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1.
The low-temperature isobaric heat capacities (C p) of β- and γ-Mg2SiO4 were measured at the range of 1.8–304.7 K with a thermal relaxation method using the Physical Property Measurement System. The obtained standard entropies (S°298) of β- and γ-Mg2SiO4 are 86.4 ± 0.4 and 82.7 ± 0.5 J/mol K, respectively. Enthalpies of transitions among α-, β- and γ-Mg2SiO4 were measured by high-temperature drop-solution calorimetry with gas-bubbling technique. The enthalpies of the α−β and β−γ transitions at 298 K (ΔH°298) in Mg2SiO4 are 27.2 ± 3.6 and 12.9 ± 3.3 kJ/mol, respectively. Calculated α−β and β−γ transition boundaries were generally consistent with those determined by high-pressure experiments within the errors. Combining the measured ΔH°298 and ΔS°298 with selected data of in situ X-ray diffraction experiments at high pressure, the ΔH°298 and ΔS°298 of the α−β and β−γ transitions were optimized. Calculation using the optimized data tightly constrained the α−β and β−γ transition boundaries in the P, T space. The slope of α−β transition boundary is 3.1 MPa/K at 13.4 GPa and 1,400 K, and that of β−γ boundary 5.2 MPa/K at 18.7 GPa and 1,600 K. The post-spinel transition boundary of γ-Mg2SiO4 to MgSiO3 perovskite plus MgO was also calculated, using the optimized data on γ-Mg2SiO4 and available enthalpy and entropy data on MgSiO3 perovskite and MgO. The calculated post-spinel boundary with a Clapeyron slope of −2.6 ± 0.2 MPa/K is located at pressure consistent with the 660 km discontinuity, considering the error of the thermodynamic data.  相似文献   

2.
We present results from low-temperature heat capacity measurements of spinels along the solid solution between MgAl2O4 and MgCr2O4. The data also include new low-temperature heat capacity measurements for MgAl2O4 spinel. Heat capacities were measured between 1.5 and 300 K, and thermochemical functions were derived from the results. No heat capacity anomaly was observed for MgAl2O4 spinel; however, we observe a low-temperature heat capacity anomaly for Cr-bearing spinels at temperatures below 15 K. From our data we calculate standard entropies (298.15 K) for Mg(Cr,Al)2O4 spinels. We suggest a standard entropy for MgAl2O4 of 80.9 ± 0.6 J mol−1 K−1. For the solid solution between MgAl2O4 and MgCr2O4, we observe a linear increase of the standard entropies from 80.9 J mol−1 K−1 for MgAl2O4 to 118.3 J mol−1 K−1 for MgCr2O4.  相似文献   

3.
Stabilities of hexagonal new aluminous (NAL) phase and Ca-ferrite-type (CF) phase were investigated on the join NaAlSiO4-MgAl2O4 in a pressure range from 23 to 58 GPa at approximately constant temperature of 1,850 K, on the basis of in situ synchrotron X-ray diffraction measurements in a laser-heated diamond-anvil cell. The results show that NAL is formed as a single phase up to 34 GPa, NAL + CF between 34 and 43 GPa, and only CF at higher pressures in 40%NaAlSiO4-60%MgAl2O4 bulk composition. On the other hand, both NAL and CF coexist below 38 and 36 GPa, and only CF was obtained at higher pressures in 60%NaAlSiO4-40%MgAl2O4 and 20%NaAlSiO4-80%MgAl2O4 composition, respectively. These results indicate that NAL appears only up to 46 GPa at 1,850 K, and CF forms continuous solid solution at higher pressures on the join NaAlSiO4-MgAl2O4. NAL has limited stability in subducted mid-oceanic ridge basalt crust in the Earth’s lower mantle and undergoes a phase transition to CF in deeper levels.  相似文献   

