?, Cl?, Cl?(H2O), Cl?(CO2), HCO3 ?, CO3 ?(H2O), CO3 ?(H2O)2, NO3 ?(H2O), NO3 ?(H2O)2, OH?(H2O), and OH?(H2O)2 ions. The analysis of the D-region rocket ion mass spectrometer measurements shows that, among these ions, O2 +, NO+, NO+(H2O), and H+(H2O) n for n = 1–7 can make the main contribution to the total positive ion number density, and O?, O2 ?, Cl?, OH?(H2O), CO3 ?, HCO3 ?, NO3 ?, ONOO?, CO4 ?, NO3 ?(H2O), NO3 ?(H2O)2, and 35Cl?(CO2) ions can be responsible for the main contribution to the total negative ion number density. Photodetachment of electrons from O?, Cl?, O2 ?, O3 ?, OH?, NO2 ?, and NO3 ?, dissociative electron photodetachment of O4 ? and OH?(H2O), and photodissociation of O3 ?, O4 ?, CO3 ?, CO4 ?, ONOO?, HCO3 ?, CO3 ?(H2O), NO3 ?(H2O), O2 +(H2O), O4 +, N4 +, NO+(H2O), NO+(H2O)2, H+(H2O) n for n = 2–4, NO+(N2), and NO+(CO2) are studied, and the photodetachment and photodissociation rate coefficients are calculated using the current state of knowledge on the cross sections of these processes and fluxes of solar radiation.  相似文献   

8.
Thermal diffusivity (D) was measured using laser-flash analysis on pristine and remelted obsidian samples from Mono Craters, California. These high-silica rhyolites contain between 0.013 and 1.10?wt% H2O and 0 to 2?vol% crystallites. At room temperature, D glass varies from 0.63 to 0.68?mm2?s?1, with more crystalline samples having higher D. As T increases, D glass decreases, approaching a constant value of ??0.55?mm2?s?1 near 700?K. The glass data are fit with a simple model as an exponential function of temperature and a linear function of crystallinity. Dissolved water contents up to 1.1?wt% have no statistically significant effect on the thermal diffusivity of the glass. Upon crossing the glass transition, D decreases rapidly near ??1,000?K for the hydrous melts and ??1,200?K for anhydrous melts. Rhyolitic melts have a D melt of ??0.51?mm2?s?1. Thermal conductivity (k?=?D·??·C P) of rhyolitic glass and melt increases slightly with T because heat capacity (C P) increases with T more strongly than density (??) and D decrease. The thermal conductivity of rhyolitic melts is ??1.5?W?m?1?K?1, and should vary little over the likely range of magmatic temperatures and water contents. These values of D and k are similar to those of major crustal rock types and granitic protoliths at magmatic temperatures, suggesting that changes in thermal properties accompanying partial melting of the crust should be relatively minor. Numerical models of shallow rhyolite intrusions indicate that the key difference in thermal history between bodies that quench to obsidian, and those that crystallize, results from the release of latent heat of crystallization. Latent heat release enables bodies that crystallize to remain at high temperatures for much longer times and cool more slowly than glassy bodies. The time to solidification is similar in both cases, however, because solidification requires cooling through the glass transition in the first case, and cooling only to the solidus in the second.  相似文献   

9.
The dependence of bulk sound speed Vφ upon mean atomic weight m and density ρ can be expressed in a single equation:
Vφ=Bρλ(m0m[12+λ(1?c)](km/sec)
Here B is an empirically determined “universal” parameter equal to 1.42, m0 = 20.2, a reference mean atomic weight for which well-determined elastic properties exist, and λ = 1.25 is a semi empirical parameter equal to γ ? 13 where γ is a Grüneisen parameter. The constant c = (? ln VM/? ln m)X, where VM is molar volume, is in general different for different crystal structure series and different cation substitutions. However, it is possible to use cFe = 0.14 for Fe2+Mg2+ and GeSi substitutions and cCa ? 1.3 for CaMg substitutional series. With these values it is pos to deduce from the above equation Birch's law, its modifications introduced by Simmons to account for Ca-bearing minerals, variations in the seismic equation of state observed by D.L. Anderson, and the apparent proportionality of bulk modulus K to VM?4.  相似文献   

