Phase relations in the system LiFMgF₂ at elevated pressures: Implications for the proposed mixed-oxide zone of the earth's mantle     

Ian Jackson 《Physics of the Earth and Planetary Interiors》1977,14(1):86-94

Solvi and liquidi for various LiFMgF₂ mixtures have been determined at pressures up to 40 kbar by differential-thermal-analysis in a piston-cylinder high-pressure device. The melting curves of pure LiF and MgF₂ were also studied and the initial slopes (dT_m/dP)_{P = 0} were found to be 11.2 and 8.3°C/kbar, respectively. The eutectic composition (LiF)_0.64(MgF₂)_0.36 is independent of pressure to 35 kbar and the eutectic temperature rises approximately 6.3°C per kbar. Initial slopes of 11°C/kbar and 35°C/kbar are inferred for the melting curves of MgO and SiO₂ (stishovite) respectively, on the basis of data for their structural analogue compounds. The observed solid solution of LiF in MgF₂ and other evidence suggest the possibility of solid solution in the system (Mg,Fe)OSiO₂ (stishovite) under mantle conditions which may have important consequences for the elastic properties of a “mixed-oxide” zone of the earth's mantle.  相似文献   

Disproportionation of Ni₂SiO₄ to stishovite plus bunsenite at high pressures and temperatures     

Lin-Gun Liu 《Earth and Planetary Science Letters》1975,24(3):357-362

Samples of Ni₂SiO₄ in both olivine and spinel phases have been compressed to pressures above 140 kbar in a diamond-anvil cell and heated to temperatures of 1400–1800°C using a continuous YAG laser. After quenching and releasing pressure, X-ray diffraction examination indicates that the samples disproportionate to a mixture of stishovite (SiO₂) and bunsenite (NiO) at pressures between 140 and 190 kbar. The exact disproportionation pressure is not certain due to transient increases in pressure during the local and rapid heating. However, thermodynamic calculations suggest that the transition pressure is about 192 ± 4 kbar at 1545°C and that the equation of the spinel-mixed oxides phase boundary isP(kbar) = 121 + (0.046 ± 0.020) T (°C).  相似文献   

Melting of some alkaline-earth and transition-metal fluorides and alkali fluoroberyllates at elevated pressures: A search for melting systematics     

Ian Jackson 《Physics of the Earth and Planetary Interiors》1977,14(2):143-164

The melting curves of the fluorides ZnF₂ and NiF₂ (rutile structure), CaF₂, SrF₂ and BaF₂ (fluorite structure), and of the fluoroberyllates Na₂BeF₄ and Li₂BeF₄ have been studied at pressures ? 40 kbar by differential thermal analysis in a piston-cylinder high-pressure device. The initial slopes (dT_m/dP)₀ of these melting curves are respectively 7.2, 5.8, 16.7, 15.2, 15.7, 15.1 and <0°C/kbar. A new Li₂BeF₄ polymorph, apparently of the olivine structure type, is stable at pressures > 10 kbar and its melting curve has an average slope of ～6.7°C/kbar. These new data and those for SiO₂, BeF₂, GeO₂, LiF and MgF₂, recently studied by Jackson, are combined with existing data for elements, ionic compounds and silicates to assess the influence of crystal structure, molar volume and the nature of interatomic bonding on the initial slopes of melting curves. It is found that the entropy of fusion (ΔS_m) is primarily a function of crystal structure while the volume change on fusion (ΔV_m) is controlled by crystal molar volume within each isostructural series. Such systematics have recently facilitated estimation of the initial slopes of the melting curves of periclase and stishovite. New and existing melting data for silicates and their analogues have been analysed and a systematic dependence of (dT_m/dP)₀ on SiO₂ concentration has been demonstrated. Possible implications of this trend for partial melting of upper-mantle garnet lherzolite are illustrated. Finally, the use of the traditional silicate-germanate and oxide-fluoride modelling schemes is reviewed in the light of information from this present study.  相似文献   

High-temperature elasticity of the fluorite-structure compounds CaF₂, SrF₂ and BaF₂     

