首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
We have investigated 44Ca self-diffusion in natural diopside single crystals (containing ∼2 atomic % Fe) at temperatures up to 1320 °C (i.e. 30 °C below the nominal melting point). Oxygen fugacity was controlled by gaseous mixtures. Diffusion profiles ranging from ∼50 to 500 nm were analysed by Rutherford Back-Scattering Spectrometry (RBS). The present results are complementary to previous studies, and show that in both synthetic (Fe-poor) and natural (Fe-rich) diopside, there are two different diffusion regimes for Ca with a transition at ∼1230±15 °C. Below this temperature diffusion is characterised by an activation enthalpy of ∼284±10 kJ/mol, while at higher temperatures it increases up to ∼1006±75 kJ/mol. These regimes are proposed to be respectively extrinsic and intrinsic. For the intrinsic regime Ca self-diffusion may involve Ca-Frenkel point defects. These are pairs of a vacancy on a M2 site and a calcium cation on an interstitial (normally unoccupied) site. The concentration of such point defects depends only on temperature, and it is especially important at very high temperatures. The activation enthalpy for intrinsic diffusion may represent the half defect formation enthalpy plus the migration enthalpy for movement through interstitial sites. For the extrinsic regime we propose Ca self-diffusion to involve extrinsic interstitial point defects with concentration proportional to ()–0.19±0.03. We suggest that for both regimes, Ca diffusion involves the well known M3 sites in the octahedral layers, as well as sites in the tetrahedral layers, that we call M4. These sites are especially convenient to explain the observed isotropic diffusion. Increasing concentration of Ca-Frenkel point defects may be related to the onset of premelting, which affects the thermodynamic properties of Fe-“free” diopside above 1250 °C. In the light of the present results, premelting is also expected to occur in natural Fe-bearing diopside and it could strongly influence its thermodynamic and transport properties. Subsequently, in deep upper mantle conditions (T≈1250 °C–1300 °C) where premelting could occur, diffusional cation exchanges with surrounding phases and diffusion controlled creep might be facilitated. Finally, our diffusion data support a previous suggestion that electrical conductivity may be electronic rather than ionic. Received: 17 December 1997 / Revised, accepted: 17 April 1998  相似文献   

2.
The interdiffusion coefficient of Mg–Fe in olivine (D Mg–Fe) was obtained at 1,400–1,600 °C at the atmospheric pressure with the oxygen fugacity of 10?3.5–10?2 Pa using a diffusion couple technique. The D Mg–Fe shows the anisotropy (largest along the [001] direction and smallest along the [100] direction), and its activation energy (280–320 kJ/mol) is ~80–120 kJ/mol higher than that estimated at lower temperatures. The D Mg–Fe at temperatures of >1,400 °C can be explained by the cation-vacancy chemistry determined both by the Fe3+/Fe2+ equilibrium and by the intrinsic point defect formation with the formation enthalpy of 220–270 kJ/mol depending on the thermodynamical model for the Fe3+/Fe2+ equilibrium in olivine. The formation enthalpy of 220–270 kJ/mol for the point defect (cation vacancy) in olivine is consistent with that estimated from the Mg self-diffusion in Fe-free forsterite. The increase in the activation energy of D Mg–Fe at >1,400 °C is thus interpreted as the result of the transition of diffusion mechanism from the transition metal extrinsic domain to the intrinsic domain at the atmospheric pressure.  相似文献   