10.
In the present study, the stable isotopes δ18O and δ2H were used for assessment of the water balance in a heterogeneously structured catchment area in the Lusatian Lignite Mining District, in particular, for estimation of the annual groundwater inflow and outflow (IGW and OGW) of Mining Lake Plessa 117. The application of stable isotopes was possible since the water exchange in the catchment area had reached steady‐state conditions after the abandonment of mining activities in 1968 and the filling of the voids and aquifers by re‐rising groundwater in the years thereafter. Diverging slopes of the Evaporation Line and the Global Meteoric Water Line manifested as evaporation from the lake catchment area. The calculated isotope water balance was compared with the commonly used surface water balance, which is unable to differentiate between IGW and OGW, and with a local groundwater model. The groundwater model calculated an IGW of about 811 000 m3 yr?1 and an OGW close to zero, whereas the isotope water balance showed fluxes of about 914 000 and 140 000 m3 yr?1, respectively. Considering the contribution of the groundwater inflow to the total annual input into the lake (ΔIT) and the mean residence time (τ), where the groundwater model and the isotope water balance calculated 42 and 47% for ΔIT and 4·3 and 3·9 years for τ, respectively, it was shown that both water balance calculation methods led to comparable results despite the differences in IGW and OGW. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

11.
The fO2 stability relations of ilmenite and ulvöspinel were determined using C-O H-N gas-flow apparatus with fO2 measured by a solid ceramic (calcia-zirconia) oxygen electrolyte cell. For Fe+TiO2 + 1/2 O2 =FeTiO3 (from 850°–1050°C), 1/2 log fO2=(−11,250/T) + 0.98 and for Fe+FeTiO3 + 1/2 O2 =Fe2TiO4 (from 850°–1210°C), 1/2 logfO2 = (−12,170/T) + 1.93. These curves lie at significantly higher values of ?O2 than determined by previous investigators (i.e., 3/4 and1/4 order of magnitude for ilmenite and ulvöspinel, respectively). In addition, for Fe+ 2TiO2 + 1/2 O2 =FeTi2O5 (1210°C), ΔGr0=−45.8 ± 0.6 kcal. The QFI curve crosses the ulvöspinel reduction curve at ∼950°C and is at lower values of fO2 below this temperature. The occurrences of fayalite reduction to SiO2 + Fe in lunar rock 14053, as well as a new finding of this assemblage in 14072, are evidence for extreme sub-solidus reduction, whereas ulvöspinel breakdown alone occurs under less reducing conditions. The ‘complete’ reduction of ulvöspinel to TiO2 + Fe occurs in 2 steps: first, to ilmenite + Fe and then, however more slowly, to rutile + Fe. Thus, the presence of ulvöspinel but lack of ilmenite reduction in lunar rocks cannot be used as evidence that the fO2 was between the associated curves — only upper limits of fO2 can be inferred.  相似文献   

12.
The elastic moduli of a synthetic single crystal of pyrope (Mg3Al2Si3O12) have been determined using a technique based on Brillouin scattering. These results are used in an evaluation of the effect of composition on the elastic properties of silicate garnet solid solution series (Mg, Fe, Mn, Ca)3 (Al, Fe, Cr)2 Si3O12. In the pyralspites (Mg FeMn aluminum garnets), for which a large amount of data is available, this analysis indicates that the bulk modulus K is independent of the Fe2+/Mg2+ ratio, which is similar to the behavior observed in olivines and pyroxenes. However, the shear modulus μ of the garnets increases by 10% from the Mg to the Fe end member, in contrast to the decrease of μ with Fe content which is observed in olivines and pyroxenes. This contrasting behavior is most probably related to the oxygen coordination of the cation site occupied by Mg2+ and Fe2+ in these different minerals.  相似文献   