Leonie E.A. Jones 《Physics of the Earth and Planetary Interiors》1977,15(1):77-89

The elastic moduli of single-crystal CaF₂, SrF₂ and BaF₂ have been determined by the ultrasonic pulse superposition technique as a function of temperature from T = 298 to T = 650°K. These new data are consistent with other data obtained by ultrasonic pulse techniques in the region of room temperature and are superior to previous high-temperature data from resonance experiments. The elastic moduli (c) are represented by quadratic functions in T over the experimental temperature range with the curvature in the same sense for all the moduli. Evaluation of the temperature derivatives of the elastic moduli at constant volume indicates that the dominant temperature effect is extrinsic for (?K_S/?T)_P and intrinsic for (?μ/?T)_P, where K_S and μ are the isotropic bulk and shear moduli, respectively. For the series CaF₂SrF₂BaF₂, |(?c/?T)_p| decreases with increasing molar volume for all moduli; however there are no theoretical or empirical grounds on which to derive a simple relationship between (?c/?T)_P and crystallographic parameters.  相似文献   

Viscosity and structural changes of albite (NaAlSi₃O₈) melt at high pressures     

I. Kushiro 《Earth and Planetary Science Letters》1978,41(1):87-90

Viscosity of anhydrous albite melt, determined by the falling-sphere method in the solid-media, piston-cylinder apparatus, decreases with increasing pressure from 1.13 × 10⁵ P at 1 atm to 1.8 × 10⁴ P at 20 kbar at 1400°C. The rate of decrease in viscosity is larger between 12 and 15 kbar than in other pressure ranges examined. The density of the quenched albite melt increases with increasing pressure of quenching from 2.38 g/cm³ at 1 atm to 2.53 g/cm³ at 20 kbar. The rate of increase in density is largest at pressures between 15 and 20 kbar. The melting curve of albite shows an inflexion at about 16 kbar. These observations strongly suggest that structural changes of albite melt would take place effectively at pressures near 15 kbar. Melt of jadeite (NaAlSi₂O₆) composition shows very similar changes in viscosity and density and a melting curve inflexion at pressures near 10 kbar. Difference in pressure for the suggested effective structural changes of albite and jadeite melts is 5–6 kbar, which is nearly the same as that between the subsolidus reaction curves nepheline + albite= 2jadeite and albite=jadeite + quartz. The structural changes of the melts are, however, continuous and begin to take place at pressures lower than those of the crystalline phases.  相似文献   

Spinel disproportionation at high pressure: calorimetric determination of enthalpy of formation of Mg₂SnO₄ and Co₂SnO₄ and some implications for silicates     

Alexandra Navrotsky  Robert B. Kasper 《Earth and Planetary Science Letters》1976,31(2):247-254

The enthalpies of formation from the oxides of Mg₂SnO₄ and Co₂SnO₄ were found by oxide melt solution calorimetry to be +1.13 ± 0.48 kcal/mol and ?2.31 ± 0.28 kcal/mol, respectively. Using these data, the slopes, ?P/?T, for disproportionation of these spinels to the component oxides at high pressure were calculated to be +30.4 ± 4.2 bar/K for Mg₂SnO₄ and ?10.3 ± 2.4 bar/K for Co₂SnO₄, in general agreement with the data of Jackson et al. (1974a,b). Using thermochemical data for the formation of olivines, for olivine-spinel transitions and for the transformation of quartz to stishovite, we calculate pressures for the disproportionation of silicate spinels to be in the range 150–200 kbar. Calculated slopes ?P/?T for the disproportionation reactions are ?10.7, ?24.9, ?11.2, and +7.6 bar/K for Mg₂SiO₄, Fe₂SiO₄, Co₂SiO₄, and Ni₂SiO₄. The large negative slope calculated for Fe₂SiO₄ results from a surprisingly large positive slope reported for the olivine-spinel transition in that compound (Akimoto et al., 1969). Further consideration of the systematic trends in the thermodynamics of spinel formation from the oxides suggests that the silicate spinels should have entropies of formation close to zero, resulting in values of ?P/?T which are zero or at most only slightly negative. This confirms the conclusion of Jackson, Liebermann, and Ringwood that values of ?P/?T for spinel disproportionation are unlikely to be more negative than ?10 bar/K and may well be slightly positive. Reaction of spinels to form other post-spinel phases, particularly ilmenite and perovskite, are discussed in terms of available thermochemical data.  相似文献   

