Spin state of ferric iron in MgSiO3 perovskite and its effect on elastic properties

Recent studies have indicated that a significant amount of iron in MgSiO₃ perovskite (Pv) is Fe³⁺ (Fe³⁺/ΣFe = 10–60%) due to crystal chemistry effects at high pressure (P) and that Fe³⁺ is more likely than Fe²⁺ to undergo a high-spin (HS) to low-spin (LS) transition in Pv in the mantle. We have measured synchrotron Mössbauer spectroscopy (SMS), X-ray emission spectroscopy (XES), and X-ray diffraction (XRD) of Pv with all iron in Fe³⁺ in the laser-heated diamond-anvil cell to over 100 GPa. Fe³⁺ increases the anisotropy of the Pv unit cell, whereas Fe²⁺ decreases it. In Pv synthesized above 50 GPa, Fe³⁺ enters into both the dodecahedral (A) and octahedral (B) sites approximately equally, suggesting charge coupled substitution. Combining SMS and XES, we found that the LS population in the B site gradually increases with pressure up to 50–60 GPa where all Fe³⁺ in the B site becomes LS, while Fe³⁺ in the A site remains HS to at least 136 GPa. Fe³⁺ makes Pv more compressible than Mg-endmember below 50 GPa because of the gradual spin transition in the B site together with lattice compression. The completion of the spin transition at 50–60 GPa increases bulk modulus with no associated change in density. This elasticity change can be a useful seismic probe for investigating compositional heterogeneities associated with Fe³⁺.     

Electronic spin transitions in iron-bearing MgSiO3 perovskite

The electronic spin state of iron in perovskites with the chemical composition Mg_0.9375Fe_0.0625SiO₃, Mg_0.8750Fe_0.1250SiO₃ and Mg_0.9375Fe_0.0625Si_0.9375Fe_0.0625O₃, have been determined at 0 K and various pressures between 40–160 GPa, using ab initio methods. The results indicate that ferric iron exhibits a wide range of spin transition pressures between about 60–160 GPa, while ferrous iron has a much narrower span, between about 130–145 GPa. In general, the lowest spin transition pressures are associated with the most energetically favorable substitution configurations, where iron atoms are closest together. The effect of spin state on calculated elastic properties is found to be small.     

Structure constraints and instability leading to the post-perovskite phase transition of MgSiO3

Recent experience with Rietveld refinement of structural analogues and literature surveys, suggests anion–anion repulsion limits the stability of the perovskite phase, including in the MgSiO₃ perovskite to post-perovskite transition. Assuming rigid octahedral coordination, still to be tested experimentally, the critical point where intra- and inter-octahedral anion–anion distances are equal provides a useful metric for predicting the pressure of the perovskite/post-perovskite transition and the Clapeyron slope of the phase boundary, once pressure and temperature derivatives of relevant structure parameters are known. The inter-octahedral anion–anion distances and the polyhedral volume ratio are rigorously formulated as a function of octahedral rotation in this work, assuming the orthorhombic (Pbnm) perovskite structure, where regular octahedra share each corner and conform to the in- and anti-phase rotation schemes designated by space group symmetry. These mathematical expressions are consistent with structure data from 70 perovskite-structured materials surveyed in the literature at ambient as well as extreme conditions and define structure constraints, such as the minimum polyhedral volume ratio, which must be reached before the phase transition to the post-perovskite structure-type can proceed. The formalism we present is general for perovskite (Pbnm) and dependent on the accuracy with which structures can be determined from, sometimes compromised, high pressure diffraction data.     

