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1.
Measurements of the heats of solution (ΔHsoln) in molten Pb2B2O5 at 708°C of anhydrous magnesian cordierites, prepared with a range of structural states, show that the enthalpy effect associated with Al/Si ordering is substantial (? 9.76 ± 1.56 kcal mole?1). Differences in the state of order between synthetic cordierites used in phase equilibrium studies and cordierites in the natural environment could lead to significant errors in the estimation of palaeo-pressures and temperatures. A continuous change of ΔHsoln with annealing time supports the suggestion of putnis (1980) that the hexagonal → orthorhombic transformation in cordierite, which can occur via a modulated structure, is truly continuous under metastable conditions. In addition, a linear relation between ΔHsoln and the logarithm of annealing time has been found, which provides some insight into the nature of the ordering mechanisms at an atomic level. Al and Si exchanges occur continuously between neighbouring tetrahedral sites with a net drift towards increasing order. No kinetic or thermochemical distinction can be made between the development of long range and short range order.The enthalpy of vitrification (~ 12 kcal mole?1) for a metastable stuffed β-quartz polymorph of cordierite composition is similar to that for pure quartz (on a per two oxygen basis), while the heat of vitrification for even the most disordered cordierite seen in this study is more than a factor of three greater (~40 kcal mole?1). This is consistent with the view that cordierite glass resembles the quartz structure more closely than the crystalline cordierite structure, and that crystallisation of the glass below ~900°C is controlled by a tetrahedral framework.  相似文献   

2.
The pressure dependence of the Raman spectrum of forsterite was measured over its entire frequency range to over 200 kbar. The shifts of the Raman modes were used to calculate the pressure dependence of the heat capacity, C v, and entropy, S, by using statistical thermodynamics of the lattice vibrations. Using the pressure dependence of C v and other previously measured thermodynamic parameters, the thermal expansion coefficient, α, at room temperature was calculated from α = K S (?T/?P) S C V/TVK T, which yields a constant value of (? ln α/? ln V)T= 6.1(5) for forsterite to 10% compression. This value is in agreement with (? ln α/? ln V)T for a large variety of materials. At 91 kbar, the compression mechanism of the forsterite lattice abruptly changes causing a strong decrease of the pressure derivative of 6 Raman modes accompanied by large reductions in the intensities of all of the modes. This observation is in agreement with single crystal x-ray diffraction studies to 150 kbar and is interpreted as a second order phase transition.  相似文献   

3.
Sabugalite has been synthesized directly from pure chemicals. From chemical, differential thermal and thermogravimetric analyses, its formula is calculated as HA1(UO2/PO4)2·16H2O. The natural relationship between hydrogen autunite, autunite and sabugalite was investigated by means of ion exchange experiments, and its infrared spectrum, electrokinetic properties and solubility studied. An increase in solubility results in a more positive zeta-potential. The cell dimensions have been determined from Guinier-Hägg diffraction data. Synthetic sabugalite crystallizes in the monoclinic system with space group C2/m and cell parameters: a=19.426 Å; b=9.843 Å; c=9.850 Å; α=γ=90°; β=96.161°; V=1,872.54 Å3 and Z=2.  相似文献   

4.
The dependence of water concentration in synthetic (Mg, Fe2+)-cordierite on the composition of the solid solution was examined in experiments that lasted for 10 days at = 200–230 MPa, t = 600–700°C, and oxygen fugacity corresponding to the Fe-FeO buffer. Mass spectrometric data indicate that the dependence of water concentration in cordierite on its Fe mole fraction Fe2+/(Fe2+ + Mg) has maxima at compositions with F = 0.2–0.3. IR diffuse reflectance spectroscopic data and data on the structural setting of H2O molecules in the structural channels of alkali-free (Mg, Fe2+)-cordierite indicate that the H-H vector of some H2O molecules (H2O-II) is perpendicular to [001] of the crystal. The dependence of the magnetic properties of synthetic (Mg, Fe2+)-cordierite was studied by static magnetization technique at 5–300 K in an external magnetic field up to 20 kOe in strength.  相似文献   

