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1.
 Enthalpies of drop solution (ΔH drop-sol) of CaGeO3, Ca(Si0.1Ge0.9)O3, Ca(Si0.2Ge0.8)O3, Ca(Si0.3Ge0.7)O3 perovskite solid solutions and CaSiO3 wollastonite were measured by high-temperature calorimetry using molten 2PbO · B2O3 solvent at 974 K. The obtained values were extrapolated linearly to the CaSiO3 end member to give ΔH drop-sol of CaSiO3 perovskite of 0.2 ± 4.4 kJ mol−1. The difference in ΔH drop-sol between CaSiO3, wollastonite, and perovskite gives a transformation enthalpy (wo → pv) of 104.4 ± 4.4 kJ mol−1. The formation enthalpy of CaSiO3 perovskite was determined as 14.8 ± 4.4 kJ mol−1 from lime + quartz or −22.2 ± 4.5 kJ mol−1 from lime + stishovite. A comparison of lattice energies among A2+B4+O3 perovskites suggests that amorphization during decompression may be due to the destabilizing effect on CaSiO3 perovskite from a large nonelectrostatic energy (repulsion energy) at atmospheric pressure. By using the formation enthalpy for CaSiO3 perovskite, phase boundaries between β-Ca2SiO4 + CaSi2O5 and CaSiO3 perovskite were calculated thermodynamically utilizing two different reference points [where ΔG(P,T )=0] as the measured phase boundary. The calculations suggest that the phase equilibrium boundary occurs between 11.5 and 12.5 GPa around 1500 K. Its slope is still not well constrained. Received: 20 September 2000 / Accepted: 17 January 2001  相似文献   

2.
 Calorimetric and P–V–T data for the high-pressure phase Mg5Al5Si6O21(OH)7 (Mg-sursassite) have been obtained. The enthalpy of drop solution of three different samples was measured by high-temperature oxide melt calorimetry in two laboratories (UC Davis, California, and Ruhr University Bochum, Germany) using lead borate (2PbO·B2O3) at T=700 ∘C as solvent. The resulting values were used to calculate the enthalpy of formation from different thermodynamic datasets; they range from −221.1 to −259.4 kJ mol−1 (formation from the oxides) respectively −13892.2 to −13927.9 kJ mol−1 (formation from the elements). The heat capacity of Mg5Al5Si6O21(OH)7 has been measured from T=50 ∘C to T=500 ∘C by differential scanning calorimetry in step-scanning mode. A Berman and Brown (1985)-type four-term equation represents the heat capacity over the entire temperature range to within the experimental uncertainty: C P (Mg-sursassite) =(1571.104 −10560.89×T −0.5−26217890.0 ×T −2+1798861000.0×T −3) J K−1 mol−1 (T in K). The P V T behaviour of Mg-sursassite has been determined under high pressures and high temperatures up to 8 GPa and 800 ∘C using a MAX 80 cubic anvil high-pressure apparatus. The samples were mixed with Vaseline to ensure hydrostatic pressure-transmitting conditions, NaCl served as an internal standard for pressure calibration. By fitting a Birch-Murnaghan EOS to the data, the bulk modulus was determined as 116.0±1.3 GPa, (K ′=4), V T,0 =446.49 3 exp[∫(0.33±0.05) × 10−4 + (0.65±0.85)×10−8 T dT], (K T/T) P  = −0.011± 0.004 GPa K−1. The thermodynamic data obtained for Mg-sursassite are consistent with phase equilibrium data reported recently (Fockenberg 1998); the best agreement was obtained with Δf H 0 298 (Mg-sursassite) = −13901.33 kJ mol−1, and S 0 298 (Mg-sursassite) = 614.61 J K−1 mol−1. Received: 21 September 2000 / Accepted: 26 February 2001  相似文献   

3.
 The monoclinic titanite-like high-pressure form of calcium disilicate has been synthesized and quenched to ambient conditions to form the triclinic low-pressure phase containing silicon in four-, five- and sixfold coordination. The enthalpy of formation of the quench product has been measured by high-temperature oxide melt calorimetry. The value obtained from samples from a series of several synthesis experiments is ΔH f = (−26.32 ± 4.27) kJ mol−1 for the formation from the component oxides, or ΔH f  = (−2482.81 ± 4.59) kJ mol−1 for the formation from the elements. The result is identical within experimental error to available estimates, although the previously predicted energy difference between the monoclinic and triclinic phases could not be verified. Received: 16 February 2000 / Accepted: 14 July 2000  相似文献   