3.
The intracrystalline diffusion rate of oxygen in diopside was constrained based on natural isotopic variations from a granulite facies marble from Cascade Slide, Adirondacks (New York, USA). The oxygen isotope compositions of the diopsides, measured as a function of grain size, are nearly constant (20.9 ± 0.3‰ vs. SMOW) over the entire measured size range (0.3–3.2 mm diameter). The δ18O values of the cores of calcite grains are 23.0‰. Temperature estimates based on the Δ18O(calcite-diopside) are 800d?C, in agreement with the highest previous thermometric estimates for these rocks. The lack of isotopic variation in the diopsides as a function of grain size requires that the oxygen intracrystalline diffusion rate in diopside from the Adirondack samples was very slow. The maximum diffusion rates (D800d?C parallel to the c-axis) were calculated with an infinite reservoir model (IRM) and a finite reservoir model (FRM) that incorporates mineral modal abundances and initial isotopic variations. For an assumed activation energy (Q) = 100 kJ/mol, the IRM diffusion rate estimate of 1.6 times 10-20cm2/s is two orders of magnitude faster than from the FRM; at Q=500kJ/mol, the D800d?C estimate for both methods is c. 5.6 times 10-20 cm2/s. The present results require that a hydrothermal fluid significantly enhances the diffusion rate of oxygen in diopside if previous data are correct. The δ18O(SMOW) and δ13C(PDB) values of the calcite, measured in situ with a CO2 laser, are 22.9 ± 0.3, 0.1±0.3‰ in the grain cores, 22.1 ±0.3, 0.2 ±0.1‰ at the grain boundaries and 21.7 ±0.4, -0.6±0.1‰ abutting diopside grains. The δ18O and δ13δC values measured conventionally are: crystal cores, 22.96, -0.95‰; abutting diopside grains, 22.38, -0.93‰; bulk, 22.79, -0.95%. Use of the bulk δ18O(calcite) values for thermometry yields unreasonably high temperatures. The lower δ18O values at the calcite grain boundaries are not due to retrograde diffusional exchange with the diopside, they are thought to be a result of a late retrograde fluid infiltration.  相似文献   

4.
The concentrations and behavior of oxygen and oxide ion were studied in silicate melts of composition CaO · MgO · xSiO2 (1.25 ≤ x ≤ 3) in the temperature range 1425 to 1575°C by cyclic voltammetry and chronopotentiometry. Electroreduction of oxygen is a reversible, 2 electron process involving dissociated oxygen atoms. The Henry's Law constant for O2 in molten diopside (CaO · MgO · 2SiO2) is 0.023 ± 0.004 mole/l atm at 1450°C. The diffusion coefficient for molecular oxygen in diopside melt is 4.5 ± .5 × 10?6 cm2/sec at 1450°C and the activation energy of diffusion is 80 ± 2 kcal/mole. Oxide ions produced by electroreduction of oxygen, rapidly dissociate silicate polymers, causing the concentration of free oxide ions in diopside melt to be buffered at a low level (4.7 ± .8 × 10?5 mole/l). The concentration of free oxide ion increases at higher proportions of metal oxides but remains at this value in more silicic melts. The rate of formation of oxide ions by polymerization in diopside melt is 0.021 ± .007 mole/l sec. Thermodynamic parameters (the standard free energy, enthalpy and entropy) for the oxidation of Ni, Co, and Zn in diopside melt in equilibrium with gaseous oxygen agree with those for solid oxide systems. The platinum reference electrode in molten diopside is a reversible, oxygen electrode.  相似文献   

5.
  相似文献   

6.
《Applied Geochemistry》2000,15(4):501-512
Using a flexible Au bag autoclave and a precision high-pressure liquid chromatography pump to control pressure, the liquid–liquid aqueous solubilities of TCE and PCE were measured as a function of temperature from 294 to 434 K (at constant pressure). The results were used to calculate the partial molal thermodynamic quantities of the organic liquid aqueous dissolution reactions: ΔḠsoln, ΔH̄soln, ΔS̄soln and ΔC̄p soln. Calculated values for these quantities at 298 K for TCE are: ΔḠsoln=11.282 (±0.003) kJ/mol, ΔH̄soln=−3.35 (±0.07) kJ/mol, ΔS̄soln=−49.07 (±0.24) J/mol K, and ΔC̄p soln=385.2 (±3.4) J/mol K. Calculated values for these quantities at 298 K for PCE are: ΔḠsoln=15.80 (±0.04) kJ/mol, ΔH̄soln=−1.79 (±0.58) kJ/mol, ΔS̄soln=−59.00 (±1.96) J/mol K and ΔC̄p soln=354.6 (±8.6) J/mol K. These thermodynamic quantities may be used to calculate the solubility of TCE and PCE at any temperature of interest. In the absence of direct measurements over this temperature range, the Henry's Law constants for TCE and PCE have been estimated using the measured aqueous solubilities and calculated vapor pressures.  相似文献   