5.
Cordierite has the ideal formula (Mg,Fe)2Al4Si5O18 .x(H2O,CO2), but it must contain some Fe3+ to account for its blue color and strong pleochroism. The site occupation and concentration of Fe3+ in two Mg-rich natural cordierites have been investigated by EPR and 57Fe Mössbauer spectroscopy. In addition, powder IR spectroscopy, X-ray diffraction, and TEM examination were used to characterize the samples. Single-crystal and powder EPR spectra indicate that Fe3+ is located on T11 in natural cordierites and not in the channels. The amount in Mg-rich cordierites is very small with an upper limit set by Mössbauer spectroscopy giving less than 0.004 cations per formula unit (pfu). Fe3+ in cordierite can, therefore, be considered insignificant for most petrologic calculations. Heat-treating cordierite in air at 1,000?°C for 2?days causes an oxidation and/or loss of Fe2+ on T11, together with an expulsion of Na+ from the channels, whereas heating at the Fe–FeO buffer produces little Fe3+ in cordierite. Heating at 1,000?°C removes all class I H2O, but small amounts of class II H2O remain as shown by the IR measurements. No evidence for channel Fe2+ or Fe3+ in the heat-treated samples was found. The blue color in cordierite arises from a broad absorption band (E//b and weaker with E//a) around 18,000?cm?1 originating from charge-transfer between Fe2+ in the octahedron and Fe3+ in the edge-shared T11 tetrahedron. It therefore appears that all natural cordierites contain some tetrahedral Fe3+. The brown color of samples heated in air may be due to the formation of very small amounts of submicroscopic magnetite and possibly hematite. These inclusions in cordierite can only be identified through TEM study.  相似文献   

6.
Complete solid solutions with the ilmenite structure from pure MgSiO3 to 75% MgSiO3·25% Al2O3 have been synthesized in the pressure region between 240 and 300 kbar at 1000–1400°C in a diamond-anvil cell coupled with laser heating. The results suggest that complete solid solutions with the ilmenite structure might be formed between MgSiO3 and Al2O3 under high pressure-high temperature conditions. The lattice parameters for the ilmenite solid solutions between MgSiO3 and Al2O3 deviate from ideality in the same manner as those found by Berry and Combs along the FeVO3–Fe2O3 join. For the ordered A2+ B4+O3 ilmenite-type compounds, co is determined primarily by the size of the relatively large A2+ cation, whereas ao depends strongly on the radii of both A2+ and B4+ cations. Such systematics might account for the fact that co and ao for the ilmenite-type MgSiO3 are respectively smaller and larger than those for Al2O3. The lattice parameters ao and molar volumes for the A23+O3 corundum-type compounds and the disordered A2+B4+O3 ilmenites follow a different trend and are, therefore, readily distinguished from the ordered A2+B4+O3 ilmenites.  相似文献   

7.
Phase relations in the system TiO2–ZrO2 were examined in the pressure range of 3.5–12?GPa at 1,800?°C, using multianvil apparatus. At 1,800?°C, TiO2 rutile transforms to αPbO2 structure at 10?GPa, and the αPbO2-type solid solution is stable in compositional range between TiO2 and about (Ti0.6, Zr0.4)O2 at 3.5–12?GPa. Combination of the present results with the published data at 0–3?GPa demonstrates that continuous solid solution with the αPbO2-type structure is stable between TiO2 and (Ti1?x , Zr x )O2 (x?≈?0.6) at 0–12?GPa. This indicates that both the αPbO2-type TiO2 and srilankite Ti2ZrO6 with the same structure belong to the continuous solid solution system though the two phases have been regarded as different minerals. With increasing ZrO2 content, lattice parameters of a- and c-axes of the αPbO2-type solid solution increase, but b-axis is almost constant or slightly decreases. At higher pressure, the αPbO2-type solid solution dissociates into two phases, αPbO2-type phase and tetragonal zirconia. Srilankite with more TiO2-rich composition than Ti2ZrO6 might be found in natural rocks derived from the deep upper mantle.  相似文献   