4.
The accepted standard state entropy of titanite (sphene) has been questioned in several recent studies, which suggested a revision from the literature value 129.3 ± 0.8 J/mol K to values in the range of 110–120 J/mol K. The heat capacity of titanite was therefore re-measured with a PPMS in the range 5 to 300 K and the standard entropy of titanite was calculated as 127.2 ± 0.2 J/mol K, much closer to the original data than the suggested revisions. Volume parameters for a modified Murgnahan equation of state: V P,T  = V 298° × [1 + a°(T − 298) − 20a°(T − 298)] × [1 – 4P/(K 298 × (1 – 1.5 × 10−4 [T − 298]) + 4P)]1/4 were fit to recent unit cell determinations at elevated pressures and temperatures, yielding the constants V 298° = 5.568 J/bar, a° = 3.1 × 10−5 K−1, and K = 1,100 kbar. The standard Gibbs free energy of formation of titanite, −2456.2 kJ/mol (∆H°f = −2598.4 kJ/mol) was calculated from the new entropy and volume data combined with data from experimental reversals on the reaction, titanite + kyanite = anorthite + rutile. This value is 4–11 kJ/mol less negative than that obtained from experimental determinations of the enthalpy of formation, and it is slightly more negative than values given in internally consistent databases. The displacement of most calculated phase equilibria involving titanite is not large except for reactions with small ∆S. Re-calculated baric estimates for several metamorphic suites yield pressure differences on the order of 2 kbar in eclogites and 10 kbar for ultra-high pressure titanite-bearing assemblages.  相似文献   

5.
 The partitioning of Mg and Fe between magnesiowüstite and ringwoodite solid solutions has been measured between 15 and 23 GPa and 1200–1600 ∘C using both Fe and Re capsule materials to vary the oxidation conditions. The partitioning results show a clear dependence on the capsule material used due to the variation in Fe3+ concentrations as a consequence of the different oxidation environments. Using results from experiments performed in Fe capsules, where metallic Fe was also added to the starting materials, the difference in the interaction parameters for the two solid solutions (W FeMg mw−W FeMg ring) is calculated to be 8.5±1 kJ mol−1. Similar experiments performed in Re metal capsules result in a value for W FeMg mw−W FeMg ring that is apparently 4 kJ higher, if all Fe is assumed to be FeO. Electron energy-loss near-edge structure (ELNES) spectroscopic analyses, however, show Fe3+ concentrations to be approximately three times higher in magnesiowüstite produced in Re capsules than in Fe capsules and that Fe3+ partitions preferentially into magnesiowüstite, with K D Fe3+ ring/mw estimated between 0.1 and 0.6. Using an existing activity composition model for magnesiowüstite, a least–squares fit to the partitioning data collected in Fe capsules results in a value for the ringwoodite interaction parameter (W FeMg ring) of 3.5±1 kJ mol−1. The equivalent regular interaction parameter for magnesiowüstite (W FeMg mw) is 12.1±1.8 kJ mol. These determinations take into account the Fe3+ concentrations that occur in both phases in the presence of metallic Fe. The free energy change in J mol−1 for the Fe exchange reaction can be described, over the range of experimental conditions, by 912 + 4.15 (T−298)+18.9P with T in K, P in kbar. The estimated volume change for this reaction is smaller than that predicted using current compilations of equation of state data and is much closer to the volume change at ambient conditions. These results are therefore a useful test of high pressure and temperature equation of state data. Using thermodynamic data consistent with this study the reaction of ringwoodite to form magnesiowüstite and stishovite is calculated from the data collected using Fe capsules. Comparison of these results with previous studies shows that the presence of Fe3+ in phases produced in multianvil experiments using Re capsules can have a marked effect on apparent phase relations and determined thermodynamic properties. Received: 13 September 2000 / Accepted: 25 March 2001  相似文献   

