Fe2SiO4 Spinel Research Articles

Ab initio Investigation of the Structure and Electronic Properties of Normal Spinel Fe2SiO4

Transition metal spinel oxides have recently been predicted to create efficient transparent conducting oxides for optoelectronic devices. These compounds can be easily tuned by doping or defect to adapt their electronic or magnetic properties. However, their cation distribution is very complex and band structures are still subject to controversy. We propose a complete density functional theory investigation of fayalite (Fe2SiO4) spinel, using Generalized Gradient Approximation (GGA) and Local Density Approximation (LDA) in order to explain the electronic and structural properties of this material. A detailed study of their crystal structure and electronic structure is given and compared with experimental data. The lattice parameters calculated are in agreement with the lattice obtained experimentally. The band structure of Fe2SiO4 spinel without Coulomb parameter U shows that the bands close to Fermi energy appear to be a band metal, with four iron d-bands crossing the Fermi level, in spite of the fact that from the experiment it is found to be an insulator.

Structural stabilities, electronic structure, optical and elastic properties of ternary Fe2SiO4 spinel: An ab initio study

Spinel-type Fe2SiO4 has been one of the materials receiving much attention in the field of new spintronics and optoelectronics materials, due to its promising physical properties of high transparency achieved experimentally. In this research, we systematically investigate the structural stabilities, elastic, optical and magnetic, properties of Fe2SiO4 spinel using the first-principles theory within generalized gradient approximation (GGA) and GGA + U frameworks. With the GGA + U method, Fe2SiO4 is shown to be stable for both spin-up and down calculations. Furthermore, incorporating the Hubbard correction parameter due to the localized Fe-3d electrons produced the bandgap value to be close to available experimental data. Our result reveals additional information on Fe2SiO4 spinel which is useful for molecular magnet and spintronics devices.

DFT+U studies of structure and optoelectronic properties of Fe2SiO4 spinel

Spinel oxides have been predicted as one of the potential materials in the transparent conducting oxides community. A detailed first-principles pseudopotential investigation was performed with a view to explain the structure and optoelectronic properties of Fe2SiO4 spinel. The band structure of Fe2SiO4 spinel without Coulomb Parameter U shows that the bands close to Fermi energy appear to be a band metal, even though experimentally it was found to be an insulator. Our result also reveals that the n-type Fe2SiO4 would be more useful for optoelectronic devices performance than that of p-type and the electrical conductivity charge concentration by valence energy band electrons is more desirable than that by conduction band holes. Further, we have shown that the dispersion together with the character of the conduction bands is part of the important features of the band structure controlling the optoelectronic application.

Si-Disordering in MgAl2O4-Spinel under High P-T Conditions, with Implications for Si-Mg Disorder in Mg2SiO4-Ringwoodite

A series of Si-bearing MgAl2O4-spinels were synthesized at 1500–1650 °C and 3–6 GPa. These spinels had SiO2 contents of up to ~1.03 wt % and showed a substitution mechanism of Si4+ + Mg2+ = 2Al3+. Unpolarized Raman spectra were collected from polished single grains, and displayed a set of well-defined Raman peaks at ~610, 823, 856 and 968 cm−1 that had not been observed before. Aided by the Raman features of natural Si-free MgAl2O4-spinel, synthetic Si-free MgAl2O4-spinel, natural low quartz, synthetic coesite, synthetic stishovite and synthetic forsterite, we infer that these Raman peaks should belong to the SiO4 groups. The relations between the Raman intensities and SiO2 contents of the Si-bearing MgAl2O4-spinels suggest that under some P-T conditions, some Si must adopt the M-site. Unlike the SiO4 groups with very intense Raman signals, the SiO6 groups are largely Raman-inactive. We further found that the Si cations primarily appear on the T-site at P-T conditions ≤~3–4 GPa and 1500 °C, but attain a random distribution between the T-site and M-site at P-T conditions ≥~5–6 GPa and 1630–1650 °C. This Si-disordering process observed for the Si-bearing MgAl2O4-spinels suggests that similar Si-disordering might happen to the (Mg,Fe)2SiO4-spinels (ringwoodite), the major phase in the lower part of the mantle transition zone of the Earth and the benchmark mineral for the very strong shock stage experienced by extraterrestrial materials. The likely consequences have been explored.