8.
 Incorporation of hydrocarbons (CH4, C2H6, C3H8, C4H10) in cordierite channels was experimentally studied at 700 °C and at pressures from 200 to 1000 MPa, (avoiding recrystallization). The maximum concentrations of hydrocarbons determined by gas chromatographic analysis are CH4=78.3, C2H6=134, C2H4 + C2H6=26, C3H8=28, C4H10=32, C5H12=23, and C6H14=7 × 10−3 wt%). According to IR spectroscopy data, the channel forms of hydrocarbons differ from the forms on the surface. As a result of interactions with the framework oxygen, normal hydrocarbons are converted to saturated oligomers and their fragments. Small amounts of water molecules of the first and second types (up to 0.3 wt%) are formed in the same way. As pressure grows from 200 to 1000 MPa, the total content of structural hydrocarbons is nearly doubled. Special runs on cordierite saturation in mixtures of hydrocarbons with water showed that low contents of hydrocarbons in natural cordierites correspond to their large concentrations in fluid. Received: 7 May 2001 / Accepted: 17 July 2001  相似文献   

9.
The heat capacity (Cp) of two synthetic spessartine samples (Sps) was measured on 20-30 mg-size samples in the temperature range 2-864 K by relaxation calorimetry (RC) and differential scanning calorimetry (DSC). The polycrystalline spessartine samples were synthesized in two different laboratories at high pressures and temperatures from glass and oxide-mixture starting materials and characterized by X-ray powder diffraction and electron-microprobe analysis. The low-temperature heat capacity data show a prominent lambda transition with a peak at 6.2 K, which is interpreted to be the result of a paramagnetic-antiferromagnetic phase transition. The DSC data around ambient T agree excellently with the RC data and can be represented by the Cp polynomial for T > 250 K:
  相似文献   

10.
Alkaline potassium permanganate oxidation of a young kerogen (lacustrine) and 34 model compounds (saturated and unsaturated fatty acids, hydroxy acid, aliphatic dicarboxylic acids, aliphatic alcohols, normal hydrocarbon, β-carotene, phenolic acids, benzenecarboxylic acids, carbohydrates, amino acids and proteins) were conducted, followed by GC and GC-MS analysis of the degradation products. The stability of the degradation products of kerogen in permanganate solution and the relationship between degradation products and kerogen building blocks were determined.The results showed that aliphatic acids C12–C16 monocarboxylic acids and C6–C10 α,ω-dicarboxylic acids) were rather susceptible to oxidation compared with benzenecarboxylic acids and the former were degraded into lower molecular weight decarboxylic acids. It was concluded that oxidation at milder conditions (60° C, 1 hr) is appropriate for qualitative and quantitative characterization of the aliphatic structure of young kerogen. It was noteworthy that benzoic acid was produced in a significant amount by oxidation of amino acids (phenylalanine) and proteins, C18-isoprenoidal ketone from phytol, and C8 and C9 α,ω-dicarboxylic acids from unsaturated fatty acids, respectively; furthermore, 2,2-dimethyl succinic and 2,2-dimethyl glutaric acids were produced from β-carotene.  相似文献   