4.
Thermal behavior of two new exhalation copper-bearing minerals, bradaczekite and urusovite, from the Great Tolbachik Fissure Eruption (1975–1976, Kamchatka Peninsula, Russia) has been studied by X-ray thermal analysis within the range 20–700°C in air. The following major values of the thermal expansion tensor have been calculated for urusovite: α11 = 10, α22 = αb = 7, α33 = 4, αV = 21 × 10−6°C−1, μ = c∧α33 = 49° and bradaczekite: α11aver = 23, α22 = 8, α33aver = 6 × 10−6°C−1, μ(c∧α33) = 73°. The sharp anisotropy of thermal deformations of these minerals, absences of phase transitions, and stability of the minerals in the selected temperature range corresponding to conditions of their formation and alteration during the posteruption period of the volcanic activity are established.  相似文献   

5.
Low-temperature isobaric heat capacities (C p ) of MgSiO3 ilmenite and perovskite were measured in the temperature range of 1.9–302.4 K with a thermal relaxation method using the Physical Properties Measurement System. The measured C p of perovskite was higher than that of ilmenite in the whole temperature range studied. From the measured C p , standard entropies at 298.15 K of MgSiO3 ilmenite and perovskite were determined to be 53.7 ± 0.4 and 57.9 ± 0.3 J/mol K, respectively. The positive entropy change (4.2 ± 0.5 J/mol K) of the ilmenite–perovskite transition in MgSiO3 is compatible with structural change across the transition in which coordination of Mg atoms is changed from sixfold to eightfold. Calculation of the ilmenite–perovskite transition boundary using the measured entropies and published enthalpy data gives an equilibrium transition boundary at about 20–23 GPa at 1,000–2,000 K with a Clapeyron slope of −2.4 ± 0.4 MPa/K at 1,600 K. The calculated boundary is almost consistent within the errors with those determined by high-pressure high-temperature in situ X-ray diffraction experiments.  相似文献   

6.
 Cation tracer diffusion coefficients, DMe *, for Me=Fe, Mn, Co and Ti, were measured using radioactive isotopes in the spinel solid solution (Ti x Fe 1−x )3−δO4 as a function of the oxygen activity. Experiments were performed at different cationic compositions (x=0, 0.1, 0.2 and 0.3) at 1100, 1200, 1300 and 1400 °C. The oxygen activity dependence of all data for DMe * at constant temperature and cationic composition can be described by equations of the type DMe *=D∘ Me[V]. CV·a O2 2/3+DMe[I] ∘·a O2 −2/3·DMe[V] ∘ and DMe[I] ∘ are constants and CV is a factor of the order of unity which decreases with increasing δ. All log DMe * vs. loga O2 curves obtained for different values of x and for different temperatures go through a minimum due to a change in the type of point defects dominating the cation diffusion with oxygen activity. Cation vacancies prevail for the cation diffusion at high oxygen activities while cation interstitials become dominant at low oxygen activities. At constant values of x, DMe[V] ∘ decreases with increasing temperature while DMe[I] ∘ increases.  相似文献   

7.
Internally consistent thermodynamic datasets available at present call for a further improvement of the data for nepheline (Holland and Powell 1988; Berman 1991). Because nepheline is a common rock-forming mineral, an attempt has been made to improve on the present state of knowledge of its thermodynamic properties. To achieve that goal, two heterogeneous reactions involving nepheline, albite, jadeite and a-quartz in the system NaAlSiO4-SiO2 have been reversed bylong duration runs in the range 460 ≤ T(°C) ≤ 960 and 10 ≤ P(kbar) ≤ 22. Given sufficiently long run times, thealbite run products approach internal equilibrium with respect to their Al,Si order-disorder states. Using appropriate thermochemical, thermophysical, and volumetric data, Landau expansion for albite, and the relevant reaction reversals, a refined thermodynamic dataset (ΔfHi0 and Si0) has been derived for nepheline, jadeite, a-quartz, albite, and monalbite. Our refined data agree very well with theircalorimetric counterparts, but have smaller uncertainties. The refined dataset for ΔfHi0 and Si0, including their uncertainties and correlation, help generate the NaAlSiO4-SiO2 phase diagram including 2a confidence interval for eachP-T curve (Fig. 5). Editorial responsibility: W. Schreyer  相似文献   