Study of lattice dynamics of Fe2SiO4- and Mg2SiO4-spinels

Fe2SiO4 is an end member of Mg-rich (Mg, Fe)2SiO4, which is believed to be a major mineral of the Earth's transition zone [1, 2]. The presence of iron has a pronounced effect on elastic and thermodynamic properties of rock-forming minerals. These properties play crucial role in the interpretation of the geophysical data and thus have a large influence on our knowledge of the earth's interior. An understanding of the elastic properties of silicate will be further helpful to the interpretation of seismological data, in particular the variation in the depth range of transition zone of earth's interior. The spinel form of magnesium-iron orthosilicate, (Mg, Fe)2SiO4, is believed to be one of the most abundant minerals in the mantle's transition zone and is found to be stable in ambient conditions and therefore, a detailed study of zone centre phonons of this stable phase of orthosilicates (Mg, Fe)2SiO4 is of high interest. Hence, in the present study, the zone centre phonons of antiferromagnetic Fe2SiO4-spinel and Mg2SiO4-spinel have been studied by using short range force constant model involving interatomic interactions upto first three neighbours. The zone centre phonons of Fe2SiO4 is compared with that of Mg2SiO4 in order to study the effect of the cation exchange on the dynamic and thermodynamic properties of (Mg, Fe)2SiO4-spinel. The calculated results are compared and analyzed with exiting experimental results.

New structure of high-pressure body-centered orthorhombic Fe2SiO4

A structural change in Fe2SiO4 spinel (ringwoodite) has been found by synchrotron powder diffraction study and the structure of a new high-pressure phase was determined by Monte-Carlo simulation method and Rietveld profile fitting of X-ray diffraction data up to 64 GPa at ambient temperature. A transition from the cubic spinel structure to a body centered orthorhombic phase ( I -Fe2SiO4) with space group Imma and Z = 4 was observed at approximately 34 GPa. The structure of I -Fe2SiO4 has two crystallographically independent FeO6 octahedra. Iron resides in two different sites of sixfold coordination: Fe1 and Fe2, which are arranged in layers parallel to (101) and (011) and are very similar to the layers of FeO6 octahedra in the spinel structure. Silicon is located in the sixfold coordination in I -Fe2SiO4. The transformation to the new high-pressure phase is reversible under decompression at ambient temperature. A martensitic transformation of each slab of the spinel structure with translation vector ![Formula][1] generates the I -Fe2SiO4 structure. Laser heating of I -Fe2SiO4 at 1500 K results in a decomposition of the material to rhombohedral FeO and SiO2 stishovite. Fe K β X-ray emission measurements at high pressure up to 65 GPa show that the transition from a high spin (HS) to an intermediate spin (IS) state begins at 17 GPa in the spinel phase. The IS electron spin state is gradually enhanced with pressure. The Fe2+ ion at the octahedral site changes the ion radius under compression at the low spin, which results in the changes of the lattice parameter and the deformation of the octahedra of the spinel structure. The compression curve of the lattice parameter of the spinel is discontinuous at ~20 GPa. The spin transition induces an isostructural change. [1]: /embed/mml-math-1.gif

Comparative ab initio study of lattice dynamics and thermodynamics ofFe2SiO4- andMg2SiO4-spinels

Lattice dynamics and thermodynamic properties of antiferromagneticFe2SiO4-spinel have been studied using density functional theory. Phonon dispersions are obtained forseveral hydrostatic pressures up to 20 GPa. They are used to calculate thermodynamic propertieswithin the quasiharmonic approximation. Comparison with ab initio results obtained forMg2SiO4-spinelis made in order to study the effect of the cation exchange on the dynamic and thermodynamic propertiesof (Mg, Fe)2SiO4-spinel. The obtained results have been compared with the available experimental data.

Compressional and shear wave velocities of Fe2SiO4 spinel at high pressure and high temperature

Ultrasonic interferometric measurements on polycrystalline Fe2SiO4 spinel were conducted simultaneously with synchrotron X-ray diffraction and X-ray imaging up to 6.5 GPa, 1073 K. The compressional and shear wave velocity data and the volume data were fitted to the third-order finite strain equations to derive the bulk and shear modulus and their pressure and temperature derivatives. The fitting results are as follows: K s0=204.5(7) GPa,=73.6(3) GPa, K′ s =4.3(3), G′=1.2(1), (∂ K s /∂ T) p =−0.027(2) GPa/K, and (∂ G/∂ T) p =−0.017(1) GPa/K. Comparison of our current results with previous data on (Mg,Fe)2SiO4 spinel with different compositions suggests that the bulk modulus (K s ) increases slightly with increasing iron content, while the shear modulus (G), in contrast, shows a dramatic decrease. However, the pressure and temperature derivatives of K s and G remain nearly constant from Mg2SiO4 to Fe2SiO4 spinel with average values of 4.2–4.4, 1.2–1.3,−0.024 GPa/K, and−0.016 GPa/K for K′ s , G′, (∂ K s /∂ T) p , and (∂ G/∂ T) p , respectively. The proposed version of equations to describe the effects of iron on the elastic moduli of ringwoodite are: K s =184.7+18.0 X Fe, and G=118.7−41.5 X Fe.