7.
The thermochemical study of a natural basic copper phosphate, pseudomalachite Cu5(PO4)2(OH)4 (Virneberg deposit, Germany), was carried out using high-temperature melt solution calorimetry method with a Tian–Calvet microcalorimeter. The enthalpy of formation of the mineral from elements was obtained to be Δ f Hel(298.15 K) =–3214 ± 13 kJ/mol. The value of the Gibbs energy of pseudomalachite formation calculated using literature data on its standard entropy is Δ f Hel°(298.15 K) =–2812 ± 13 kJ/mol.  相似文献   

8.
The thermochemical study of natural hydrous calcium and iron phosphate, anapaite Ca2Fe(PO4)2 · 4H2O (Kerch iron ore deposit, Crimea, Russia), was carried out using high-temperature melt solution calorimetry with a Tian-Kalvet microcalorimeter. The enthalpy of formation of the mineral from elements was obtained to be Δ f Hel°(298.15 K) =–4812 ± 16 kJ/mol. The values of the standard entropy and the Gibbs energy of anapaite formation are S°(298.15 K) = 404.2 J/K mol and Δ f Gel°(298.15 K) =–4352 ± 16 kJ/mol, respectively.  相似文献   

9.
Based on the expert review of literature data on the thermodynamic properties of species in the Cl-Pd system, stepwise and overall stability constants are recommended for species of the composition [PdCl n ]2 ? n , and the standard electrode potential of the half-cell PdCl 4 2? /Pd(c) is evaluated at E 298,15° = 0.646 ± 0.007 V, which corresponds to Δ f G 298.15° = ?400.4 ± 1.4 kJ/mol for the ion PdCl 4 2? (aq). Derived from calorimetric data, Δ f H 298.15° PdCl 4 2? (aq) = ?524.6 ± 1.6 kJ/mol and Δ f H 298.15° Pd2+(aq) = 189.7 ± 2.6 kJ/mol. The assumed values of the overall stability constant of the PdCl 4 2? ion and the standard electrode potential of the PdCl 4 2? /Pd(c) half-cell correspond to Δ f G 298.15° = 190.1 ± 1.4 kJ/mol and S 298.15° = ?94.2 ± 10 J/(mol K) for the Pd2+(aq) ion.  相似文献   

10.
The heat capacity of synthetic andradite garnet (Ca3Fe2Si3O12) was measured between 9.6 and 365.5 K by cryogenic adiabatic calorimetry and from 340 to 990 K by differential scanning calorimetry. At 298.15 K Cop,m and Som are 351.9 ± 0.7 and 316.4 ± 2.0 J/(mol·K), respectively.Andradite has a λ-peak in Cop,m with a maximum at 11.7 ± 0.2 K which is presumably associated with the antiferromagnetic ordering of the magnetic moments of the Fe3+ ions. The Gibbs free energy of formation, ΔfGom (298.15 K) of andradite is −5414.8 ± 5.5 kJ/mol and was obtained by combining our entropy and heat capacity data with the known breakdown of andradite to pseudowollastonite and hematite at ≈ 1410 to 1438 K. From a reexamination of the calcite + quartz = wollastonite equilibrium data we obtained ΔfHom (298.15 K) = − 1634.5 ± 1.8 kJ/mol for wollastonite.Between 300 and 1000 K the molar heat capacity of andradite can be represented by the equation Cop,m = 809.24 - 7.025 × 10−2T− 7.403 × 103T−0.5 − 6.789 × 105T−2. We have also used our thermochemical data for andradite to estimate the Gibbs free energy of formation of hedenbergite (CaFeSi2O6) for which we obtained ΔfGom (298.15 K) = −2674.3 ± 5.8 kJ/mol.  相似文献   