13.
The “anomalous” layer in the lowermost mantle, identified as D″ in the notation of K.E. Bullen, appears in the PREM Earth model as a 150 km-thick zone in which the gradient of incompressibility with pressure, dKdP, is almost 1.6, instead of 3.2 as in the overlying mantle. Since PREM shows no accompanying change in density or density gradient, we identify D″ as a thermal boundary layer and not as a chemically distinct zone. The anomaly in dKdP is related to the temperature gradient by the temperature dependence of Ks, for which we present a thermodynamic identity in terms of accessible quantities. This gives the numerical result (?Ks/?T)P=?1.6×107 Pa K?1 for D″ material. The corresponding temperature increment over the D″ range is 840 K. Such a layer cannot be a static feature, but must be maintained by a downward motion of the lower mantle toward the core-mantle boundary with a strong horizontal flow near the base of D″. Assuming a core heat flux of 1.6 × 1012 W, the downward speed is 0.07 mm y?1 and the temperature profile in D″, scaled to match PREM data, is approximately exponential with a scale height of 73 km. The inferred thermal conductivity is 1.2 W m?1 K?1. Using these values we develop a new analytical model of D″ which is dynamically and thermally consistent. In this model, the lower-mantle material is heated and softened as it moves down into D″ where the strong temperature dependence of viscosity concentrates the horizontal flow in a layer ～ 12 km thick and similarly ensures its removal via narrow plumes.  相似文献   

14.
A new phase which is much denser than the component oxides of spinel (MgAl2O4) was synthesised at loading pressures greater than 250 kbar and at about 1000°C in a diamond-anvil press coupled with laser heating. The new phase (ε-MgAl2O4) was indexed on the basis of an orthorhombic cell with a = 8.507 ± 0.004, b = 2.740 ± 0.003, c = 9.407 ± 0.005Å, and Z = 4 at room temperature and 1 bar pressure. Thus the molar volume for ε-MgAl2O4 at the above conditions was calculated to be 33.01 ± 0.07 cm3, which is 10.3% less than that of the mixture corundum plus periclase. The dense phase of spinel found in shock-wave experiments can be reasonably interpreted as ε-MgAl2O4, and this may be a potentially important mineral component of the earth's lower mantle. The new structure may also provide a possible candidate for the dense phases of Fe3O4 and Mg2SiO4 which were found by shock experiments.  相似文献   

15.
Volume measurements for magnesiowüstite (Mg0.6Fe0.4)O, were carried out up to pressures of 10.1 GPa in the temperature range 300–1273 K, using energy-dispersive synchrotron X-ray diffraction. These data allow reliable determination of the temperature dependence of the bulk modulus and good constraint on the thermal expansitivity at ambient pressure which was previously not known for magnesiowüstite. From these data, thermal and elastic parameters were derived from various approaches based on the Birch–Murnaghan equation of state (EOS) and on the relevant thermodynamic relations. The results from three different equations of state are remarkably consistent. With (∂KT/∂P)T fixed at 4, we obtained K0=158(2) GPa, (∂KT/∂T)P=−0.029(3) GPa K−1, (∂KT/∂T)V=−3.9(±2.3)×10−3 GPa K−1, and αT=3.45(18)×10−5+1.14(28)×10−8T. The K0, (∂KT/∂T)P, and (∂KT/∂T)V values are in agreement with those of Fei et al. (1992) and are similar to previously determined values for MgO. The zero pressure thermal expansitivity of (Mg0.6Fe0.4)O is found to be similar to that for MgO (Suzuki, 1975). These results indicate that, for the compositional range x=0–0.4 in (Mg1−xFex)O, the thermal and elastic properties of magnesiowüstite exhibit a dependence on the iron content that is negligibly small, within uncertainties of the experiments. They are consequently insensitive to the Fe–Mg partitioning between (Mg, Fe)SiO3 perovskite and magnesiowüstite when applied to compositional models of the lower mantle. With the assumption that (Mg0.6Fe0.4)O is a Debye-like solid, a modified equation of heat capacity at constant pressure is proposed and thermodynamic properties of geophysically importance are calculated and tabulated at high temperatures.  相似文献   