Synthesis of γ-Mg₂SiO₄     

Eiji Ito  Yoshito Matsui  Kaichi Suito  Naoto Kawai 《Physics of the Earth and Planetary Interiors》1974,8(4):342-344

Magnesium orthosilicate with spinel structure (γ-Mg₂SiO₄) was synthesized at about 250 kbar and 1000°C. Unit cell dimension was established to be 8.076 ± 0.001Å. X-ray powder diffraction pattern revealed a significant difference between γ-Mg₂SiO₄ and other γ-M₂SiO₄ spinels (M = Fe, Co, and Ni) in the intensities of (111) and (331) reflections, both of which are virtually absent in the Mg₂SiO₄ spinel. This feature could be thoroughly understood by the calculation of the intensities for several silicate spinels.  相似文献   

High-pressure phases of Co₂GeO₄, Ni₂GeO₄, Mn₂GeO₄ and MnGeO₃: implications for the germanate-silicate modeling scheme and the Earth's mantle     

Lin-Gun Liu 《Earth and Planetary Science Letters》1976,31(3):393-396

In a diamond-anvil press coupled with YAG laser heating, the spinels of Co₂GeO₄ and Ni₂GeO₄ have been found to disproportionate into their isochemical oxide mixtures at about 250 kbar and 1400–1800°C in the same manner as their silicate analogues. At about the same P-T conditions MnGeO₃ transforms to the orthorhombic perovskite structure (space group Pbnm); the lattice parameters at room temperature and 1 bar are a₀ = 5.084 ± 0.002, b₀ = 5.214 ± 0.002, and c₀ = 7.323 ± 0.003Å with Z = 4 for the perovskite phase. The zero-pressure volume change associated with the ilmenite-perovskite phase transition in MnGeO₃ is ?6.6%. Mn₂GeO₄ disproportionates into a mixture of the perovskite phase of MnGeO₃ plus the rocksalt phase of MnO at P = 250kbar and T = 1400–1800°C. The concept of utilizing germanates as high-pressure models for silicates is valid in general. The results of this study support the previous conclusion that the lower mantle comprises predominantly the orthorhombic perovskite phase of ferromagnesian silicate.  相似文献   

Melting of forsterite Mg₂SiO₄ up to 15 GPa     

Eiji Ohtani  Mineo Kumazawa 《Physics of the Earth and Planetary Interiors》1981,27(1):32-38

The melting curve of forsterite has been studied by static experiment up to a pressure of 15 GPa. Forsterite melts congruently at least up to 12.7 GPa. The congruent melting temperature is expressed by the Kraut-Kennedy equation in the following form: T_m(K)=2163 (1+3.0(V₀ ? V)/V₀), where the volume change with pressure was calculated by the Birch-Managhan equation of state with the isothermal bulk modulus K₀ = 125.4 GPa and its pressure derivative K′ = 5.33. The triple point of forsterite-β-Mg₂SiO₄-liquid will be located at about 2600°C and 20 GPa, assuming that congruent melting persists up to the limit of the stability field of forsterite. The extrapolation of the previous melting data on enstatite and periclase indicates that the eutectic composition of the forsterite-enstatite system should shift toward the forsterite component with increasing pressure, and there is a possibility of incongruent melting of forsterite into periclase and liquid at higher pressure, although no evidence on incongruent melting has been obtained in the present experiment.  相似文献   

Pyroxene-garnet solid-solution equilibria in the systems Mg₄Si₄O₁₂Mg₃Al₂Si₃O₁₂ and Fe₄Si₄O₁₂Fe₃Al₂Si₃O₁₂ at high pressures and temperatures     