6.
Experimental clinopyroxenes synthesized at 850–1500 °C and 0–60 kbar in the CMS and CMAS-Cr systems and in more complex lherzolitic systems have been used to calibrate a Cr-in-Cpx barometer and an enstatite-in-Cpx thermometer for Cr-diopsides derived from garnet peridotites. The experiments cover a wide range of possible natural peridotitic compositions, from fertile pyrolite to refractory, high-Cr lherzolite. The barometer is based on the Cr exchange between clinopyroxene and garnet. Pressure is formulated as a function of temperature and clinopyroxene composition:
where a CaCrTs Cpx=Cr−0.81·Cr#·(Na+K) and Cr#= , with elements in atoms per 6 oxygens. This formulation reproduces the experimental pressures to ±2.3 kbar (1σ) and has a temperature dependence (1.2–2.4 kbar/50 °C, varying with composition) that is weaker than that of the widely used Al-in-Opx barometer (2–3 kbar/50 °C). The enstatite-in-Cpx thermometer includes corrections for the effect of minor components and is formulated as
where K)). The thermometer reproduces the experimental temperatures to ±30 °C (1σ). The uncertainties of the present formulations are comparable to, or better than, those of the most widely used thermobarometers for garnet peridotites. P-T estimates obtained for diamond-bearing and graphite-bearing lherzolite xenoliths and peridotitic clinopyroxene inclusions in kimberlitic and lamproitic diamonds confirm the reliability of the thermobarometer. Cr-diopside thermobarometry appears to be a potential tool for obtaining information on the thermal state of the upper mantle and the extent of mantle sampling by deep-seated magmas. We consider the Cr-in-Cpx barometer to be the best alternative to the Al-in-Opx barometer for the evaluation of pressure conditions of equilibration of natural garnet lherzolites. P-T conditions of equilibration can be directly retrieved from the composition of Cr-diopside alone, thus allowing application to partially altered xenoliths, inclusions in diamonds, and loose grains from sediments. We foresee application of the present thermobarometer to evaluation of the diamond potential of kimberlite and lamproite provinces and in diamond exploration where Cr-diopside from deep mantle sources is preserved in the surficial weathering environment. Received: 16 August 1999 / Accepted: 17 March 2000  相似文献   

7.
The heat capacity of synthetic pretulite ScPO4(c) was measured by adiabatic calorimetry within a temperature range of 12.13–345.31 K, and the temperature dependence of the pretulite heat capacity at 0–1600 K was derived from experimental and literature data on H 0(T)-H 0(298.15 K) for Sc orthophosphate. This dependence was used to calculate the values of the following thermodynamic functions: entropy, enthalpy change, and reduced Gibbs energy. They have the following values at 298.15 K: C p 0 (298.15 K) = 97.45 ± 0.06 J K−1 mol−1, S 0(298.15 K) = 84.82 ± 0.18 J K−1 mol−1, H 0(298.15 K)-H 0(0) = 14.934 ± 0.016 kJ mol−1, and Φ 0(298.15 K) = 34.73 ± 0.19 J K−1mol−1. The enthalpy of formation Δ f H 0(ScPO4, 298.15 K) = − 1893.6 ± 8.4 kJ mol−1.  相似文献   

8.
Summary  Transmission M?ssbauer spectra of synthetic Ca-free P21/c Mg0.22Fe0.78SiO3 clinopyroxene were collected at temperatures in the range 4.2 to 745 K and in an external magnetic field of 60 kOe at 180 K. The magnetic order-disorder transition temperature was determined by M?ssbauer thermoscanning to be 21 ± 3 K. Above this temperature, all M?ssbauer spectra consist of a superposition of two doublets, respectively produced by Fe2+ ions at an almost regular octahedral M1 site and at a more distorted octahedral M2 site. The temperature variation of the Fe2+ center shifts were analyzed using the Debye model for the lattice vibrations. The characteristic M?ssbauer temperatures were found to be 356 K ± 35 K for M1 and 333 K ± 25 K for M2. From the external field (60 kOe) M?ssbauer spectrum recorded at 180 K, the principal component V zz of the electric field gradient (EFG) was determined to be positive for both sites but precise values for the magnitudes of the asymmetry parameters η of the EFG could not be determined. The temperature variations of the M1 and M2 quadrupole splittings ΔE Q(T ) are consistent with the higher distortion of the M2 octahedra. Using the crystal-field model to interpret ΔE Q(T ), the energy gaps δ1 and δ2 of the first excited electronic states within the 5D orbital term were estimated to be 410 ± 50 cm−1 and 730 ± 50 cm−1 for M1, and δ1 = 1050 ± 75 cm−1 for M2. Received May 29, 2000;/revised version accepted July 13, 2001  相似文献   