Solubility of FeO in (Mg,Fe)SiO3 perovskite and the post-perovskite phase transition

Phase relations in Mg_0.5Fe_0.5SiO₃ and Mg_0.25Fe_0.75SiO₃ were investigated in a pressure range from 72 to 123 GPa on the basis of synchrotron X-ray diffraction measurements in situ at high-pressure and -temperature in a laser-heated diamond-anvil cell (LHDAC). Results demonstrate that Mg_0.5Fe_0.5SiO₃ perovskite is formed as a single phase at 85–108 GPa and 1800–2330 K, indicating a high solubility of FeO in (Mg,Fe)SiO₃ perovskite at high pressures. Post-perovskite appears coexisting with perovskite in Mg_0.5Fe_0.5SiO₃ above 106 GPa at 1410 K, the condition very close to the post-perovskite phase transition boundary in pure MgSiO₃. The coexistence of perovskite and post-perovskite was observed to 123 GPa. In addition, post-perovskite was formed coexisting with perovskite also in Mg_0.25Fe_0.75SiO₃ bulk composition at 106–123 GPa. In contrast to earlier experimental and theoretical studies, these results show that incorporation of FeO stabilizes perovskite at higher pressures. This could be due to a larger ionic radius of Fe²⁺ ion, which is incompatible with a small Mg²⁺ site in the post-perovskite phase.     

The equation of state of CaSiO3 perovskite to 108 GPa at 300 K

Pressure–volume measurements have been performed for CaSiO₃ perovskite to 108 GPa at 300 K using NaCl and argon pressure-transmitting media, and energy dispersive X-ray diffraction (EDXD) in a diamond-anvil cell (DAC). By determining a parameter that is the product of the elastic anisotropy, S, and the uniaxial stress component, t, for each data point, we define the stress condition of the sample. For different points at the same pressure in a temperature-quenched sample, the St value can differ by as much as a factor of 5, indicating heterogeneity in the stress condition. This may be responsible for the large scatter of earlier P–V measurements in the DAC which in general used a large diameter X-ray beam. Also, the St value provides insight into the elastic anisotropy, S, of CaSiO₃ perovskite and platinum. The sign of S (positive) for CaSiO₃ perovskite agrees with first principles calculations but the magnitude may be inconsistent. A new compression curve at 300 K was obtained for CaSiO₃ perovskite by using those data points which represent the most nearly hydrostatic conditions. It is observed that the data points with high St values yield larger volumes than the points with small St values at a given pressure. By selecting the data points having low St values (St≤0.005), combining with lower pressure large volume press (LVP) measurements and fitting to third order Birch–Murnaghan equation of state (EOS), we find that CaSiO₃ perovskite is more compressible (V₀=45.58±0.05 ų, K_T0=236±4 GPa, and K_T0′=3.9±0.2 GPa) than suggested by previous studies. The density and bulk modulus of CaSiO₃ perovskite at lower mantle pressures and 300 K are 1–3% greater and 5–15% smaller, respectively, than found in previous studies. This study demonstrates that defining the stress state of the sample is crucial to obtain an accurate 300 K compression curve for unquenchable high-pressure phases.     

Simultaneous measurements of X-rays and electrons during a pulsating aurora

The PULSAUR II rocket was launched from Andøya Rocket Range at 23.43 UT on 9 February 1994 into a pulsating aurora. In this paper we focus on the observations of precipitating electrons and auroral X-rays. By using models it is possible to deduce the electron energy spectrum from X-ray measurements. Comparisons are made between the deduced electron fluxes and the directly measured electron fluxes on the rocket. We found the shape of the observed and the deduced electron spectra to fit very well, with almost identical e-folding energies in the energy range from 10 keV to 60–80 keV. For the integrated fluxes from 10.8 to 250 keV, we found a discrepancy of 30%. By combining two models, we have found a good method of deducing the electron precipitation from X-ray measurements. The discrepancies between calculations and measurements are in the range of the uncertainties in the measurements.     

Accuracy in measurements and the temperature and volume dependence of thermoelastic parameters

To obtain the temperatureT and volumeV (or pressureP) dependence of the Anderson-Grüneisen parameter _T , measurements with high sensitivity are required. We show two examples:P, V, T measurements of NaCl done with the piston cylinder and elasticity measurements of MgO using a resonance method. In both cases, the sensitivity of the measurements leads to results that provide information about _T (,T), where V/V ₀ andV ₀ is the volume at zero pressure. We demonstrate that determination of _T leads to understanding of the volume and temperature dependence ofq=( ln / lnV) _T over a broadV, T range, where is the Grüneisen ratio.     