11.
Solution calorimetric measurements compared with solubility determinations from the literature for the same samples of gibbsite have provided a direct thermochemical cycle through which the Gibbs free energy of formation of [Al(OH)4 aq?] can be determined. The Gibbs free energy of formation of [Al(OH)4 aq?] at 298.15 K is ?1305 ± 1 kJ/mol. These heat-of-solution results show no significant difference in the thermodynamic properties of gibbsite particles in the range from 50 to 0.05 μm.The Gibbs free energies of formation at 298.15 K and 1 bar pressure of diaspore, boehmite and bayerite are ?9210 ± 5.0, ?918.4 ± 2.1 and ?1153 ± 2 kJ/mol based upon the Gibbs free energy of [A1(OH)4 aq?] calculated in this paper and the acceptance of ?1582.2 ± 1.3 and ?1154.9 ± 1.2 kJ/mol for the Gibbs free energy of formation of corundum and gibbsite, respectively.Values for the Gibbs free energy formation of [Al(OH)2 aq+] and [AlO2 aq?] were also calculated as ?914.2 ± 2.1 and ?830.9 ± 2.1 kJ/mol, respectively. The use of [AlC2 aq?] as a chemical species is discouraged.A revised Gibbs free energy of formation for [H4SiO4aq0] was recalculated from calorimetric data yielding a value of ?1307.5 ± 1.7 kJ/mol which is in good agreement with the results obtained from several solubility studies.Smoothed values for the thermodynamic functions CP0, (HT0 - H2980)T, (GT0 - H2980)T, ST0 - S00, ΔH?,2980 kaolinite are listed at integral temperatures between 298.15 and 800 K. The heat capacity of kaolinite at temperatures between 250 and 800 K may be calculated from the following equation: CP0 = 1430.26 ? 0.78850 T + 3.0340 × 10?4T2 ?1.85158 × 10?4T212 + 8.3341 × 106 T?2.The thermodynamic properties of most of the geologically important Al-bearing phases have been referenced to the same reference state for Al, namely gibbsite.  相似文献   

12.
The thermodynamic properties of carnegieite and NaAlSiO4 glass and liquid have been investigated through C p determinations from 10 to 1800 K and solution-calorimetry measurements. The relative entropies S 298-S0 of carnegieite and NaAlSiO4 glass are 118.7 and 124.8 J/mol K, respectively. The low-high carnegieite transition has been observed at 966 K with an enthalpy of transition of 8.1±0.3 kJ/mol, and the enthalpy of fusion of carnegieite at the congruent melting point of 1799 K is 21.7±3 kJ/mol. These results are consistent with the reported temperature of the nepheline-carnegieite transition and available thermodynamic data for nepheline. The entropy of quenched NaAlSiO4 glass at 0 K is 9.7±2 J/mol K and indicates considerable ordering among AlO4 and SiO4 tetrahedra. In the liquid state, progressive, temperature-induced Si, Al disordering could account for the high configurational heat capacity. Finally, the differences between the entropies and heat capacities of nepheline and carnegieite do not seem to conform to current polyhedral modeling of these properties  相似文献   