9.
The low-pressure stability of clinohumite has been investigated in phase-equilibrium experiments on the reaction forsterite + brucite = clinohumite. The reaction was bracketed between 2.45 and 2.84 GPa at 650 °C, extending to between 1.37 and 1.57 GPa at 850 °C. At temperatures above the reaction brucite = periclase + vapour, the reaction clinohumite = forsterite + vapour was bracketed between 1.27 and 1.52 GPa at 900 °C, rising to between 1.90 and 2.00 GPa at 1000 °C. The position of the reaction forsterite + brucite = clinohumite is ∼0.5 GPa below the position determined in previous work, the difference arising either from pressure uncertainties in both studies, from enhanced reaction to clinohumite in this study due to the presence of excess brucite in the starting material, or from different concentrations of defects in the two samples. The brackets on the reaction were combined with other measured and estimated thermodynamic data for clinohumite to determine its enthalpy of formation and entropy, in a revised version of the thermodynamic dataset of Holland and Powell (1998). The values obtained were ΔH f =−9607.29±3.05 kJ mol−1, S=445 J mol−1 K−1. These data were used to calculate positions of other reactions involving clinohumite. The calculations suggest a larger stability field for clinohumite than implied by the results of previous experimental studies, indicating a need for more high-pressure phase-equilibrium studies to provide better thermodynamic data. Received: 30 April 1999 / Accepted: 8 November 1999  相似文献   

10.
 Thermodynamic properties of high-pressure minerals that are not recoverable from synthesis experiments by conventional quenching methods (“unquenchable” phases) usually are calculated from equation of state data and phase diagram topologies. The present study shows that, with cryogenic methods of recovery and sample treatment, phases with a suitable decomposition rate can be made accessible to direct thermodynamic measurements. A set of samples of Ca(OH)2-II has been synthesized in a multianvil device and subsequently recovered by cooling the high-pressure assembly with liquid nitrogen. Upon heating from liquid nitrogen to room temperature, the material transformed back to Ca(OH)2-I. The heat effect of this backtransformation was measured by differential scanning calorimetry. A commercial differential scanning calorimeter (Netzsch DSC 404), modified to allow sample loading at liquid nitrogen temperature was used to heat the material from −150 to +200 °C at rates varying between 5 and 15 °C min−1. The transformation started around −50 °C very gradually, and peaked at about 0 °C. To obtain a baseline correction, each sample was scanned under exactly the same conditions after the backtransformation was complete. Because of the relative sluggishness, onset and offset temperatures were not well defined as compared to fast (e.g., melting) reactions. To aid in integration, the resulting signals were successfully fitted using a generic asymmetric peak model. The enthalpy of backtransformation was determined to be ΔH =−10.37 ± 0.50 kJ mol−1. From previous in situ X-ray diffraction experiments, the location of the direct transformation in P-T space has been constrained to 5.7 ± 0.4 GPa at 500 °C (Kunz et al. 1996). With the reaction volume known from the same study, and assuming that ΔC p of the transformation remains negligible between the conditions of our measurements and 500 °C, our result gives an estimate of the entropy of transition and the P-T slope of the reaction curve. To a first approximation, the values ΔS = −16.00 ± 0.65 J(mol · K)−1 and dP/dT = 0.0040 ± 0.0002 GPa/K have been determined. These results need to be refined by equation of state data for Ca(OH)2-II. Received: 30 December 1999 / Accepted: 10 April 2000  相似文献   