M. Akaogi  S. Akimoto 《Physics of the Earth and Planetary Interiors》1977,15(1):90-106

Pyroxene-garnet solid-solution equilibria have been studied in the pressure range 41–200 kbar and over the temperature range 850–1,450°C for the system Mg₄Si₄O₁₂Mg₃Al₂Si₃O₁₂, and in the pressure range 30–105 kbar and over the temperature range 1,000–1,300°C for the system Fe₄Si₄O₁₂Fe₃Al₂Si₃O₁₂. At 1,000°C, the solid solubility of enstatite (MgSiO₃) in pyrope (Mg₃Al₂Si₃O₁₂) increases gradually to 140 kbar and then increases suddenly in the pressure range 140–175 kbar, resulting in the formation of a homogeneous garnet with composition Mg₃(Al_0.8Mg_0.6Si_0.6)Si₃O₁₂. In the MgSiO₃-rich field, the three-phase assemblage of β- or γ-Mg₂SiO₄, stishovite and a garnet solid solution is stable at pressures above 175 kbar at 1,000°C. The system Fe₄Si₄O₁₂Fe₃Al₂Si₃O₁₂ shows a similar trend of high-pressure transformations: the maximum solubility of ferrosilite (FeSiO₃) in almandine (Fe₃Al₂Si₃O₁₂) forming a homogeneous garnet solid solution is 40 mol% at 93 kbar and 1,000°C.If a pyrolite mantle is assumed, from the present results, the following transformation scheme is suggested for the pyroxene-garnet assemblage in the mantle. Pyroxenes begin to react with the already present pyrope-rich garnet at depths around 150 km. Although the pyroxene-garnet transformation is spread over more than 400 km in depth, the most effective transition to a complex garnet solid solution takes place at depths between 450 and 540 km. The complex garnet solid solution is expected to be stable at depths between 540 and 590 km. At greater depths, it will decompose to a mixture of modified spinel or spinel, stishovite and garnet solid solutions with smaller amounts of a pyroxene component in solution.  相似文献   

Compression and phase behavior of solid CO₂ to half a megabar     

Lin-gun Liu 《Earth and Planetary Science Letters》1984,71(1):104-110

CO₂ has been investigated up to 514 kbar at23 ± 2°C by both optical and in situ X-ray diffraction studies using a diamond-anvil pressure cell. CO₂ solidifies in an unknown structure in the pressure range 5 to 23 kbar, and transforms to ordinary dry-ice structure above 23 kbar at room temperature. Isothermal compression data for dry ice have been obtained above about 24 kbar. These appear to be the first data at room temperature known in the literature. The data fitted to the Birch equation of state yieldK₀ = 29.3 ± 1.0kbar andK₀^′ = 7.8 assuming the volume of the hypothetical dry ice at zero-pressure and room temperature is 31.4 ± 0.2 cm³/mole. The isothermal bulk modulus(K₀) thus derived is consistent with the compression data and compressibilities for dry ice obtained at low temperatures using dilatometry and ultrasonic techniques, respectively, reported in the literature. By comparing shock-wave data for relevant materials, it is suggested that CO₂ is not likely to transform to one of the crystalline forms of SiO₂ which is otherwise expected from empirical grounds, but may instead decompose into C (diamond) + O₂, at high pressures.  相似文献   

High-pressure disproportionation of Co₂SiO₄ spinel and implications for Mg₂SiO₄ spinel     

Lin-Gun Liu 《Earth and Planetary Science Letters》1975,25(3):286-290

Co₂SiO₄ spinel has been found to disproportionate into its isochemically mixed oxides with rocksalt and rutile structures at pressures between 170 and 190 kbar and temperatures between 1400 and 1800°C in a diamond-anvil press. The exact disproportionation pressure is not certain due to transient increases in pressure during the local and rapid heating by a continuous YAG laser. The slope of the phase boundary between the spinel phase and the mixed oxides is calculated to be?33 ± 20bar/deg. This negative slope is consistent with the observed anomalously large entropy of CoO (relative to its isostructural oxides) in entropy vs.(MV)^?1/2 systematics, whereM is the formula weight andV the molar volume. The sign of the slope for a phase boundary in the disproportionation of spinel depends on the values of entropy of the rocksalt oxides as well as the inverse character exhibited in the spinel phases. The normal entropy of MgO suggests that the phase boundary for the disproportionation of Mg₂SiO₄ spinel has positive slope.  相似文献   