8.
 The magnetic behavior of the Jahn-Teller structure braunite, (Mn2+ 1−yM y )(Mn3+ 6− x Mx)SiO12, is strongly influenced by the incorporation of elements substituting manganese. Magnetic properties of well-defined synthetic samples were investigated in dependence on the composition. The final results are presented in magnetic phase diagrams. To derive the necessary data, ac susceptibility and magnetization of braunites with the substitutional elements M = Mg, Fe, (Cu+Ti) and Cu were measured. Whereas the antiferromagnetic ordering temperature, T N , of pure braunite is hardly affected by the substitution of nonmagnetic Mg, it is rapidly suppressed by the substitution of magnetic atoms at the Mn positions. Typically for a concentration (x, y) ≥ 0.7 of the substituted elements, a spin glass phase occurs in the magnetic phase diagrams. Additionally, for the braunite system with Fe3+ substitutions, we observe in the concentration range 0.2 < x< 0.7 a double transition from the paramagnetic state, first to the antiferromagnetic state, followed by a transition to a spin glass state at lower temperatures. The unusual change of the magnetic properties with magnetic substitution at the Mn positions is attributed to the peculiar antiferromagnetic structure of braunite, which has been resolved recently. Received: 19 April 2001 / Accepted: 6 September 2001  相似文献   

9.
Phase relations in the system NaAlSiO4-NaGaSiO4 to 945° C at 1 kbar P(H2O) are dominated by stability of Na(Al,Ga)SiO4 with the beryllonite-type structure. The nepheline structure is restricted to NaAlSiO4-rich compositions at moderate and high temperature. Structure-composition relationships are controlled by space-fitting requirements of both framework and cavity cations, as in related systems. The two-phase (nepheline-type+beryllonite-type) field has been delineated from the end-member NaAlSiO4 composition up to the peritectic point at about 945° C (and 60 mol% NaGaSiO4), using a volume-composition relationship for the beryllonite-type phase, phase appearance, and electron microprobe analysis. At end-member NaGaSiO4 composition, the beryllonite-type phase is stable to the melting point (902±5° C). At end-member NaAlSiO4 composition, the beryllonite-type?nepheline-type transformation occurs at 348±2° C, and is associated with an increase in molar volume of 2.4% and enthalpy of 5170±40J·mol?1. Thus, end-member NaAlSiO4 nepheline, and probably all sub-potassic nephelines as well, are metastable at very-low geological temperatures.  相似文献   

10.
 Synthetic aegirine LiFeSi2O6 and NaFeSi2O6 were characterized using infrared spectroscopy in the frequency range 50–2000 cm−1, and at temperatures between 20 and 300 K. For the C2/c phase of LiFeSi2O6, 25 of the 27 predicted infrared bands and 26 of 30 predicted Raman bands are recorded at room temperature. NaFeSi2O6 (with symmetry C2/c) shows 25 infrared and 26 Raman bands. On cooling, the C2/c–P21/c structural phase transition of LiFeSi2O6 is characterized by the appearance of 13 additional recorded peaks. This observation indicates the enlargement of the unit cell at the transition point. The appearance of an extra band near 688 cm−1 in the monoclinic P21/c phase, which is due to the Si–O–Si vibration in the Si2O6 chains, indicates that there are two non-equivalent Si sites with different Si–O bond lengths. Most significant spectral changes appear in the far-infrared region, where Li–O and Fe–O vibrations are mainly located. Infrared bands between 300 and 330 cm−1 show unusually dramatic changes at temperatures far below the transition. Compared with the infrared data of NaFeSi2O6 measured at low temperatures, the change in LiFeSi2O6 is interpreted as the consequence of mode crossing in the frequency region. A generalized Landau theory was used to analyze the order parameter of the C2/c–P21/c phase transition, and the results suggest that the transition is close to tricritical. Received: 21 January 2002 / Accepted: 22 July 2002  相似文献   