11.
  The diffusivity of water has been investigated for a haplogranitic melt of anhydrous composition Qz28Ab38Or34 (in wt %) at temperatures of 800–1200°C and at pressures of 0.5–5.0 kbar using the diffusion couple technique. Water contents of the starting glass pairs varied between 0 and 9 wt %. Concentration-distance profiles for the different water species (molecular water and hydroxyl groups) were determined by near-infrared microspectroscopy. Because the water speciation of the melt is not quenchable (Nowak 1995; Nowak and Behrens 1995; Shen and Keppler 1995), the diffusivities of the individual species can not be evaluated directly from these profiles. Therefore, apparent chemical diffusion coefficients of water (D water) were determined from the total water profiles using a modified Boltzmann-Matano analysis. The diffusivity of water increases linearly with water content <3 wt % but exponentially at higher water contents. The activation energy decreases from 64 ± 10 kJ/mole for 0.5 wt % water to 46 ± 5 kJ/mole for 4 wt % water but remains constant at higher water contents. A small but systematic decrease of D water with pressure indicates an average activation volume of about 9 cm3/mole. The diffusivity (in cm2/s) can be calculated for given water content (in wt %), T (in K) and P (in kbar) by
in the ranges 1073 K ≤ T ≤ 1473 K; 0.5 kbar ≤ P≤ 5␣kbar; 0.5 wt % ≤ C water ≤ 6 wt %. The absence of alkali concentration gradients in the glasses after the experiments shows that interdiffusion of alkali and H+ or H3O+ gives no contribution to the transport of water in aluminosilicate melts. The H/D interdiffusion coefficients obtained at 800°C and 5 kbar using glass pieces with almost the same molar content of either water or deuterium oxide are almost identical to the chemical diffusivities of water. This indicates that protons are transported by the neutral component H2O under these conditions. Received: 26 March 1996 / Accepted: 23 August 1996  相似文献   

12.
The expansivity of supercooled diopside liquid has been determined using techniques of container-based dilatometry. Two thermal strategies have been employed, one in which the sample is brought to volumetric equilibrium by long-duration dwells at low temperatures (817 °C) and one in which scanning dilatometry of the sample has been performed at somewhat higher temperatures (890–913 °C). The results of both experiments yield a supercooled liquid expansivity for diopside liquid in the temperature range of 817–913 °C of 84.4 ± 2.8 × 10−4 cm3/mol K. The expansivity is 65% higher than that obtained for diopside melt obtained at superliquidus temperatures using the double bob Archimedean method. Combined fitting of the new low temperature, volume–temperature data from the present study and the superliquidus data from the literature has been performed. The combined fit yields the following equations for the volume–temperature relationship of diopside liquid (T=temperature in °C):
The standard error of the fit using both equations reproduces the volume–temperature data for diopside liquid within experimental error. This result reconciles the disparate values of expansivity measured at low temperatures in the supercooled state and at superliquidus temperatures and confirms the temperature-dependence of the expansivity of diopside liquid. Comparison with previous low temperature estimates of melt volume and expansivity are discussed in light of these new results. Received: 18 November 1999 / Accepted: 24 January 2000  相似文献   

13.
In the Grt-Bt-Sil restitic xenoliths of El Joyazo (Cerro de Hoyazo), hercynitic spinel is a minor phase commonly associated with biotite. The possible reaction relationships among biotite and spinel are studied in reaction textures developed around biotites at their contact with patches of fibrolitic sillimanite and rhyolitic melt. In these textures, resorbed biotite crystals about 1 mm long are rimmed by a layer of glass <200 μm thick containing spinel and ilmenite; the same glass also fills embayments in biotite. Spinel forms euhedral crystals <100 μm in size, and ilmenite occurs as smaller anhedral crystals or needles, often intergrown with spinel. The homogeneous felt-like melt-sillimanite aggregate (“mix”) is richest in glass close to the reaction rim around biotite. Plagioclase and garnet are located >5 mm away from the reaction texture. Biotite is chemically zoned. Cores (Bt 1 ) have XMg=0.35 ± 0.02 and Ti=0.58 ± 0.01 atoms; whereas the outer rims (Bt 2 ) have XMg=0.45 ± 0.01 and Ti up to 0.68 atoms. The hercynite-rich spinel (Spl) has low ZnO content (<0.80 wt%) and XMg=0.26 ± 0.04. The chemical compositions of the mix aggregate represent linear combinations between sillimanite and a silica-rich melt. This melt (melt 1 ) is different from that of the layer around biotite (melt 2 ), which is also richer in Ca and alkalis. Garnet rims (Grt) have low Ca and Mn, and XMg=0.14. Plagioclase is characterized by large homogeneous cores (Pl 1 , An31 ± 2) and more calcic rims (Pl 2 , An49 ± 6). Matrix analysis in the 9-component (Al-Ca-Fe-K-Mg-Mn-Na-Si-Ti), 9-phase (Bt1-Bt2-Grt-Spl-Ilm-melt2-mix-Pl1-Pl2) system provides the mass balance (in mole units):
This relationship is in excellent agreement with the observed textures and hence is considered a good model for the incongruent melting of biotite in the xenoliths. The mass-balance indicates that melt production is dominated by the availability of K from biotite, and that garnet and plagioclase must be involved as reactants, so that the reaction volume is larger than the melt production site. The melting of biotite, constrained at T=900–950°C and P ≥ 5 kbar, is not a terminal reaction, as its variance in the reduced 8-component multisystem is ≥3. Received: 1 June 1999 / Accepted: 8 February 2000  相似文献   