11.
The crystal structure of α-CaSi2O5 synthesized at conditions of 1500°C and 10 GPa, has been solved and refined in centrosymmetric space group P , using single crystal X-ray diffraction data. The composition (Z=4) and unit cell are Ca1.02Si1.99O5 by EPMA analysis and a=7.243(2) Å, b=7.546(4) Å, c=6.501(4) Å, α=81.43(5)°, β=84.82(4)°, γ=69.60(3)°, V=329.5(3) Å3, yielding the density value, 3.55 g/cm3. The structure is closely related to that of titanite, CaTiSiO5 and features the square-pyramid five-fold coordination of silicon by oxygen. The ionic radius for five-coordinated Si calculated from the bond distances is 0.33 Å. The substantial deviation of valence sum for Ca indicates the existence of local strain and the instability of α-CaSi2O5 at room pressure.  相似文献   

12.
The nucleation kinetics of binary solid solutions, with general formula BxC1−xA, crystallising from aqueous solution can be described using a generalised expression for the nucleation rate: the function, J(x), in which supersaturation, interfacial free energy and other parameters of the classical nucleation rate equation are considered as functions of the solid composition. As an example, we studied the behaviour of such J(x) functions for the case of the (Ba,Sr)SO4 and (Ba,Sr)CO3 solid solutions. J(x) functions are very sensitive to slight changes in the composition of the aqueous solution, which result in strong modifications of the nucleation kinetics. The implications of the relationship between supersaturation and nucleation rate functions for the general nucleation behaviour in solid solution-aqueous solution (SS-AS) systems are discussed. Finally, we present a method for constructing non-equilibrium Roozeboom diagrams based on the nucleation kinetics in SS-AS systems. Our Roozeboom diagrams calculated for different departures from equilibrium conditions are consistent with previous experimental work and they can be used to predict actual distribution coefficients.  相似文献   

13.
Relative-enthalpy measurements have been made on the hexagonal, tetragonal, glass and liquid phases of GeO2. The glass transition is very sensitive to the impurity content, with a T g ranging from 980 K for a pure product to 780 K for a Li-doped sample with 0.06 mol % Li. The relative C p change at T g of about 5% increases with the impurity content as a result of lower glass transition temperatures. Above 298 K the derived heat capacities are similar for all forms, with slightly higher values for the amorphous phases and two C p cross-overs at 400 and 1000 K between the hexagonal and tetragonal modifications. For both GeO2 and SiO2 the coordination state markedly affects C p and the entropy below 300 K, where the properties are much lower for the tetragonal than for the hexagonal modifications, i.e., S 298 = 39.7 vs 55.3 J/mole K and 27.8 vs 41.4 J/ mole K for GeO2 and SiO2, respectively. The high-temperature C p's of coesite and stishovite are likely similar to those of the low-pressure SiO2 forms. Finally, these results, low-temperature C p data and enthalpy-of-solution measurements have been used to derive a consistent set of thermodynamic properties for the GeO2 modifications.  相似文献   

14.
Crystal structure data are presented for seven synthetic samples of disordered zirconium-titanate solid solution (Zr,Ti)O2, ranging in composition from x Ti=0.43 to 0.67, thus covering compounds such as ZrTiO4, Zr5Ti7O24, and ZrTi2O6 (srilankite). The compounds, synthesized at high temperatures and various pressures in their respective stability fields, are well crystallized and of homogeneous composition. The resulting structure data are less scattered compared to previous studies that were based on compounds synthesized metastably at low temperatures and room pressure. The compounds have the structure of scrutinyite (α-PbO2) with space group Pbcn, Z=4, unit cell parameters a=4.8495(3) Å, b=5.4635(3) Å, c=5.0462(3) Å at x Ti=0.425 to a=4.7112(2) Å, b=5.4944(1) Å, c=4.9962(1) Å at x Ti=0.666. The first structure refinement of pure, synthetic srilankite is presented, which is in good agreement with that of the natural counterpart. Structural trends observed in disordered zirconium-titanate solid solution along the binary join ZrO2–TiO2 are relatively smooth and continuous, except for rapid lengthening of an unshared octahedral edge which is anomalously short in scrutinyite-structure TiO2. The shortness of this edge may explain the observed instability of this structure with the relatively small Ti as the dominant cation. With increasing Zr content, the average cation position moves off-centre inside the octahedron, away from two shared edges, which permits the 12 closest cation–cation distances in the structure to become more equal. The shortening of the b dimension with increasing amount of the larger cation Zr decreases the distance between octahedral Zr and two additional oxygens in an adjacent chain of edge-sharing octahedra, implying that the Zr environment is evolving towards eightfold coordination. If the two additional oxygens are considered as part of the Zr coordination polyhedron, the bonding topology of tetragonal zirconia is obtained. The compositional evolution of the cell parameters, Zr atomic coordinates and Zr coordination environment is consistent with the idea that the structure is evolving towards that of tetragonal ZrO2. Group-theoretical relationships between scrutinyite, tetragonal zirconia, baddeleyite and fluorite structures show that the sequence of structures fluorite > tetragonal zirconia > scrutinyite > baddeleyite are all related by potentially diffusionless phase transitions driven by wavelike displacements of the oxygen substructure. The scrutinyite and tetragonal structures can act outside their stability fields as “transition states” between the structures on either side.  相似文献   