High-pressure phase transformations in the system CaSiO₃-Al₂O₃     

Lin-Gun Liu 《Earth and Planetary Science Letters》1979,43(2):331-335

Phase assemblages for five selected compositions in the system CaSiO₃-Al₂O₃ have been investigated in the pressure range 100–300 kbar and at about 1000°C in a diamond-anvil press coupled with laser heating. At pressures below about 250 kbar, the assemblage of grossularite plus corundum is stable for compositions containing more than 25 mole% Al₂O₃. Above about 250 kbar, phase assemblages for the latter compositions are truncated by those in the join CaAl₂O₄-SiO₂. Garnet solid solutions are stable between about 10 and 25 mole% Al₂O₃. Grossularite transforms to a new tetragonal form at pressures greater than about 250 kbar, but the stability field for the garnet solid solutions extends to pressures up to about 300 kbar. The perovskite modification appears to be stable at pressures above about 150 kbar, but is probably limited to nearly pure CaSiO₃ composition. Phase behaviour for calcium-bearing silicates or aluminosilicates in the lower mantle are apparently more complicated than was suggested earlier.  相似文献   

Phase relations of titan-phlogopite,K₂Mg₄TiAl₂Si₆O₂₀(OH)₄: a refractory phase in the upper mantle?     

W.C. Forbes  M.F.J. Flower 《Earth and Planetary Science Letters》1974,22(1):60-66

Experimental data on the stability of titan-phlogopite [K₂Mg₄TiAl₂Si₆O₂₀(OH)₄] are presented which show it to be stable to substantially higher temperatures than normal phlogopite [K₂Mg₆Al₂Si₆O₂₀(OH)₄]. A qualitative model to explain the role of titan-phlogopite during magma generation is put forward. Breakdown of titan-phlogopite during melting at depth (> 150km) on subducted lithospheric slabs is believed responsible for the concomitant increase of K and Ti observed in magmas erupted during orogenic volcanism. At lower pressures (up to about 10 kbar) beneath mid-oceanic ridges, titan-phlogopite is predicted to behave as a refractory phase during partial melting in the mantle, especially if H₂O-excess conditions pertain, although at higher pressures in this environment it would almost certainly behave as a low-melting component.  相似文献   

High-pressure decompositions in cobalt and nickel silicates     

Eiji Ito 《Physics of the Earth and Planetary Interiors》1975,10(1):88-93

High-pressure stability relations in cobalt and nickel silicates have been studied over the pressure range 130–330 kbar employing a double-staged split-sphere-type high-pressure apparatus.γ-Co₂SiO₄ and γ-Ni₂SiO₄ decompose directly into their constituent oxide mixtures (rocksalt plus stishovite) 175 kbar and 280 kbar, respectively. The result that γ-Ni₂SiO₄ has a wider stability field in pressure than γ-Co₂SiO₄, is consistent with simple crystal-field theory.The experimental precision is high enough to show that the decomposition boundary of γ-Co₂SiO₄ has a positive slope (dP/dT > 0) and a preliminary determination of the boundary curve is P(kbar) = 0.065 T (°C) + 110.No positive evidence for the existence of high-pressure forms of CoSiO₃ and NiSiO₃ has been obtained in these quenching experiments, and they finally decompose into constituent oxide mixtures as in the cases of orthosilicates.  相似文献   

Elastic properties of polycrystalline diopside (CaMgSi2O6)     

Robert C. Liebermann  D.J. Mayson 《Physics of the Earth and Planetary Interiors》1976,11(3):P1-P4

A polycrystalline specimen of clinopyroxene diopside has been hot-pressed at P = 15 kbar and T = 850°C in a piston-cylinder apparatus. Compressional (ν_P) and shear (ν_S) velocities are determined as a function of pressure to 7.5 kbar at room temperature by an ultrasonic pulse transmission technique. The velocities at 7.5 kbar are ν_P = 8.06 km/sec and ν_S = 4.77 km/sec. These data are consistent with velocity-density trends for orthopyroxenes due to the compensating effects of the monoclinic structure (positive) and Ca content (negative). With the addition of the new data for diopside, it is possible to calculate directly the velocities of various upper-mantle mineral assemblages.  相似文献   