14.
Pb diffusion in rutile   总被引：16，自引：0，他引：16  
Diffusion of Pb was measured in natural and synthetic rutile under dry, 1 atmosphere conditions, using mixtures of Pb titanate or Pb sulfide and TiO2 as the sources of diffusant. Pb depth profiles were then measured with Rutherford Backscattering Spectrometry (RBS). Over the temperature range 700–1100 °C, the following Arrhenius relation was obtained for the synthetic rutile: D=3.9 × 10−10exp(−250 ± 12 kJ mol−1/RT) m2s−1. Results for diffusion in natural and synthetic rutile were quite similar, despite significant differences in trace element compositions. Mean closure temperatures calculated from the diffusion parameters are around 600 °C for rutile grains of ∼100 μm size. This is about 100 °C higher than rutile closure temperature determinations from past field-based studies, suggesting that rutile is more resistant to Pb loss through volume diffusion than previously thought. Received: 28 June 1999 / Accepted: 29 December 1999  相似文献   

15.
Low-temperature heat capacity measurements for MgCr2O4 have only been performed down to 52 K, and the commonly quoted third-law entropy at 298 K (106 J K−1 mol−1) was obtained by empirical extrapolation of these measurements to 0 K without considering the magnetic or electronic ordering contributions to the entropy. Subsequent magnetic measurements at low temperature reveal that the Néel temperature, at which magnetic ordering of the Cr3+ ions in MgCr2O4 occurs, is at ∼15 K. Hence a substantial contribution to the entropy of MgCr2O4 has been missed. We have determined the position of the near-univariant reaction MgCr2O4+SiO2=MgSiO3+Cr2O3. The reaction, which has a small positive slope in P-T space, has been bracketed at 100 K intervals between 1273 and 1773 K by reversal experiments. The reaction is extremely sluggish, and lengthy run times with a flux (H2O, BaO-B2O3 or K2O-B2O3) are needed to produce tight reversal brackets. The results, combined with assessed thermodynamic data for Cr2O3, MgSiO3 and SiO2, give the entropy and enthalpy of formation of MgCr2O4 spinel. As expected, our experimental results are not in good agreement with the presently available thermodynamic data. We obtain Δ f H ∘ 298=−1759.2±1.5 kJ mol−1 and S ∘ 298=122.1±1.0 J K−1 mol−1 for MgCr2O4. This entropy is some 16 J K−1 mol−1 more than the calorimetrically determined value, and implies a value for the magnetic entropy of MgCr2O4 consistent with an effective spin quantum number (S') for Cr3+ of 1/2 rather than the theoretical 3/2, indicating, as in other spinels, spin quenching. Received: 9 May 1997 / Accepted: 28 July 1997  相似文献   

16.
 Iron tracer diffusion experiments in diopside have been performed using natural and synthetic single crystals of diopside, and stable iron tracers enriched in 54Fe, at temperatures in the range 950–1100 °C, total pressure 1 atm, for times up to 29 days. Iron isotope diffusion profiles were determined with an ion microprobe. For experiments performed at log pO2 = −13, in directions parallel to the c axis and the b axis of two natural, low iron (Fe ∼ 1.8 at %) diopsides, the data obey a single Arrhenius relationship of the form D = 6.22−5.9 +49.6×10−15 exp(−161.5 ± 35.0 kJ mol−1/RT) m2 s−1. A single datum for iron diffusion in iron-free, single-crystal diopside at 1050 °C, is approximately 1 order of magnitude slower than in the natural crystals. The pO2 dependence of iron diffusion in natural crystals at 1050 °C (power exponent = 0.229 ± 0.036) indicates a vacancy mechanism; this is consistent with the results of unpublished atomistic simulation studies. There is no evidence of anisotropy for iron diffusion in diopside. Received: 16 March 1999 / Accepted: 10 April 2000  相似文献   