Temperature-dependent magnetic susceptibility behaviour of spinelloid and spinel solid solutions in the systems Fe2SiO4?Fe3O4 and (Fe,Mg)2SiO4?Fe3O4

The magnetic behaviour and Curie temperatures (TC) of spinelloids and spinels in the Fe3O4–Fe2SiO4 and Fe3O4–(Mg,Fe)2SiO4 systems have been determined from magnetic susceptibility (k) measurements in the temperature range −192 to 700 °C. Spinelloid II is ferrimagnetic at room temperature and the k measurements display a characteristic asymmetric hump before reaching a TC at 190 °C. Spinelloid V from the Mg-free system is paramagnetic at room temperature and hysteresis loops at various low temperatures indicate a ferri- to superparamagnetic transition before reaching the TC. The TC shows a non-linear variation with composition between −50 and −183 °C with decreasing magnetite component (XFe3O4). The substitution of Mg in spinelloid V further decreases TC. Spinelloid III is paramagnetic over nearly the total temperature range. Ferrimagnetic models for spinelloid II and spinelloid V are proposed. The TC of Fe3O4–Fe2SiO4 spinel solid solutions gradually decrease with increasing Si content. Spinel is ferrimagnetic at least to a composition of XFe3O4=0.20, constraining a ferrimagnetic to antiferromagnetic transition to occur at a composition of XFe3O4<0.20. A contribution of the studied ferrimagnetic phases for crustal anomalies on the Earth can be excluded because they lose their magnetization at relatively low temperatures. However, their relevance for magnetic anomalies on other planets (Mars?), where these high-pressure Fe-rich minerals could survive their exhumation or were formed by impacts, has to be considered.

Seismic Evidence for Olivine Phase Changes at the 410- and 660-Kilometer Discontinuities

The view that the seismic discontinuities bounding the mantle transition zone at 410- and 660-kilometer depths are caused by isochemical phase transformations of the olivine structure is debated. Combining converted-wave measurements in East Asia and Australia with seismic velocities from regional tomography studies, we observe a correlation of the thickness of, and wavespeed variations within, the transition zone that is consistent with olivine structural transformations. Moreover, the seismologically inferred Clapeyron slopes are in agreement with the mineralogical Clapeyron slopes of the (Mg,Fe)2SiO4 spinel and postspinel transformations.

Mg-Fe partitioning between silicate spinel and magnesiow�stite at high pressure: experimental determination and calculation of phase relations in the system Mg 2 SiO 4 -Fe 2 SiO 4

Mg-Fe partitioning experiments between (Mg,Fe)2SiO4 spinel and (Mg,Fe)O magnesiowustite were carried out at pressures of 17–21.3 GPa at temperatures of 1400 and 1600 °C, using a multi-anvil apparatus, in order to determine interaction parameters of spinel and magnesiowustite solid solutions and also to constrain the equilibrium boundaries of the postspinel transition in the Fe-rich side in the system Mg2SiO4-Fe2SiO4. The obtained values of the interaction parameters were 3.4 ± 1.5 and 13.9 ± 1.4 kJ mol−1, respectively, for spinel and magnesiowustite solid solutions at 19 GPa and 1600 °C. The partitioning data in the system Mg2SiO4-Fe2SiO4 at 1400 and 1600 °C showed that the transition boundary between spinel and the mixture of magnesiowustite and stishovite has a negative dP/dT slope. Using the above interaction parameters and available thermodynamic data of the Mg2SiO4 and Fe2SiO4 end members, the transition boundaries of spinel to the mixture of magnesiowustite and stishovite were calculated. Within the uncertainties of the data used, the calculated boundaries are in good agreement with the boundaries at 1400 and 1600 °C experimentally determined in this study. The dissociation boundary of Fe2SiO4 spinel to wustite and stishovite, calculated from the thermodynamic data, has a negative slope of −1.5 ± 0.6 MPa K−1.