15.
The adsorption of hydrogen sulfide (ΓH2S) and protons (ΓH+) on the surface of crystalline sulfur was investigated experimentally in H2S-bearing solutions at temperatures of 25, 50, and 70°C, NaCl concentrations of 0.1 and 0.5 mol/dm−3 and log CH+ values in the range −2.3 to −5. At all temperatures, the dominant process on the surface of the sulfur was deprotonation, and the average values of ΓH2S were very close to the highest values determined for ΓH+. This finding, combined with the lack of detectable proton adsorption in H2S-free solutions, suggests that proton adsorption/desorption on the surface of sulfur occurs through formation of ≡ S − H2S complexes in the presence of H2S.We propose that this complexation represents sulfidation of the sulfur surface, a process analogous to hydroxylation of oxide surfaces, and that the sulfidation can be described by the reaction: ≡ S + H2S = ≡SSH20 β° The deprotonation of the ≡ SH° complex occurs via the reaction: ≡ SSH20 = ≡SSH− + H+ β− Values of 2.9, 2.8, and 2.9 (± 0.23) were obtained for −log β− at 25, 50, and 70°C, respectively. These data were employed to estimate the second dissociation constant for hydrogen sulfide in aqueous solutions using the extrapolation method proposed by Schoonen and Barnes (1988) and yielded corresponding values for the constant of 17.4 ± 0.3, 15.7, and 14.5, respectively. The value for 25°C is in very good agreement with the experimentally determined values of Giggenbach (1971) at 17 ± 0.1; Meyer et al. (1983) at 17 ± 1; Licht and Manassen (1987) at 17.6 ± 0.3; and Licht et al. (1990) at 17.1 ± 0.3.  相似文献   

16.
《Organic Geochemistry》1987,11(4):251-264
Particulate matter, sediment trap, and surface sediment samples collected in freshwater Lake Haruna were studied to understand early diageesis of organic materials in the water column and in bottom sediments. The samples were analyzed for biomarkers, including aliphatic and aromatic hydrocarbons, fatty alcohols, saturated and unsaturated fatty acids, β- and ω-hydroxyacids, and α,ω-dicar☐ylic acids. Decreases in concentrations of autochthonous saturated C12–C19 fatty acids and polyunsaturated C18 acids relative to TOC occured with the settling of organic matter ot the lake bottom, whereas the amounts of terrestial saturated C20–C30 acids remained almost constant. Conversely, the concentrations of monosaturated fatty acids, branched chain fatty acids, and β- and ω-hydroxyacids, which are probably produced by microbial activity, increased. These results indicate that preferential degradation of algal lipids accompanies microbial resynthesis of lipids during settling, however, terrigenous lipids are relatively stable.  相似文献   

