High-pressure transformations in silicates,germanates, and titanates with ABO3 stoichiometry

High-pressure phase transformations were investigated for two silicates, MgSiO₃ and ZnSiO₃; six germanates, MGeO₃ and six titanates, MTiO₃ (M=Ni, Mg, Co, Zn, Fe, and Mn) at about 1,000°C and pressures up to ca. 30 GPa. CoGeO₃ was found to assume the ilmenite form. The ilmenite phases were confirmed to transform in the following schemes: to perovskite in MgSiO₃ and MnGeO₃, to corundum in MgGeO₃ and ZnGeO₃, to rocksalt plus rutile in ZnSiO₃ and CoGeO₃ and to rocksalt plus TiO₂ (possibly of some denser structure) in NiTiO₃, MgTiO₃, CoTiO₃, ZnTiO₃ and FeTiO₃. In the case of FeTiO₃, the corundum form appeared as an intermediate phase. The possibility that the corundum type MnTiO₃ might transform to some denser modification could not be excluded. The compound NiGeO₃ was nonexistent throughout the pressure range studied. High-pressure phases of ABO₃ (A=Ni, Mg, Co, Zn, Fe, and Mn; B=Si, Ge and Ti) are summarized, and those stabilized at pressures higher than 20 GPa are discussed.     

High-pressure phase transformations in the MgFe2O4 and Fe2O3–MgSiO3 systems

 The crystal structure of MgFe₂O₄ was investigated by in situ X-ray diffraction at high pressure, using YAG laser annealing in a diamond anvil cell. Magnesioferrite undergoes a phase transformation at about 25 GPa, which leads to a CaMn₂O₄-type polymorph about 8% denser, as determined using Rietveld analysis. The consequences of the occurrence of this dense MgFe₂O₄ form on the high-pressure phase transformations in the (MgSi)_0.75(Fe^III)_0.5O₃ system were investigated. After laser annealing at about 20 GPa, we observe decomposition to two phases: stishovite and a spinel-derived structure with orthorhombic symmetry and probably intermediate composition between MgFe₂O₄ and Mg₂SiO₄. At pressures above 35 GPa, we observe recombination of these products to a single phase with Pbnm perovskite structure. We thus conclude for the formation of Mg₃Fe₂Si₃O₁₂ perovskite. Received: 27 March 2000 / Accepted: 1 October 2000     

Calorimetric study of perovskite solid solutions in the CaSiO3–CaGeO3 system

 Enthalpies of drop solution (ΔH _drop-sol) of CaGeO₃, Ca(Si_0.1Ge_0.9)O₃, Ca(Si_0.2Ge_0.8)O₃, Ca(Si_0.3Ge_0.7)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     

High-pressure transformations of ilmenite to perovskite, and lithium niobate to perovskite in zinc germanate

In-situ X-ray powder diffraction measurements conducted under high pressure confirmed the existence of an unquenchable orthorhombic perovskite in ZnGeO₃. ZnGeO₃ ilmenite transformed into perovskite at 30.0 GPa and 1300±150 K in a laser-heated diamond anvil cell. After releasing the pressure, the lithium niobate phase was recovered as a quenched product. The perovskite was also obtained by recompression of the lithium niobate phase at room temperature under a lower pressure than the equilibrium phase boundary of the ilmenite–perovskite transition. Bulk moduli of ilmenite, lithium niobate, and perovskite phases were calculated on the basis of the refined X-ray diffraction data. The structural relations among these phases are considered in terms of the rotation of GeO₆ octahedra. A slight rotation of the octahedra plays an important role for the transition from lithium niobate to perovskite at ambient temperature. On the other hand, high temperature is needed to rearrange GeO₆ octahedra in the ilmenite–perovskite transition. The correlation of quenchability with rotation angle of GeO₆ octahedra for other germanate perovskites is also discussed.     

Ab initio study of the high-pressure behavior of CaSiO3 perovskite

Using density functional simulations, within the generalized gradient approximation and projector-augmented wave method, we study structures and energetics of CaSiO₃ perovskite in the pressure range of the Earths lower mantle (0–150 GPa). At zero Kelvin temperature the cubic CaSiO₃ perovskite structure is unstable in the whole pressure range, at low pressures the orthorhombic (Pnam) structure is preferred. At 14.2 GPa there is a phase transition to the tetragonal (I4/mcm) phase. The CaIrO₃-type structure is not stable for CaSiO₃. Our results also rule out the possibility of decomposition into oxides.