TEM observations of several spinel‐garnet assemblies: toward the rheology of the transition zone

ABSTRACTIn order to better identify the mineral phase which controls the rheology of the transition zone (between 410 and 660 km depth) transmission electron microscopy observations were made on several coexisting spinel‐garnet assemblies: alkremite xenolith; pyrope‐rich – MgO:1.1Al2O3 spinel assembly deformed at 1173K, 800 MPa in a Griggs apparatus; (Mg,Fe)3(Al,Mg,Si)2Si3O12 majorite – (Mg,Fe)2SiO4 spinel assembly synthesized in a laser heated diamond anvil cell. It was found that garnet crystals systematically remain undeformed while spinel crystals are plastically deformed. These results are in accord with the assumption that the rheology of majorite is stronger than the rheology of spinel, in the conditions of the transition zone.

Mössbauer spectroscopy of quenched high-pressure phases: Investigating the Earth's interior

New phases formed at high pressure and temperature can be successfully quenched to ambient conditions if the kinetics of the back transformation are slow. One important application of this technique is to mineral physics — the combination of microscopic measurements of the structural, physical and chemical properties of high-pressure minerals measured in the laboratory with macroscopic geophysical and geochemical data to produce a unified description of the Earth's interior. The dominant high-pressure phases inferred to be present in the Earth's mantle have been synthesised using a multi-anvil press to correlate structural and chemical information obtained from Mossbauer data with bulk geophysical and geochemical data to determine properties of the mantle, such as the oxidation state. Results from experiments on the major transition zone minerals β−(Mg, Fe)2SiO4, (Mg, Fe)SiO3 garnet and γ−(Mg, Fe)2SiO4 spinel include the discovery of significant Fe3+ in these phases synthesised in equilibrium with metallic iron and excess silica, implying that the oxygen fugacity (fO2) of the transition zone must be substantially lower than the upper mantlefO2. Mossbauer spectra of the lower mantle phase (Mg, Fe)SiO3 perovskite show that Fe2+ occupies the large distorted site almost exclusively, and that significant Fe3+ is present at the minimum fO2 stability limit. Low temperature spectra indicate a phase transition possibly corresponding to a distortion of the perovskite structure. Mossbauer spectra of FexO quenched from high pressure indicate that the Fe3+ content of samples in equilibrium with metallic iron is small, implying that the minimum amount of Fe3+ in the lower mantle (Mg, Fe)O at lower mantle conditions is also small. Mossbauer spectra from samples of (Mg, Fe)O synthesised at different temperatures andfO2 conditions show that the Fe3+ content can be reliably measured, and that it varies significantly withfO2. Implications of these results to properties of the Earth's interior are discussed.

Pressure-volume-temperature behavior of ?-Fe2SiO4 (spinel) based on static compression measurements at 400� C

Thirteen energy-dispersive x-ray diffraction spectra for γ-Fe2SiO4 (spinel) collected in situ at 400° C and pressures to 24 GPa constitute the basis for an elevated-temperature static compression isotherm for this important high-pressure phase. A Murnaghan regression of these molar volume measurements yields 177.3 (±17.4) GPa and 5.4(±2.5) for the 400° C, room pressure values of the isothermal bulk modulus (K P 0) and its first pressure derivative (K′ P 0), respectively. When compared to the room-Tdeterminations of K P 0 available in the literature, our 400° C K P 0 yields -4.1 (±6.2)×10-2 GPa/degree for the average value of (∂K/∂T) P 0 over the temperature interval 25° C<T<400° C.

地球惑星物質の衝撃誘起相転移

Shock-induced phase transformation mechanims of silicate minerals are discussed. The main part of this review is devoted to a description of TEM observation of shock-produced veins in naturally shocked ordinary chondrites containing high-pressure minerals (i.e. (Mg, Fe)2SiO4 spinel, (Mg, Fe) SiO3 garnet, (Ca, Na, K) (Al, Si)4O8 hollandite and (Mg, Fe) SiO3 perovskite). Data from laboratory shock experimets are also referenced. The high pressure minerals in the shock veins are interpreted as quenched materials from shock-produced high-pressure and temperature (ca. 23-28 GPa, 2400-2800°C) silicate melts along the shear deformation bands in the meteorites.

超高圧高温地球科学最前線

Current interest of high-pressure earth scientists is shifting from upper mantle to lower mantle, from mantle to core, from quenching experiments to high-pressure, high-temperature in-situ measurements, and from static phase equilibrium to large scale dynamical motion. In this paper, frontier report is given on several topics. A precise version of the olivine-modified spinel-spinel-post spinel transformation diagram in the system Mg2SiO4Fe2SiO4 and of the pyroxene-garnet-ilmenite-perovskite transformation diagram in the system Mg4Si4O12-Mg3Al2Si3O12 is presented. The 670km seismic discontinuity is well-explained by the dissociation of (Mg, Fe) 2SiO4 spinel to (Mg, Fe) SiO3 perovskite and (Mg, Fe) O magnesiowüstite in pyrolitic mantle. Based on the phase diagram, the temperature at the depth of 655km is estimated about 1600°C. Effect of the olivine-modified spinel-spinel-post spinel transformations on the dynamics of the descending slab (plate) is discussed in relevance to the deep focus earthquakes. Recent progress of high-pressure, high-temperature research in the system Fe-H, Fe-FeO and Fe-SiO2 is outlined with reference to the core formation process in the proto-earth and the composition of the present earth's core. Solubility of hydrogen, oxygen and silicon into molten iron increases remarkably at very high pressure up to 24 GPa. Oxygen and hydrogen might be the chief light elements in the earth's outer core. The silicon content in the core depends on the depth of the magma ocean of the proto-earth.