17.
The thermal expansivity of liquid GeO2 at temperatures just above the glass transition has been obtained using a combination of scanning calorimetry and dilatometry. The calorimetric and dilatometric curves of c p and dV/dT are normalized to the temperature derivative of fictive temperature versus temperature using the method of Webb et al. (1992). This normalization, based on the equivalence of relaxation parameters for volume and enthalpy, allows the completion of the dilatometric trace across the glass transition to yield liquid expansivity and volume. The values of liquid volume and expansivity obtained in this study are combined with high temperature densitometry determinations of the liquid volume of GeO2 by Sekiya et al. (1980) to yield a temperature-volume relation for GeO2 melt from 660 to 1400 °C. Liquid GeO2 shows a strongly temperature-dependent liquid molar expansivity, decreasing from 20.27 × 10?4 cm3 mol?1°C?1 to 1.97 × 10?4cm3 mol?1 °C?1 with increasing temperature. The coefficient of volume thermal expansion (α v ) decreases from 76.33 × 10?6 °C?1 to 2.46 × 10?6 °C?1 with increasing temperature. A qualitatively similar volume-temperature relationship, with α v decreasing from 335 × 10?6 °C?1 to 33 × 10?6 °C?1 with increasing temperature, has been observed previously in liquid B2O3. The determination of the glass transition temperature, liquid volume, liquid and glassy expansivities and heat capacities in this study, combined with compressibility data for glassy and liquid GeO2 from the literature (Soga 1969; Kurkjian et al. 1972; Scarfe et al. 1987) allows the calculation of the Prigogine-Defay ratio (Π), c p -c v and the thermal Grüneisen parameter (γ th) for GeO2. From available data on liquid SiO2 it is concluded that liquid GeO2 is not a good analog for the low pressure properties of liquid SiO2.  相似文献   

18.
The mineral chemistry of cordierites from three different sanidinite facies localities-1) volcanic xenoliths from the Eifel, Germany; 1) buchites of the Blaue Kuppe, Germany; 3) paralavas from the Bokaro coalfield, India-is characterized by unusually high potassium contents up to 1.71 wt%, equivalent to 0.22 K atoms per formula unit (p.f.u.) based on 18 oxygens. Significantly, these cordierites are either hexagonal highcordierites (indialites) with =0 or exhibit intermediate -values 0<<0.20 relative to well Al,Si-ordered orthorhombic low-cordierite. Based on microprobe analyses, the predominant substitutional mechanism for alkali incorporation is Alk[Channel]+Al[4] for +Si[4], thus leading to Al/Si-ratios deviating considerably from the value 4:5 in ideal cordierite M2[Al4Si5O18]. The most highly substituted cordierite from Blaue Kuppe is about (K0.22Na0.07)[Ch](Mg1.33Fe 0.66 2+ )[6][Al4.16Si4.79O18]. Bokaro cordierites are further characterized by obvious (Al+Si)-deficiencies against the ideal value of 9.0 p.f.u., a tendency of which is apparent in most Blaue Kuppe analyses as well. As the tetrahedral deficiencies are often equivalent to excess cations in the octahedra, we assume that ferric iron fills up the remaining tetrahedral sites, again linked with the introduction of potassium according to K+Fe3+ for +Si. In comparison with the available experimental data, these natural potassic cordierites are considered stable high-temperature phases regarding their compositions, but not their structural states. Although the substitution KAl for Si in Mg-cordierite is known to lower the maximum -value to be attained, the hexagonal nature of the cordierites must be due to very rapid crystallization and subsequent quenching. The higher -values of the Blaue Kuppe cordierites might be caused by their topotactic origin from preexisting biotite. The complicated twin and domain patterns of the hexagonal Eifel and Bokaro cordierites as observed in thin section could perhaps be attributed to structural modulations as postulated recently for hexagonal cordierite shortly after its growth.  相似文献   