13.
Reversals for the reaction 2 annite+3 quartz=2 sanidine+3 fayalite+2 H2O have been experimentally determined in cold-seal pressure vessels at pressures of 2, 3, 4 and 5?kbar, limiting annite +quartz stability towards higher temperatures. The equilibrium passes through the temperature intervals 500–540°?C (2?kbar), 550–570°?C (3?kbar), 570–590°?C (4?kbar) and 590–610°?C (5?kbar). Starting materials for most experiments were mixtures of synthetic annite +fayalite+sanidine+quartz and in some runs annite+quartz alone. Microprobe analyses of the reacted mixtures showed that the annites deviate slightly from their ideal Si/Al ratio (Si per formula unit ranges between 2.85 and 2.92, AlVI between 0.06 and 0.15). As determined by Mössbauer spectroscopy, the Fe3+ content of annite in the assemblage annite+fayalite +sanidine+quartz is around 5–7%. The experimental data were used to extract the thermodynamic standard state enthalpy and entropy of annite as follows: H 0 f,?Ann =?5125.896±8.319 [kJ/mol] and S 0 Ann=432.62±8.89 [J/mol/K] (consistent with the Holland and Powell 1990 data set), and H 0 f,Ann =?5130.971±7.939 [kJ/mol] and S 0 Ann=424.02±8.39 [J/mol/K] (consistent with the TWEEQ data base, Berman 1991). The preceeding values are close to the standard state properties derived from hydrogen sensor data of the redox reaction annite=sanidine+magnetite+H 2 (Dachs 1994). The experimental half-reversal of Eugster and Wones (1962) on the annite +quartz breakdown reaction could not be reproduced experimentally (formation of annite from sanidine+fayalite+quartz at 540°?C/1.035?kbar/magnetite-iron buffer) and probable reasons for this discrepancy remain unclear. The extracted thermodynamic standard state properties of annite were used to calculate annite and annite+quartz stabilities for pressures between 2 and 5?kbar.  相似文献   

14.
Steady-state dissolution rates of diopside are measured as a function of solution saturation state using a titanium flow-through reactor at pH 7.5 and temperature ranging from 125 to 175°C. Diopside dissolved stoichiometrically under all experimental conditions and rates were not dependent on sample history. At each temperature, rates continuously decreased by two orders of magnitude as equilibrium was approached and did not exhibit a dissolution plateau of constant rates at high degrees of undersaturation. The variation of diopside dissolution rates with solution saturation can be described equally well with a ion exchange model based on transition state theory or pit nucleation model based on crystal growth/dissolution theory from 125 to 175°C. At 175°C, both models over predict dissolution rates by two orders of magnitude indicating that a secondary phase precipitated in the experiments. The ion exchange model assumes the formation of a Si-rich, Mg-deficient precursor complex. Lack of dependence of rates on steady-state aqueous calcium concentration supports the formation of such a complex, which is formed by exchange of protons for magnesium ions at the surface. Fit to the experimental data yields where the Mg-H exchange coefficient, n = 1.39, the apparent activation energy, E a = 332 kJ mol-1, and the apparent rate constant, k = 1041.2 mol diopside cm-2 s-1. Fits to the data with the pit nucleation model suggest that diopside dissolution proceeds through retreat of steps developed by nucleation of pits created homogeneously at the mineral surface or at defect sites, where homogeneous nucleation occurs at lower degrees of saturation than defect-assisted nucleation. Rate expressions for each mechanism (i) were fit to where the step edge energy (α) for homogeneously nucleated pits were higher (275 to 65 mJ m-2) than the pits nucleated at defects (39 to 65 mJ m-2) and the activation energy associated with the temperature dependence of site density and the kinetic coefficient for homogeneously nucleated pits (Eb-homogeneous = 2.59 × 10-16 mJ K-1) were lower than the pits nucleated at defects (Eb-defect assisted = 8.44 × 10-16 mJ K-1).  相似文献   