Equations of state of CaSiO3 Perovskite: a molecular dynamics study

The molar volumes and bulk moduli of CaSiO₃ perovskite are calculated in the temperature range from 300 to 2,800 K and the pressure range from 0 to 143 GPa using molecular dynamics simulations that employ the breathing shell model for oxygen and the quantum correction in addition to the conventional pairwise interatomic potential models. The performance of five equations of state, i.e., the Keane, the generalized-Rydberg, the Holzapfel, the Stacey–Rydberg, and the third-order Birch–Murnaghan equations of state are examined using these data. The third-order Birch–Murnaghan equation of state is found to have a clear tendency to overestimate the bulk modulus at very high pressures. The Stacey–Rydberg equation of state degrades slightly at very high pressures along the low-temperature isotherms. In comparison, the Keane and the Holzapfel equations of state remain accurate in the whole temperature and pressure range considered in the present study. K ₀′ derived from the Holzapfel equation of state also agrees best with that calculated independently from molecular dynamics simulations. The adiabatic bulk moduli of CaSiO₃ perovskite along lower mantle geotherms are further calculated using the Keane and the Mie-Grüneisen–Debye equations of state. They are found to be constantly higher than those of the PREM by ~5%, and also very similar to those of the MgSiO₃ perovskite. Our results support the view that CaSiO₃ perovskite remains invisible in the Earth’s lower mantle.     

High-pressure perovskites on the join CaTiO3-FeTiO3

An exploratory high-pressure study of the join CaTiO₃-FeTiO₃ has uncovered two intermediate perovskites with the compositions CaFe₃Ti₄O₁₂ and CaFeTi₂O₆. These perovskites have ordering of Ca²⁺ and Fe²⁺ on the A sites. Both of these perovskites are unusual in that the A sites containing Fe²⁺ are either square planar or tetrahedral, due to the particular tilt geometries of the octahedral frameworks. For CaFe₃Ti₄O₁₂, the structure has been refined from a powder using the Rietveld technique. This compound is a cubic double perovskite (SG Im $\bar 3$ , a = 7.4672 Å), isostructural with NaMn₇O₁₂. Fe²⁺ is in a square-planar A site (similar to Mn³⁺ in NaMn₇O₁₂) with Fe-O = 2.042(3) Å, with distant second neighbors in a rectangle at Fe-O = 2.780(6) Å. Calcium is in a distorted icosahedron with Ca-O =2.635(5) Å. CaFeTi₂O₆ crystallizes in a unique tetragonal double perovskite structure (SG P4₂/nmc, a = 7.5157(2), c = 7.5548(2)), with A-site iron in square-planar (Fe-O = 2.097(2) Å) and tetrahedral (Fe-O = 2.084(2) Å) coordination, again with distant second neighbor oxygens near 2.8 Å. Rietveld refinement was also performed for the previously known perovskite-related form of FeTiO₃ recovered from high pressure (lithium niobate type). This compound is trigonal R3c, with a = 5.1233(1) and c = 13.7602(2). The ordered perovskites appear to be stable at 1215 GPa and CaFe₃Ti₄O₁₂ is found as low as 5 GPa. Thus these perovskites may be important to upper mantle mineralogy, particularly in kimberlites. These compounds are the first known quenchable perovskites with large amounts of A-site ferrous iron, and add greatly to the known occurrences of ferrous iron in perovskites.     

Stability relations in the system CaSiO3-CaMnSi2O6-CaFeSi2O6

In the system CaSiO₃-CaMnSi₂O₆-CaFeSi₂O₆ extensive miscibility gaps between pyroxenoids and clinopyroxenes are observed. The miscibility gap between Mn-bustamite and Mn-wollastonite has been determined experimentally by a hydrothermal technique between 400° and 1200° C at P _f= 2 kbar. Further experiments have been performed at P _f=9 kbar, which revealed a shifting of the miscibility gap towards more Ca-rich compositions. The bustamite phase is stabilized by high pressures and the wollastonite structure is the stable phase at high temperatures.Similar phase relations as along the join CaSiO₃-CaMnSi₂O₆ exist along the join CaSiO₃-CaFeSi₂O₆ but with a more extensive two-phase field of bustamite-clinopyroxene.Possible phase relations along the joins CaSiO₃-CaMnSi₂O₆, CaSiO₃-CaFeSi₂O₆ and CaFeSi₂O₆-CaMnSi₂O₆ are given in temperature-composition diagrams for low pressures, based on natural and experimental data.     