19.
Thermal expansion properties of synthetic orthopyroxenes (Fe0.20Mg0.80)SiO3, (Fe0.40Mg0.60)SiO3, (Fe0.50Mg0.50)SiO3, (Fe0.75Mg0.25)SiO3 and (Fe0.83Mg0.17)SiO3 were systematically studied by means of single-crystal x-ray diffraction in the temperature range from 296 to 1300 K. The measurements of unit cell dimensions as a function of temperature reveal that the a and c dimensions and the unit cell volume V increase nonlinearly with a positive curvature with rising temperature, whereas the b dimension behaves differently, depending on the total Fe content. For Mg-rich orthopyroxenes (Fe/(Fe+Mg)<30%), the b dimension expands similarly as the a and c dimensions, but it exhibits a nonlinear increase with a negative curvature for orthopyroxenes with Fe/(Fe+Mg)>30%. Together with the high temperature neutron diffraction data on enstatite (MgSiO3) (McMullan, Haga and Ghose, unpublished) and x-ray diffraction data on ferrosilite (FeSiO3) (Sueno et al. 1976), the measured unit cell dimensions were analyzed in terms of the Grüneisen theory of thermal expansion. The linear thermal expansion coefficients α a and α c both increase as temperature is elevated, with α c increasing faster, while α b changes gradually from increasing for Mg-rich orthopyroxenes to decreasing for Fe-rich orthopyroxenes. The relative magnitudes of linear thermal expansion coefficients are always in the order α b >α c >α a between 300 and 500 K, but at higher temperatures, the order changes to α c >α b >α a for Mg-rich orthopyroxenes and α c >α a >α b for Fe-rich ones. The linear thermal expansion behavior is interpreted on the basis of the structural mechanical model of Weidner and Vaughan (1982). The anomalous behavior of α b is mainly attributed to the changes in the Fe2+ population at the M2 site and the relative stiffness of the M2(Fe2+)-O bonds compared to the M2(Mg2+)-O bonds. The volume thermal expansion coefficients are nonlinear functions of temperature and lie between 23 and 49×10?6/K. The previously reported results of mean volume thermal expansion coefficients appear to represent the α V values characteristic of higher temperatures compared to our results. The thermal Debye temperatures are composition-dependent, decreasing linearly from 812 (MgSiO3) to 561 K (FeSiO3), and are systematically higher than the corresponding acoustic Debye temperatures. The Grüneisen parameters range from 0.85 to 0.89 and do not seem to vary with composition. The linear compressibilities derived from thermal expansion and elastic moduli data agree very well. The pressure derivatives of the isothermal bulk modulus (dK0/dP) are also composition-dependent and decrease from 11.2 (MgSiO3) to 8.77 (FeSiO3). Such large values indicate possible anomalous elastic behavior of orthopyroxenes at high pressures in the Earth's upper mantle.  相似文献   

20.
Hydrotalcite-like solid solutions have been synthesized by coprecipitation in basic solutions with variable SO 4 2? /CO 3 2? ratios. Chemical determination of CO 3 2? in the interlayer was impossible because of the presence of minor hydromagnesite. SO 4 2? was determined both by chemical analysis and X-ray photoelectron spectroscopy (XPS), the two methods giving similar results. A Raman spectrometry gave additional data on the SO 4 2? /CO 3 2? ratio. Then, the stoichiometry of the anionic interlayers, X s , X c , and X OH were determined, and the influence of X s on the c′ parameter (increasing from c′=7.97 Å to c′=8.63 Å between X s =0 and X s =1) was characterized. In addition, a partitioning curve of SO 4 2? and CO 3 2? between aqueous solutions and hydrotalcite-like compounds was established. Its general shape strongly suggests a miscibility gap between a sulfate-rich end and a carbonate-rich solid solution (maximum SO 4 2? /CO 3 2? about 0.2). This result explains why most of the hydrotalcites synthesized during experimental alteration of basaltic glasses by sea-water (a sulfate-rich solution) are CO 3 2? -rich solid solutions.  相似文献   

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