11.
Crystals of hydronium jarosite were synthesized by hydrothermal treatment of Fe(III)–SO4 solutions. Single-crystal XRD refinement with R1=0.0232 for the unique observed reflections (|Fo| > 4F) and wR2=0.0451 for all data gave a=7.3559(8) Å, c=17.019(3) Å, Vo=160.11(4) cm3, and fractional positions for all atoms except the H in the H3O groups. The chemical composition of this sample is described by the formula (H3O)0.91Fe2.91(SO4)2[(OH)5.64(H2O)0.18]. The enthalpy of formation (Hof) is –3694.5 ± 4.6 kJ mol–1, calculated from acid (5.0 N HCl) solution calorimetry data for hydronium jarosite, -FeOOH, MgO, H2O, and -MgSO4. The entropy at standard temperature and pressure (So) is 438.9±0.7 J mol–1 K–1, calculated from adiabatic and semi-adiabatic calorimetry data. The heat capacity (Cp) data between 273 and 400 K were fitted to a Maier-Kelley polynomial Cp(T in K)=280.6 + 0.6149T–3199700T–2. The Gibbs free energy of formation is –3162.2 ± 4.6 kJ mol–1. Speciation and activity calculations for Fe(III)–SO4 solutions show that these new thermodynamic data reproduce the results of solubility experiments with hydronium jarosite. A spin-glass freezing transition was manifested as a broad anomaly in the Cp data, and as a broad maximum in the zero-field-cooled magnetic susceptibility data at 16.5 K. Another anomaly in Cp, below 0.7 K, has been tentatively attributed to spin cluster tunneling. A set of thermodynamic values for an ideal composition end member (H3O)Fe3(SO4)2(OH)6 was estimated: Gof= –3226.4 ± 4.6 kJ mol–1, Hof=–3770.2 ± 4.6 kJ mol–1, So=448.2 ± 0.7 J mol–1 K–1, Cp (T in K)=287.2 + 0.6281T–3286000T–2 (between 273 and 400 K).  相似文献   

12.
In the olivine crystal structure, cations are distributed over two inequivalent octahedral sites, M1 and M2. Kinetics of cation exchange between the two octahedral sites in (Co0.1Mg0.9)2SiO4 single crystal have been studied in the temperature range from 600 to 800°C by monitoring the time evolution of the absorbance of Co2+ ions in M1 or M2 sites using optical spectroscopy after rapid temperature jumps. It was found from such temperature-jump induced relaxation experiments that with increasing temperature the absorbance of Co2+ ions in the M1 site decreases while that in the M2 site increases. This indicates a tendency of Co2+ cations to populate the M2 site with increasing temperatures and vice versa. The experimental relaxation data can be modeled using a triple exponential equation based on theoretical analysis. Activation energies of 221 ± 4 and 213 ± 10 kJ/mol were derived from relaxation experiments on the M2 site and M1 site, respectively, for the cation exchange processes in (Co0.1Mg0.9)2SiO4 olivine. Implications for cation diffusion at low temperatures are discussed.  相似文献   

13.
A pristine magnetite (Fe3O4) specimen was studied by means of Neutron Powder Diffraction in the 273–1,073 K temperature range, in order to characterize its structural and magnetic behavior at high temperatures. An accurate analysis of the collected data allowed the understanding of the behavior of the main structural and magnetic features of magnetite as a function of temperature. The magnetic moments of both tetrahedral and octahedral sites were extracted by means of magnetic diffraction up to the Curie temperature (between 773 and 873 K). A change in the thermal expansion coefficient around the Curie temperature together with an increase in the oxygen coordinate value above 700 K can be observed, both features being the result of a change in the thermal expansion of the tetrahedral site. This anomaly is not related to the magnetic transition but can be explained with an intervened cation reordering, as magnetite gradually transforms from a disordered configuration into a partially ordered one. Based on a simple model which takes into account the cation-oxygen bond length, the degree of order as a function of temperature and consequently the enthalpy and entropy of the reordering process were determined. The refined values are ΔH0 = −23.2(1.7) kJ mol−1 and ΔS0 = −16(2) J K−1 mol−1. These results are in perfect agreement with values reported in literature (Mack et al. in Solid State Ion 135(1–4):625–630, 2000; Wu and Mason in J Am Ceramic Soc 64(9):520–522, 1981).  相似文献   