饱和多孔微极介质的波动方程及其势函数方程

土是由一定尺寸大小颗粒所构成的多孔介质,具有明显的颗粒特性,当土颗粒间的孔隙被流体（如水或油）充满时则成为饱和土.利用微极理论和Biot波动理论的研究成果,把饱和土中多孔固体骨架部分近似地视为微极介质,孔隙中的流体部分视为质点介质,获得饱和多孔微极介质的弹性波动方程.借鉴Greetsma理论,建立了饱和多孔微极介质弹性本构方程力学参数与相应单相介质弹性参数的相互关系,使饱和多孔微极介质弹性波动方程中的物理参数具有明确的物理意义,易于在试验中确定.运用场论理论把饱和多孔微极介质的波动方程简化为势函数方程,建立了饱和多孔微极介质中五种弹性波的弥散方程,数值分析了五种简谐体波在无限饱和多孔微极介质中的传播特性. 结果表明,P1波、P2波和剪切S1波的波速弥散曲线与经典饱和多孔介质基本相同,当频率小于临界频率ω0时旋转纵波θ波和横波S2波不存在,当频率大于临界频率ω0时,θ波和S2波的传播速度随频率增加而减小.     

Incoming solar radiation measurements in Iraq and their presentation

Summary Measurements of the total incoming solar radiation on a horizontal surface have been recorded since 1967 at a network of actinometric stations in Iraq. The instrument used at each of these stations is a bimetallic actinograph of the Robitzsch type. The stations at Mosul, Baghdad and Nasiriya have been selected, to represent respectively the northern, central and southern climatic zones of Iraq.This paper examines from the available records at these stations, until August 1973, the differnt aspects of radiation climatology which are important in solar energy utilizations. The mean annual, monthly and daily radiation; the number of days in which the radiation has exceeded certain given limits and the periods of successive days having radiation less than 100 and 250 cal cm^–2 day^–1 have been studied and presented in tabular and graphical forms suitable for direct application.To architects and engineers interested in the availability of solar radiation on vertical and inclined surfaces, the hourly radiation values for Baghdad have been studied for the different months of the year and the corresponding isopleths on the horizontal have been drawn.     

High-pressure phase transformations and isothermal compression in CaTiO3 (perovskite)

A polycrystalline CaTiO₃ (perovskite) was investigated under static pressures up to 38 GPa and temperatures up to 1000°C by using a diamond anvil pressure cell, a YAG laser, and the ruby fluorescence pressure calibration system. In situ x-ray diffraction data reveal that at room temperature, the orthorhombic CaTiO₃(I) transforms into a hexagonal CaTiO₃(II) at ∼ 10 GPa with a volume of change of 1.6%. At 1000°C, the orthorhombic CaTiO₃(I) first transforms into a tetragonal CaTiO₃(III) at 8.5 GPa and then transforms further into a hexagonal CaTiO₃(II′) at ∼ 15 GPa with molar volume changes of 0% and 1.6%, respectively. All three high-pressure polymorphs found in this study are nonquenchable.Isothermal compressibility of the orthorhombic CaTiO₃ was derived from measurements under truly hydrostatic environments (i.e., ⩽ 10.4 GPa). By assuming K^′₀ = 5.6 obtained ultrasonically on SrTiO₃ perovskite, the value of the bulk modulus (K₀) was calculated with the Birch-Murnaghan equation to be 210 ± 7 GPa.     

Elastic and thermal properties of fluoride and oxide analogues in the rocksalt,fluorite, rutile and perovskite structures

Fluorides are considered as models for the physical properties of oxides on the basis of Goldschmidt's crystal chemical arguments. The well-established bulk modulus (K)-volume (V) relationship KV = constant is sddddhown to hold for fluorides and oxides belonging to the four isostructural series. The bulk moduli of equivolume oxides and fluorides are scaled as 4S², where S = Z^O/2Z^F is the ratio of the effective unit charges and is approximately 77% for all of the crystal structures. The fluorides have distinctly lower melting and Debye temperatures which offers the possibility of using these compounds as models for the high-temperature elastic behaviour of their oxide analogues.     