15.
The short term (2–40 days) dissolution of enstatite, diopside, and tremolite in aqueous solution at low temperatures (20–60°C) and pH 1–6 has been studied in the laboratory by means of chemical analyses of reacting solutions for Ca2+, Mg2+, and Si(OH)4 and by the use of X-ray photoelectron spectroscopy (XPS) for detecting changes in surface chemistry of the minerals. All three minerals were found to release silica at a constant rate (linear kinetics) providing that ultrafine particles, produced by grinding, were removed initially by HF treatment. All three also underwent incongruent dissolution with preferential release of Ca and/or Mg relative to Si from their outermost surfaces. The preferential release of Ca, but not Mg for diopside at pH 6 was found by both XPS and solution chemistry verifying the theoretical prediction of greater mobility of cations located in M2 structural sites. Loss mainly from M2 sites also explains the degree of preferential loss of Mg from enstatite at pH 6; similar structural arguments apply to the loss of Ca and Mg from the surface of tremolite. In the case of diopside and tremolite initial incongruency was followed by essentially congruent cation-plus-silica dissolution indicating rapid formation of a constant-thickness, cation-depleted surface layer. Cation depletion at elevated temperature and low pH (~ 1) for enstatite and diopside was much greater than at low temperature and neutral pH, and continued reaction resulted in the formation of a surface precipitate of pure silica as indicated by solubility calculations, XPS analyses, and scanning electron microscopy.From XPS results at pH 6, model calculations indicate a cation-depleted altered surface layer of only a few atoms thickness in all three minerals. Also, lack of shifts in XPS peak energies for Si, Ca, and Mg, along with undersaturation of solutions with respect to all known Mg and Ca silicate minerals, suggest that cation depletion results from the substitution of hydrogen ion for Ca2+ and/or Mg2+ in a modified silicate structure and not from the precipitation of a new, radically different surface phase. These results, combined with findings of high activation energies for dissolution, a non-linear dependence on aH+ for silica release from enstatite and diopside, and the occurrence of etch pitting, all point to surface chemical reaction and not bulk diffusion (either in solution or through altered surface layers) as the rate controlling mechanism of iron-free pyroxene and amphibole dissolution at earth surface temperatures.  相似文献   

16.
 Calcium self-diffusion rates in natural calcite single crystals were experimentally determined at 700 to 900° C and 0.1 MPa in a stream of CO2. Diffusion coefficients (D) were determined from 42Ca concentration profiles measured with an ion microprobe. The Arrhenius parameters yield an activation energy (Q)=382±37 kJ/mol and pre-exponential factor (D0)=0.13 m2/s, and there is no measurable anisotropy. Calcium grain boundary diffusion rates were experimentally determined in natural (Solnhofen) limestone and hot-pressed calcite aggregates at 650° to 850° C and 0.1 to 100 MPa pressure. The Solnhofen limestone was first pre-annealed for 24 h at 700° C and 100 MPa confining pressure under anhydrous conditions to produce an equilibrium microstructure for the diffusion experiments. Values for the product of the grain boundary diffusion coefficient (D′) and the effective grain boundary diffusion width (δ) were determined from 42Ca concentration profiles measured with an ion microprobe. The results show that there is no measurable difference between D′δ values obtained for pre-annealed Solnhofen samples at 0.1 and 100 MPa or between hot-pressed calcite aggregates and pre-annealed Solnhofen samples. The temperature dependence for calcium grain boundary diffusion in Solnhofen samples annealed at 0.1 MPa is described by the Arrhenius parameters D′ 0δ=1.5×10−9 m3/s and Q=267±47 kJ/mol. Comparison of the results of this study with previously published data show that calcium is the slowest volume diffusing species in calcite. The calcium diffusivities measured in this study place constraints on several geological processes that involve diffusive mass transfer including diffusion-accommodated mechanisms in the deformation of calcite rocks. Received: 19 December 1994/Accepted: 30 June 1995  相似文献   

17.
The paper reports pioneering data on the calorimetrically determined enthalpy of formation from elements of cuspidine, Ca fluordiorthosilicate Ca4Si2O7F2, from the Tyrny-Auz Mo–W deposit in Kabardino- Balkaria, Russia. The data were obtained by high-temperature melt solution calorimetry. The determined value is ΔfHel° (298.15 K) =–5190 ± 13 kJ/mol. The paper reports estimated S°(298.15 K) and ΔfGel° (298.15 K) of cuspidine.  相似文献   