High-pressure crystal chemistry of MgSiO3 perovskite

A high-pressure single-crystal x-ray diffraction study of perovskite-type MgSiO₃ has been completed to 12.6 GPa. The compressibility of MgSiO₃ perovskite is anisotropic with b approximately 23% less compressible than a or c which have similar compressibilities. The observed unit cell compression gives a bulk modulus of 254 GPa using a Birch-Murnaghan equation of state with K set equal to 4 and V/V ₀ at room pressure equal to one. Between room pressure and 5 GPa, the primary response of the structure to pressure is compression of the Mg-O and Si-O bonds. Above 5 GPa, the SiO₆ octahedra tilt, particularly in the [bc]-plane. The distortion of the MgO₁₂ site increases under compression. The variation of the O(2)-O(2)-O(2) angles and bondlength distortion of the MgO₁₂ site with pressure in MgSiO₃ perovskite follow trends observed in GdFeO₃type perovskites with increasing distortion. Such trends might be useful for predicting distortions in GdFeO₃-type perovskites as a function of pressure.     

High-pressure phase transition in MnTiO3 from the ilmenite to the LiNbO3 structure

Crystals of a high-pressure phase of MnTiO₃ have been synthesized at pressures of 60 kbar using the SAM-85 cubic-anvil high pressure apparatus. Although all crystals examined were twinned on (10 \(\bar 1\) \(\bar 2\) ), a set of diffraction intensities that are essentially unaffected by the twinning were obtained. Three possible structure models were considered: (1) the corundum (completely disordered Mn and Ti), (2) the partially-disordered ilmenite, and (3) the LiNbO₃ structures. The R factors of the corundum and the disordered ilmenite models were much larger than that of LiNbO₃. Using structure factors unaffected by twinning, the final LiNbO₃-type refinement gave R _w=0.037 and R=0.034. The averaged bond lengths for Mn-O and Ti-O were consistent with ones calculated using Shannon and Prewitt's (1969) radii. The study concludes that MnTiO₃ II actually has an ordered LiNbO₃-type structure rather than the disordered one as reported previously. From the analysis of the two MnTiO₃ structures, the transition can be related to a cation reordering process, in which half of the cations participate, accompanied by the rotation of oxygens to accommodate the cations.     

Oxygen deficient perovskites in the system CaSiO3–CaAlO2.5 and implications for the Earth’s interior

Oxygen deficient perovskites of the system CaSiO₃–CaAlO_2.5 have been synthesised at high-pressure and -temperature conditions relevant to the Earth’s transition zone in order to investigate their stabilities in the Earth’s mantle and determine structural properties associated with vacancy incorporation. Two polysomes of thermodynamically stable defect perovskites with Ca(Al_0.4Si_0.6)O_2.8 and Ca(Al_0.5Si_0.5)O_2.75 stoichiometry have been identified. The ordering of oxygen defects into pseudo-cubic (111) layers results in well-ordered ten- or eightfold superstructures, respectively. At all other compositions examined, a metastable formation of perovskites has been observed instead, which are assumed to grow initially disordered. These are now characterised by tiny domains, formed due to subsequent ordering of vacancies along various pseudo-cubic {111} layers. Both ordered defect perovskites show a large P–T stability field ranging from about 9–18 GPa and 4–12 GPa, respectively. Microstructural TEM analyses revealed the presence of growth and ferroelastic twins, which indicate a phase transition from rhombohedral to monoclinic symmetry during quenching. Electron energy loss spectroscopy of Si and Al K edges point at the presence of tetrahedral, octahedral and maybe some pentacoordinated silicon, whereas aluminium is predominantly octahedrally coordinated with minor fractions in lower coordination. Observed properties are interpreted in terms of a new structural model, explaining the observed phase transition and formation of different twin laws as well as giving reasons for the development of such large superstructures. With respect to phase relations of the transition zone, the potential occurrence of such defect perovskites in the Earth’s interior is discussed.     