Synthesis and crystal-chemical characterization of MgSiO3 perovskite

The orthorhombic MgSiO₃ perovskite has been synthesized with the aid of a double-stage split-sphere-type high-pressure apparatus at about 280 kbar and 1000°C. The unit cell dimensions are: a = 4.7754(3)Å, b = 4.9292(4)Å and c = 6.8969(5)Å with the probable space group Pbnm. Calculated density is 4.108 g cm^?3. Crystal structure determination has been carried out by means of both the geometrical simulation (DLS) technique and the ordinary powder X-ray analysis. The results indicate that the MgSiO₃ perovskite is closer to the ideal perovskite than ScAlO₃ perovskite.     

Application of the Lorenz equations for studying thermal convection in the lithosphere

Stationary solutions including wave solutions with constant amplitudes are found for nonlinear equations of thermal convection in a layer with nonlinear rheology. The solution is based on the Fourier expansion of unknown velocities and temperatures with only the first and first two terms retained in the velocity and temperature series, respectively. This method, which can be regarded as the Lorenz method, yields the Lorenz equations that fairly well describe the thermal convection in a layer with Newtonian rheology if the Rayleigh number is not very large. The obtained generalization of the Lorenz equations to the case of an integral (having a memory) nonlinear rheology implies that only the first term is retained in the Fourier series for the stress components, i.e., the nonlinear rheological equation is harmonically linearized. However, in the Fourier series of temperature, it is essential to keep the second term: this term, which is independent of the horizontal coordinate, models the thermal boundary layer that characterizes the developed convection. We constructed the bifurcation curves that describe the stationary convection in the nonlinear integral medium simulating the rheology of the mantle, and analyzed the stability of stationary convective flows. The Lorenz method is applied to study small-scale thermal convection in the lithosphere of the Earth.     

Simultaneous interplanetary scintillation and optical measurements of the acceleration of the slow solar wind

Simultaneous observations of the slow solar wind off the southeast limb of the Sun were made in May 1999 using optical measurements from the C2 and C3 LASCO coronagraphs on board the SOHO spacecraft and radio-scattering measurements from the MERLIN and EISCAT facilities. The observations show the slow solar wind accelerating outwards from 4.5 solar radii (R), reaching a final velocity of 200–300 km s^-1 by 25–30 R. The acceleration profile indicated by these results is more gentle than the average profile seen in earlier LASCO observations of larger scale features, but is within the variation seen in these studies.     

电离层不规则结构漂移的GPS测量及其初步结果

本文阐述了利用GPS接收机台阵测量到的闪烁和TEC变化率ROT快速起伏图样估计F层不规则结构漂移的原理和方法,并利用实测数据估计了静日和暴时电离层不规则结构的水平漂移速度.短间距台网和超短间距台链观测实例的计算结果表明,暴时武汉地区引起TEC快速起伏的电离层不规则结构沿纬圈向西漂移,21∶30至03∶00 LT,西向漂移速度在约40 m/s至130 m/s的范围内变化;在桂林地区,磁静日午夜前后引起L波段电波闪烁的电离层不规则结构沿纬圈向东漂移,漂移速度从约70 m/s下降到约55 m/s,磁扰日午夜前不规则结构向西漂移,速度从约150 m/s下降到约50 m/s,午夜后转为向东漂移,速度从约25 m/s上升到约65 m/s.文中还提出了由单站多卫星观测估计F层不规则结构漂移的设想.实例分析与计算结果表明,利用单站多卫星观测估计电离层不规则结构漂移是一种合理可行的方法.     