16.
Lasaga's model [Lasaga, A.C., 1983. Geospeedometry: an extension of geothermometry. In: Saxena, S.K. (Ed.), Kinetics and Equilibrium in Mineral Reactions. Spring, New York, pp. 82–114.] to estimate cooling rate (s=dT/dt), as other analytical formulations available in the literature for instance the work of Dodson, M.H., 1976 [Dodson, M.H., 1976. Kinetic processes and thermal history of slowly cooling solids. Nature, 259, 551–553; Dodson, M.H., 1986. Closure profiles in cooling systems. Mat. Sci. Forum 7, 145–154.] can be difficult to apply to natural rocks for two reasons: (i) heavy numerical computations; (ii) the choice of the appropriate set of diffusion data. We propose a new formulation of Lasaga's equation which is simpler to use and less tight in the choice of a particular experimental data set. It is based on `frozen in' diffusion profiles in mineral pairs which are chemically isolated from the rest of the host rock. In this model, ions are exchanged by solid-state diffusion through the common surface of coexisting minerals. Our method approximates the shape of the elemental concentration profiles after cooling to an error function (with an effective diffusion coefficient Deff), as most of the variation of C(x) from rim (x=0) to core (x=d) occurs over a distance 0<x<a (a is the `characteristic diffusion length' of the profile: a≈3√(Defft)). Our analytical formula to calculate s is straightforward to use. It allows excellent estimates of s as long as 2a<d when the grain size is large compared to diffusion distances. For small grain sizes, we give another analytical formula that evaluates s at the value strial. We show by how much strial underestimates s. Expressing cooling rates s requires diffusion parameters Do and E (the pre-exponential factor and activation energy of diffusion). Available experimental data of (Do,E) are scattered due to different experimental conditions (T, fO2, mineral compositions). Therefore geospeedometric results range widely. It had been long noticed that the (Do,E) pairs are linearly correlated. This correlation can be expressed as the `compensation law' [Hart, S.R., 1981. Diffusion compensation in natural silicates. Geochim. Cosmochim. Acta 45, 200–215.]. We give the compensation law for Fe/Mg interdiffusion in two minerals used in geospeedometry, olivines and garnets. We show that implementing the `compensation law' into the analytical expression of cooling rate gives final s value consistent with all the (Do,E) experimental data sets and consequently greatly reduces the uncertainty on cooling rate (s or strial).  相似文献   

17.
The melting curve of perovskite MgSiO3 and the liquidus and solidus curves of the lower mantle were estimated from thermodynamic data and the results of experiments on phase changes and melting in silicates.The initial slope of the melting curve of perovskite MgSiO3 was obtained as dTm/dP?77 KGPa?1 at 23 GPa. The melting curve of perovskite was expressed by the Kraut-Kennedy equation as Tm(K)=917(1+29.6ΔVV0), where Tm?2900 K and P?23 GPa; and by the Simon equation, P(GPa)?23=21.2[(Tm(K)2900)1.75?1].The liquidus curve of the lower mantle was estimated as Tliq ? 0.9 Tm (perovskite) and this gives the liquidus temperature Tliq=7000 ±500 K at the mantle-core boundary. The solidus curve of the lower mantle was also estimated by extrapolating the solidus curve of dry peridotite using the slope of the solidus curve of magnesiowüstite at high pressures. The solidus temperature is ～ 5000 K at the base of the lower mantle. If the temperature distribution of the mantle was 1.5 times higher than that given by the present geotherm in the early stage of the Earth's history, partial melting would have proceeded into the deep interior of the lower mantle.Estimation of the density of melts in the MgOFeOSiO2 system for lower mantle conditions indicates that the initial melt formed by partial fusion of the lower mantle would be denser than the residual solid because of high concentration of iron into the melt. Thus, the melt generated in the lower mantle would tend to move downward toward the mantle-core boundary. This downward transportation of the melt in the lower mantle might have affected the chemistry of the lower mantle, such as in the D″ layer, and the distribution of the radioactive elements between mantle and core.  相似文献   