Back transformation and oxidation of (Mg, Fe)2SiO4 spinels at high temperatures

Based on the in situ and temperature-quench X-ray measurements, the back transformation in the (Mg, Fe)2SiO4-spinels has been characterized in terms of the transformation temperature (T r ),mechanism and kinetics of the transformation, and of the end product(s), with specific emphasis on the effect of oxygen on this transformation. The in situ measurements were conducted to 900° C in vacuum (10-4 to 10-5 torr) and to 600° C in air using synchrotron radiation (SR) at Stanford Synchrotron Radiation Laboratory (SSRL). In the quench-type measurements, samples were heated in air to 1100° C, quenched and examined at ambient conditions using the conventional X-ray diffraction facilities. Important results are (1) in vacuum, all the spinels convert back into the olivine phase, with their T r decreasing with increasing iron content; (2) the spinel → olivine back transformation is a nucleation and growth type of transformation and can be described quantitatively using the Avrami equation; (3) in air, the (Mg, Fe)2SiO4-spinels with 0.2 mole fraction Fe or more are all oxidized, and the composition and phase of the end products depend upon the temperature and the starting composition; and (4) the oxidation of the iron-rich (Mg, Fe)2SiO4-spinels in air occurs at ∼350–400° C, which is significantly lower than its T r (∼ 300° C) in vacuum.

Crystal structures of Ni2SiO4 and Fe2SiO4 as a function of temperature and heating duration

X-ray structure refinements of Ni2SiO4 and Fe2SiO4 spinels have been made as a function of temperature and heating duration by intensity measurements at high temperatures and room pressure. The lattice parameters of Ni2SiO4 spinel linearly increased with temperature up to 1,000° C. However, Fe2SiO4 spinel exhibited a nonlinear thermal expansion and was converted to a polycrystalline mixture of spinel and olivine by heating of less than one-hour at 800° C.

Eutectoid phase transformation of olivine and spinel into perovskite and rock salt structures

The 700-km seismic discontinuity which defines the top of the Earth's lower mantle is generally attributed to the transformation of the upper mantle (Mg, Fe)2 SiO4 spinel into magnesiowustite (Mg, Fe)O and (Mg, Fe)SiO3 with perovskite structure1. Large-volume high-pressure experiments2 have provided important thermodynamical information on this transition, but its mechanisms and kinetics are still unknown. Here we report microstructural observations of the products of the transformation of (Mg, Fe)2SiO4 olivine and spinel and Ca2GeO4 olivine, effected in a laser-heated diamond anvil cell (DAC). The observations were made by transmission electron microscopy (TEM), a technique that has already proved useful in investigating the olivine-spinel transition in the DAC3,4. We find that, in all cases, the transformed product has a eutectoid-like appearance (as in pearlitic carbon steels), with alternating lamellae of the rock salt and perovskite phases. The thickness and spacing of the lamellae increase with temperature until a granular structure forms. These observations suggest that the transformation starts as a eutectoid transformation, whose physical mechanisms might be as effective in the Earth's mantle as in the DAC.

Effects of composition and high pressures on the electrical conductivity of Fe-rich (Mg, Fe)2SiO4 olivines and spinels

Synthetic olivines, with composition Fa50, Fa75 and Fa100, have been transformed into spinel in a laser-heated diamond-cell at pressures from 70 to 200 kbar and at a luminance temperature of about 1,200° C. The electrical conductivity σ was measured, at room temperature and up to 200 kbar, on olivine (Lacam 1982; 1983) and spinel (present study). The data obtained permit the following conclusions: a) Sample nature effect: under the same conditions (composition, pressure), the σ of spinel is more than three orders of magnitude of the σ of olivine. b) Composition effect: there are more than three orders of magnitude between the values of σ for spinels derived from initial compositions of Fa50 and Fa100, respectively. c) Pressure effect: The P-effect on σ is greater for olivines than for spinels.