14.
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   

15.
It is shown the possibility to determine the coordination of paramagnetic ions in disordered solid structures, e.g., in barium borate glasses. For this purpose the electron paramagnetic resonance (EPR) method was used to study α-and β-BaB2O4 crystals and glasses of 45·BaO × 55·B2O3 and 40·BaO × 60·B2O3 (mol%) composition activated by Ag+ and Pb2+ ions. After the samples were exposed to X-rays at 77 K, different EPR centers were observed in them. In α-and β-BaB2O4 crystals and glasses the EPR centers Ag2+, Ag0, Pb+, Pb3+, and hole centers of O− type were studied. The EPR parameters of these centers and their arrangement in crystal structure were determined. It is shown that Pb3+ ions in β-BaB2O4 crystals occupy Ba2+ position in an irregular polyhedron from the eight oxygen, whereas in α-BaB2O4 crystals they occupy Bа2 position in a sixfold coordination. Pb+ ions in α-BaB2O4 crystals occupy Bа1 position in a ninefold coordination from oxygen. In barium borate glasses, Pb3+ ions were studied in coordination polyhedron from six oxygen atoms and in a polyhedron from nine to ten oxygen atoms. It is assumed that the established difference in the structural position of Pb3+ ions in glasses is due to their previous incorporation in associative cation–anion complexes (AC) and “free” structure-forming cations (FC). Computer simulations have been performed to analyze the stability of specific associative complexes and to compare their bond lengths with experimental data.  相似文献   

16.
A new synchrotron X-ray diffraction study of chromium oxide Cr2O3 (eskolaite) with the corundum-type structure has been carried out in a Kawai-type multi-anvil apparatus to pressure of 15 GPa and temperatures of 1873 K. Fitting the Birch–Murnaghan equation of state (EoS) with the present data up to 15 GPa yielded: bulk modulus (K 0,T0), 206 ± 4 GPa; its pressure derivative K′0,T , 4.4 ± 0.8; (?K 0,T /?T) = ?0.037 ± 0.006 GPa K?1; a = 2.98 ± 0.14 × 10?5 K?1 and b = 0.47 ± 0.28 × 10?8 K?2, where α 0,T  = a + bT is the volumetric thermal expansion coefficient. The thermal expansion of Cr2O3 was additionally measured at the high-temperature powder diffraction experiment at ambient pressure and α 0,T0 was determined to be 2.95 × 10?5 K?1. The results indicate that coefficient of the thermal expansion calculated from the EoS appeared to be high-precision because it is consistent with the data obtained at 1 atm. However, our results contradict α 0 value suggested by Rigby et al. (Brit Ceram Trans J 45:137–148, 1946) widely used in many physical and geological databases. Fitting the Mie–Grüneisen–Debye EoS with the present ambient and high-pressure data yielded the following parameters: K 0,T0 = 205 ± 3 GPa, K′0,T  = 4.0, Grüneisen parameter (γ 0) = 1.42 ± 0.80, q = 1.82 ± 0.56. The thermoelastic parameters indicate that Cr2O3 undergoes near isotropic compression at room and high temperatures up to 15 GPa. Cr2O3 is shown to be stable in this pressure range and adopts the corundum-type structure. Using obtained thermoelastic parameters, we calculated the reaction boundary of knorringite formation from enstatite and eskolaite. The Clapeyron slope (with \({\text{d}}P/{\text{d}}T = - 0.014\) GPa/K) was found to be consistent with experimental data.  相似文献   