18.
The ion microprobe at Johnson Space Center has been calibrated for in situ water determinations on a 10-μm scale over the range 0.2 wt.% H2O to 1.8, 6.8, and 3.7 wt.%, for basaltic, albitic, and rhyolitic glasses, respectively. The basalt glass calibration curve differs substantially from those of albite and rhyolite glasses, indicating a need to carefully match composition and/or melt structure between H2O standards and unknowns.A value for the diffusivity of water as a function of concentration and time has been calculated from water diffusion profiles measured in rhyolite glasses prepared at 850°C and 700 barsPt(H2O) [1]. Transient diffusion into a semi-infinite medium is described by the equation:?(φ/2)?¸/?φ=?(Dw?¸/?φ)/?φ #x003B8;=1, φ=0, θ→ 0, θ→∞, wherex =distance from the cylinder edge,t =time,C0 =initial concentration,Cs =concentration at the edge,C =concentration at x,θ = C ? C0/Cs ? C0,φ = x/t1/2, andDw =diffusivity of water. An iterative technique has been used to calculate solutions to the diffusion equation as a function ofDw [2]. Comparison of these solutions with the ion probe data indicate that, for0.2wt.% ≤ C ≤ 3.7wt.%H2O,Dw can be described by an exponential function of θ, of the formDw = D0exp(bθ), withD0 (i.e.,Dw at 0.2%) = (0.8?2.2) × 10?8 cm2/s and2 ≤ b ≤ 4.  相似文献   

19.
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   

20.
Pyroxene-garnet solid-solution equilibria have been studied in the pressure range 41–200 kbar and over the temperature range 850–1,450°C for the system Mg4Si4O12Mg3Al2Si3O12, and in the pressure range 30–105 kbar and over the temperature range 1,000–1,300°C for the system Fe4Si4O12Fe3Al2Si3O12. At 1,000°C, the solid solubility of enstatite (MgSiO3) in pyrope (Mg3Al2Si3O12) increases gradually to 140 kbar and then increases suddenly in the pressure range 140–175 kbar, resulting in the formation of a homogeneous garnet with composition Mg3(Al0.8Mg0.6Si0.6)Si3O12. In the MgSiO3-rich field, the three-phase assemblage of β- or γ-Mg2SiO4, stishovite and a garnet solid solution is stable at pressures above 175 kbar at 1,000°C. The system Fe4Si4O12Fe3Al2Si3O12 shows a similar trend of high-pressure transformations: the maximum solubility of ferrosilite (FeSiO3) in almandine (Fe3Al2Si3O12) forming a homogeneous garnet solid solution is 40 mol% at 93 kbar and 1,000°C.If a pyrolite mantle is assumed, from the present results, the following transformation scheme is suggested for the pyroxene-garnet assemblage in the mantle. Pyroxenes begin to react with the already present pyrope-rich garnet at depths around 150 km. Although the pyroxene-garnet transformation is spread over more than 400 km in depth, the most effective transition to a complex garnet solid solution takes place at depths between 450 and 540 km. The complex garnet solid solution is expected to be stable at depths between 540 and 590 km. At greater depths, it will decompose to a mixture of modified spinel or spinel, stishovite and garnet solid solutions with smaller amounts of a pyroxene component in solution.  相似文献   

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