17.
Equilibrium volumes and expansivities of three liquids in the system anorthite (CaAl2Si2O8)–diopside (CaMgSi2O6) have been derived from dilatometric measurements of the equilibrium length of samples in the glass transition range. The typical temperature range of 40 K for the measurements is limited at low temperature by the very long times necessary to reach structural equilibrium and at high temperature by the penetration of the rod used to measure sample dilatation. Despite such narrow intervals, the expansivities are determined to better than 3% thanks to the high precision with which length changes are measured. The coefficient of volume thermal expansion (1/V dV/dT) of the fully relaxed liquid just above the glass transition is found to decrease linearly from diopside composition (139 ± 4 × 10−6 K−1) to anorthite composition (59 ± 2 × 10−6 K−1). These values are greater than those determined for the same liquids at superliquidus temperatures, demonstrating that expansivities of silicate melts may decrease markedly with increasing temperature. A predictive model based upon partial molar volumes which vary as a linear function of the logarithm of temperature is proposed. Received: 25 February 2000 / Accepted: 29 May 2000  相似文献   

18.
The heat capacity (C p ) of dmitryivanovite synthesized with a cubic press was measured in the temperature range of 5–664 K using the heat capacity option of a physical properties measurement system and a differential scanning calorimeter. The entropy of dmitryivanovite at standard temperature and pressure (STP) was calculated to be 110.1 ± 1.6 J mol−1 K−1 from the measured C p data. With the help of new phase equilibrium experiments done at 1.5 GPa, the phase transition boundary between krotite and dmitryivanovite was best represented by the equation: P (GPa) = −2.1825 + 0.0025 T (K). From the temperature intercept of this phase boundary and other available thermodynamic data for krotite and dmitryivanovite, the enthalpy of formation and Gibbs free energy of formation of dmitryivanovite at STP were calculated to be −2326.7 ± 2.1 and −2,208.1 ± 2.1 kJ mol−1, respectively. It is also inferred that dmitryivanovite is the stable CaAl2O4 phase at STP and has a wide stability field at high pressures whereas the stability field of krotite is located at high temperatures and relatively low pressures. This conclusion is consistent with natural occurrences (in Ca–Al-rich inclusions) of dmitryivanovite and krotite, where the former is interpreted as the shock metamorphic product of originally present krotite.  相似文献   

19.
The heat capacity of eskolaite Cr2O3(c) was determined by adiabatic vacuum calorimetry at 11.99–355.83 K and by differential calorimetry at 320–480 K. Experimental data of the authors and data compiled from the literature were applied to calculate the heat capacity, entropy, and the enthalpy change of Cr2O3 within the temperature range of 0–1800 K. These functions have the following values at 298.15 K: C p 0 (298.15) = 121.5 ± 0.2 J K−1mol−1, S 0(298.15) = 80.95 ± 0.14 J K−1mol−1, and H 0(298.15)-H 0(0) = 15.30±0.02 kJ mol−1. Data were obtained on the transitions from the antiferromagnetic to paramagnetic states at 228–457 K; it was determined that this transition has the following parameters: Neel temperature T N = 307 K, Δ tr S = 6.11 ± 0.12 J K−1mol−1 and δ tr H = 1.87 ± 0.04 kJ mol−1.  相似文献   

20.
The heat capacity of xenotime YPO4(c) was measured by adiabatic calorimetry at 4.78–348.07 K. Our experimental and literature data on H 0(T)-H 0(298.15 K) of Y orthophosphate were utilized to derive the C p 0(T) function of xenotime at 0–1600 K, which was then used to calculate the values of thermodynamic functions: entropy, enthalpy change, and reduced Gibbs energy. These functions assume the following values at 298.15 K: C p 0 (298.15 K) = 99.27 ± 0.02 J K−1 mol−1, S 0(298.15 K) = 93.86 ± 0.08 J K−1 mol−1, H 0(298.15 K) − H 0(0) = 15.944 ± 0.005 kJ mol−1, Φ0(298.15 K) = 40.38 ± 0.08 J K−1 mol−1. The value of the free energy of formation Δ f G 0(YPO4, 298.15 K) is −1867.9 ± 1.7 kJ mol−1.  相似文献   

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