Melting relationships in CaO-CO₂ and MgO-CO₂ to 36 kilobars with comments on CO₂ in the mantle     

A.J. Irving  P.J. Wyllie 《Earth and Planetary Science Letters》1973,20(2):220-225

The melting curves of CaCO₃ and MgCO₃ have been extended to pressures of 36 kb by experiments in piston-cylinder apparatus. At 30 kb, the melting temperatures of calcite and magnesite are 1610°C and 1585°C, respectively. New data for the magnesite dissociation reaction permit the location of an invariant point for the assemblage magnesite + periclase + liquid + vapor near 26 kb-1550°C. New data are also presented for the calcite-aragonite transition at 800°C, 950°C and 1100°C. At pressures above 36–50 kb, calcite and magnesite melt at temperatures lower than the solidus of dry mantle peridotite. Natural and experimental evidence suggests that carbon dioxide in the Earth's mantle could be present in a variety of forms: (a) a free vapor phase, (b) vapor dissolved in silicate magma, (c) crystalline carbonate, (d) carbonatite liquid, (e) carbon-bearing silicate analogs, or (f) carbonato-silicates (such as scapolite, spurrite, tilleyite, and related compounds).  相似文献   

Measurement of the adiabats of quartz,forsterite, and magnesium oxide at high pressures and high temperatures and adiabatic gradient in the mantle     

Reinhard Boehler 《Physics of the Earth and Planetary Interiors》1982,29(1):105-107

The geophysically-important adiabat (?T/?P)_s has been measured at pressures up to 50 kbar and temperatures up to 1000°C. A simple power law describes the relationship between (?T/?P)_s and the compression of the material. The power is independent of the material and of the temperature within the uncertainty. This consistency in the power allows the extrapolation of the adiabat to pressure and temperature conditions of the mantle of the earth. The adiabatic gradient is shown to be significantly smaller than the melting gradient.  相似文献   

High-pressure X-ray diffraction studies on β- and γ-Mg₂SiO₄     

Seishiro Mizukami  Akihito Ohtani  Naoto Kawai  Eiji Ito 《Physics of the Earth and Planetary Interiors》1975,10(2):177-182

Pressure effects on the lattice parameters of β- and γ-Mg₂SiO₄ have been measured at room temperature and at pressures up to 100 kbar using a multi-anvil high-pressure X-ray diffraction apparatus. The volume changes (ΔV/V₀) at 90 kbar are 5.4 · 10^?2 and 4.2 · 10^?2 for β- and γ-Mg₂SiO₄, respectively. Isothermal bulk moduli at zero pressure have been calculated from least-square fits of the data to straight lines. They turn out to be 1.66 ± 0.4 and 2.13 ± 0.1 Mbar for β- and γ-Mg₂SiO₄, respectively. The α → γ transition obeys Wang's linear V_φ?ρ relation but the α → β transition does not.  相似文献   

Synthesis of a perovskite-type polymorph of CaSiO₃     

Lin-Gun Liu  A.E. Ringwood 《Earth and Planetary Science Letters》1975,28(2):209-211

Synthetic crystalline (wollastonite) and glass forms of CaSiO₃ have been compressed to loading pressures above 160 kbar and heated to about 1500° C by a laser in a diamond-anvil cell. After cooling, an X-ray diffraction study carried out whilst the sample was maintained at high pressure revealed that it had transformed to a cubic perovskite-type 3olymorph with a = 3.485 ± 0.008A?. After release of pressure, however, the sample showed a mixture of glass plus a few weak lines corresponding to ε-CaSiO₃ which is thus interpreted as a retrogressive transition product. The density of the perovskite polymorph of CaSiO₃ is about 9.2% greater than that of an isochemical mixture of CaO + SiO₂ (stishovite) at about 160 kbar.  相似文献