High-temperature compression experiments of CaSiO3 perovskite to lowermost mantle conditions and its thermal equation of state

In order to examine pressure–volume–temperature (P–V–T) relations for CaSiO₃ perovskite (Ca-perovskite), high-temperature compression experiments with in situ X-ray diffraction were performed in a laser-heated diamond anvil cell (DAC) to 127 GPa and 2,300 K. We also employed an external heating system in the DAC in order to obtain P–V data at a moderate temperature of 700 K up to 113 GPa, which is the reference temperature for constructing an equation of state. The P–V data at 700 K were fitted to the second-order Birch–Murnaghan equation of state, yielding K _700,1bar = 207 ± 4 GPa and V _700,1bar = 46.5 ± 0.1 Å³. Thermal pressure terms were evaluated in the framework of the Mie–Grüneisen–Debye model, yielding γ _700,1bar = 2.7 ± 0.3, q _700,1bar = 1.2 ± 0.8, and θ _700,1bar = 1,300 ± 500 K. A thermodynamic thermal pressure model was also employed, yielding α_700,1bar = 5.7 ± 0.5 × 10^?5/K and (?K/?T) _V  = ?0.010 ± 0.004 GPa/K. Computed densities along a lower mantle geotherm demonstrate that Ca-perovskite is denser than the surrounding lower mantle, suggesting that Ca-perovskite-rich rocks do not rise up through the lower mantle. One of such rocks might be a residue of partial melting of subducted mid-oceanic ridge basalt (MORB) at the base of the mantle. Since the partial melt is FeO-rich and therefore denser than the mantle, all the components of subducted MORB may not return to shallow levels.     

Premelting and calcium mobility in gehlenite (Ca2Al2SiO7) and pseudowollastonite (CaSiO3)

 Premelting effects in gehlenite (Ca₂Al₂SiO₇) have been studied by Raman spectroscopy and calorimetry, and in gehlenite and pseudowollastonite (CaSiO₃) by electrical conductivity. The enthalpy of premelting of gehlenite is 17.3 kJ mol⁻¹ and represents 9% of the reported enthalpy of fusion, which is in the range of the reported fraction of other minerals. The Raman and electrical conductivity experiments at high temperatures, for gehlenite and pseudowollastonite, show that the premelting effects of both compositions are associated with enhanced dynamics of calcium atoms near the melting point. This conclusion agrees with the results obtained for other minerals like diopside, but contrasts with those found for sodium metasilicate in which the weaker bonding of sodium allows the silicate framework to distort near the melting temperature and deform in such a way to prefigure the silicate entities present in the melt. Received: 30 April 2002 / Accepted: 7 August 2002 Acknowledgements We thank Y. Linard for help with DSC measurements and two anonymous reviewers for their constructive comments. This work has been partly supported by the EU Marie-Curie fellowship contract no. HPMF-CT-1999-00329, the CNRS-Carnegie Institution of Washington program PICS no.192, and the NSF grants EAR-9614432 and EAR-9901886 to B.O.M.     

High-pressure behavior of MnGeO3 and CdGeO3 perovskites and the post-perovskite phase transition

The stability and high-pressure behavior of perovskite structure in MnGeO₃ and CdGeO₃ were examined on the basis of in situ synchrotron X-ray diffraction measurements at high pressure and temperature in a laser-heated diamond-anvil cell. Results demonstrate that the structural distortion of orthorhombic MnGeO₃ perovskite is enhanced with increasing pressure and it undergoes phase transition to a CaIrO₃-type post-perovskite structure above 60 GPa at 1,800 K. A molar volume of the post-perovskite phase is smaller by 1.6% than that of perovskite at equivalent pressure. In contrast, the structure of CdGeO₃ perovskite becomes less distorted from the ideal cubic perovskite structure with increasing pressure, and it is stable even at 110 GPa and 2,000 K. These results suggest that the phase transition to post-perovskite is induced by a large distortion of perovskite structure with increasing pressure.     

High-pressure behaviour and equation of state of calcite, CaCO3

The high-pressure response of the cell parameters of calcite, CaCO₃, has been investigated by single crystal X-ray diffraction. The unit cell parameters have been refined from 0 to 1.435?GPa, and the linear and volume compressibilities have been measured as β _a =2.62(2)?×?10^?3?GPa^?1,β _c =7.94(7)?×?10^?3?GPa^?1, β _v =13.12?×?10^?3?GPa^?1. The bulk modulus has been obtained from a fit to the Birch-Murnaghan equation of state, giving K ₀=73.46?±?0.27?GPa and V ₀=367.789 ±?0.004?ų with K′=4. Combined with earlier data for magnesite, ankerite and dolomite, these data suggest that K ₀ V ₀ is a constant for the Ca-Mg rhombohedral carbonates.     

HPA-S高压湿法消解仪消解油类样品——标准方法ASTM C 1234-98(2004)

分析油类样品中的痕量元素有多种方法,例如采用有机溶剂稀释后用光谱法直接测定,在稳定乳液系统中测定,以及氧弹燃烧等常规消解方法;