18.
Chemical interdiffusion of Fe–Mg along the c-axis [001] in natural diopside crystals (X Di = 0.93) was experimentally studied at ambient pressure, at temperatures ranging from 800 to 1,200 °C and oxygen fugacities from 10?11 to 10?17 bar. Diffusion couples were prepared by ablating an olivine (X Fo = 0.3) target to deposit a thin film (20–100 nm) onto a polished surface of a natural, oriented diopside crystal using the pulsed laser deposition technique. After diffusion anneals, compositional depth profiles at the near surface region (~400 nm) were measured using Rutherford backscattering spectroscopy. In the experimental temperature and compositional range, no strong dependence of D Fe–Mg on composition of clinopyroxene (Fe/Mg ratio between Di93–Di65) or oxygen fugacity could be detected within the resolution of the study. The lack of fO2-dependence may be related to the relatively high Al content of the crystals used in this study. Diffusion coefficients, D Fe–Mg, can be described by a single Arrhenius relation with $$D^{{{\text{Fe}} - {\text{Mg}}}} = 2. 7 7\pm 4. 2 7\times 10^{ - 7} {\text{exp(}}-3 20. 7\pm 1 6.0{\text{ kJ}}/{\text{mol}}/{\text{RT)m}}^{ 2} /{\text{s}}.$$ D Fe–Mg in clinopyroxene appears to be faster than diffusion involving Ca-species (e.g., D Ca–Mg) while it is slower than D Fe–Mg in other common mafic minerals (spinel, olivine, garnet, and orthopyroxene). As a consequence, diffusion in clinopyroxene may be the rate-limiting process for the freezing of many geothermometers, and compositional zoning in clinopyroxene may preserve records of a higher (compared to that preserved in other coexisting mafic minerals) temperature segment of the thermal history of a rock. In the absence of pervasive recrystallization, clinopyroxene grains will retain compositions from peak temperatures at their cores in most geological and planetary settings where peak temperatures did not exceed ~1,100 °C (e.g., resetting may be expected in slowly cooled mantle rocks, many plutonic mafic rocks, or ultra-high temperature metamorphic rocks).  相似文献   

19.
Grain growth experiments have been performed at 1 atm on fine grain size (<10 μm) synthetic olivine (Fo91) aggregates at various temperatures (1200° to 1400° C), oxygen fugacities (10-4 to 10-11 atm) and total anneal times (10, 30, 60, 100 and 200 h). The rate of grain growth increased with increasing temperature and with increasing oxygen fugacity. The presence of a second phase (residual porosity), introduced during sample fabrication, has a significant effect on grain growth, with evolution in grain size paralleled by changes in the size and frequency of the pores. When the grain growth data were fit to a growth law G n ?G O n =κ0 tf 0 m 2e?Q/RT, the growth exponents fall in the range of n=4 to 5, suggesting that grain growth may be controlled by the coalescence of the second phase. The evolution in pore size and frequency may occur either by the transport of the ionic species constituting olivine between the pores or by the movement of the pores themselves along the grain boundaries and edges. Thus, the rate of growth of the pores and grains is probably limited by diffusion of the slowest ionic species constituting olivine (magnesium, iron, silicon, or oxygen) moving along the fastest path for that species (through the lattice, along the grain boundaries, around the surface of the moving pores, or through the vapor phase in the pores). Activation energies for grain growth of Q=290 ± 20 kJ/mol and 345 ± 25 kJ/mol were calculated from our results for n=4 and 5, respectively. These activation energies preclude vapor-phase transport and iron diffusion along grain boundaries but do not otherwise permit a discrimination between the rate limiting species or path. The oxygen fugacity exponent of m ≈0.12 suggests that lattice diffusion does not control the grain growth. However, the lack of data for magnesium, iron, silicon and oxygen surface and grain boundary diffusion in olivine makes definitive determination of the mechanism controlling grain growth difficult.  相似文献   

20.
Using the method of direct synthesis calorimetry, we determined the standard enthalpy of formation of PtSb (stumpflite), Δ f H°298.15 (PtSb, cr) =–105.16 ± 0.84 kJ/mol and PdSb2 (geversite), Δ f H°298.15 (PtSb2,cr) =–160.92 ± 0.84 kJ/mol. Isothermal (298.15 K, p = 1 bar) phase diagrams were computed for the Pt–Sb–S and Pt–Sb–O ternary systems in the coordinates composition of the Pt–Sb binary system versus fugacity of a gaseous volatile component (O2, S2).  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号