17.
High pressure in situ synchrotron X-ray diffraction experiment of strontium orthophosphate Sr3(PO4)2 has been carried out to 20.0 GPa at room temperature using multianvil apparatus. Fitting a third-order Birch–Murnaghan equation of state to the P–V data yields a volume of V 0 = 498.0 ± 0.1 Å3, an isothermal bulk modulus of K T  = 89.5 ± 1.7 GPa, and first pressure derivative of K T ′ = 6.57 ± 0.34. If K T ′ is fixed at 4, K T is obtained as 104.4 ± 1.2 GPa. Analysis of axial compressible modulus shows that the a-axis (K a  = 79.6 ± 3.2 GPa) is more compressible than the c-axis (K c  = 116.4 ± 4.3 GPa). Based on the high pressure Raman spectroscopic results, the mode Grüneisen parameters are determined and the average mode Grüneisen parameter of PO4 vibrations of Sr3(PO4)2 is calculated to be 0.30(2).  相似文献   

18.
Summary The complexation of aluminium(III) and silicon(IV) was studied in a simplified seawater medium (0.6 M Na(Cl)) at 25 °C. The measurements were performed as potentiometric titrations using a hydrogen electrode with OH − ions being generated coulometrically. The total concentrations of Si(IV) and Al(III) respectively [Si tot ] and [Al t ot], and −log[H +] were varied within the limits 0.3 < [Si tot ] < 2.5 mM, 0.5 < [Al tot ] < 2.6 mM, and 2 ≤ -log[H +] ≤ 4.2. Within these ranges of concentration, evidence is given for the formation of an AlSiO(OH) 3 2+ complex with a formation constant log β1,1-1 = −2.75 ± 0.1 defined by the reaction Al 3++Si (OH)4 ↔ AlOSi(OH) 3 2+ +H + An extrapolation of this value to I=0 gives log β1,1-1 = −2.30. The calculated value of log K (Al 3++SiO(OH) 3 − ↔ AlOSi(OH) 3 2+ ) = 6.72 (I=0.6 M) can be compared with corresponding constants for the formation of AlF 2+ and AlOH 2+ , which are equal to 6.16 and 8.20. Obviously, the stability of these Al(III) complexes decreases within the series OH −>SiO(OH) 3 −  > F −  相似文献   

19.
The electrical conductivity of (Mg0.93Fe0.07)SiO3 ilmenite was measured at temperatures of 500–1,200 K and pressures of 25–35 GPa in a Kawai-type multi-anvil apparatus equipped with sintered diamond anvils. In order to verify the reliability of this study, the electrical conductivity of (Mg0.93Fe0.07)SiO3 perovskite was also measured at temperatures of 500–1,400 K and pressures of 30–35 GPa. The pressure calibration was carried out using in situ X-ray diffraction of MgO as pressure marker. The oxidation conditions of the samples were controlled by the Fe disk. The activation energy at zero pressure and activation volume for ilmenite are 0.82(6) eV and −1.5(2) cm3/mol, respectively. Those for perovskite were 0.5(1) eV and −0.4(4) cm3/mol, respectively, which are in agreement with the experimental results reported previously. It is concluded that ilmenite conductivity has a large pressure dependence in the investigated P–T range.  相似文献   

20.
Vibrational density of states of the NaAlSi2O6 jadeite and NaAlSiO4 calcium ferrite (CF)-type, and SiO2 stishovite is calculated as a function of pressure up to 50 GPa using density functional perturbation theory. The calculated frequencies are used to determine the thermal contribution to the Helmholtz free energy within the quasi-harmonic approximation and to derive the equation of state and several thermodynamic properties of interest. A dissociation of jadeite into a mixture of a CF-type phase and stishovite is predicted to occur at 23.4 GPa and 1,800 K with a positive Clapeyron slope of 2.8 MPa/K. Elastic anisotropy for jadeite, the CF-type phase, and stishovite also computed clearly shows that stishovite and the CF-type phase are the most anisotropic and isotropic in these three phases, respectively.  相似文献   

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