Simultaneous optical and radar measurements of meteors using the Poker Flat Incoherent Scatter Radar

Coordinated Poker Flat Incoherent Scatter Radar (PFISR) and imaging observations are examined in order to study the radar scattering signatures of optically visible meteors. Preliminary observations in 2006 revealed that optical meteors were detected by PFISR. A recent campaign in the winter of 2009, employed a radar mode optimized for meteor Doppler shifts. This paper presents a case study from 21 January 2009. Seven out of the 338 meteors observed with PFISR were also detected optically. Six out of those seven were detected by the side lobes of the radar and not the main beam. A positive correlation was found between the corrected backscattered radar power and the optical brightness of the meteors, as well as between optical brightness and absolute speed. Meteors originating in the east had higher speeds and more glancing incidence angles, while meteors originating in the north had lower speeds and more direct incidence angles.     

The early thermal and magnetic state of the cratered highlands of Mars

Surface heat flows are calculated from elastic lithosphere thicknesses for the heavy cratered highlands of Mars, in terms of the fraction of the surface heat flow derived from crustal heat sources. Previous heat flow estimations for Mars used linear thermal gradients, which is equivalent to ignoring the existence of heat sources within the crust. We compute surface heat flows following a methodology that relates effective thickness and curvature of an elastic plate with the strength envelope of the lithosphere, and assuming crustal heat sources homogeneously distributed in a radioactive element-rich layer 20 or 60 km thick. The obtained results show that the surface heat flow increases with the proportion of heat sources within the crust, and with the decrease of both radioactive element-rich layer thickness and surface temperature. Also, the results permit us to calculate representative temperatures for the crust base, rock strength for the upper mantle, and lower and upper limits to the crustal magnetization depth and intensity, respectively. For Terra Cimmeria, an effective elastic thickness of 12 km implies between 30% and 80% of heat sources located within the crust. In this case the uppermost mantle would be weak at the time of loading, and temperatures in the lower crust cold enough to favor unrelaxed crustal thickness variations and to permit deep Curie depths in the highlands, as suggested by the observational evidence.     

Weathering processes in baked sediments and their effects on archaeomagnetic field-intensity measurements

Thellier-type measurements of ancient field intensity on specimens of clay and other sediments, which were apparently well-baked and oxidised in ancient times, often fail to give consistent results over part or even most of the blocking-temperature spectrum. It is suggested that post-baking chemical alteration, or weathering, leading to the presence of hydrated Fe minerals, is a major cause of non-ideal behaviour in such material. The behaviour of hypothetical specimens containing either goethite or lepidocrocite can be predicted using simple models, and actual examples are given from two sites in southern Australia which show some similarities with the predicted model behaviour. The results of the Thellier measurements, after interpretations, agree closely with previously published results from the northern hemisphere for the period 4700-4200 yr. B.P. The presence of hydrated minerals may not be readily detected by methods other than the Thellier technique and, if so, would result in estimates of the ancient geomagnetic field strength that are systematically too low.     

Elasticity of the ilmenite - perovskite phase transformation in CdTiO3

Ultrasonic data for the velocities of the ilmenite and perovskite polymorphs of CdTiO₃ have been determined as a function of pressure to 7.5 kbar at room temperature for polycrystalline specimens hot-pressed at pressures up to 25 kbar. This transition is characterized by the following velocity (ν)-density (?) relationships: (1) the changes in compressional (ν_p) and bulk sound (ν_?) velocities are comparable in percentage magnitude to the density jump, while the shear (ν_s) velocity jump is three times greater than that for ?; (2) (ν_p/ν_s) decreases across the transition from the low- to high-pressure phase; and (3) low slopes (linear or logarithmic) on ν-? diagrams. The (ν_p/ν_s) behaviour for the ilmenite-perovskite transformation is unusual for the transitions studied in our laboratory. The observed relationships (1) and (2) are typical of the elasticity behaviour across phase transformations which involve increases in cation-anion co-ordination and in nearest-neighbour interatomic distances, such as those exhibited by CdTiO₃ in transforming from the ilmenite to the perovskite phase. Elasticity systematics for isostructural sequences are used to estimate the bulk moduli of the perovskite polymorphs of CaSiO₃ (2.7 Mbar) and MgSiO₃ (2.8 Mbar).