Mg2SiO4 Olivine Research Articles

Lattice thermal conductivity of Mg2SiO4 olivine and its polymorphs under extreme conditions

Lattice thermal conductivity of Mg2SiO4 olivine and its polymorphs under extreme conditions

The core structure of screw dislocations with [001] Burgers vector in Mg 2 SiO 4 olivine

In this study, we report atomistic calculations of the core structure of screw dislocations with [001] Burgers vector in Mg2SiO4 olivine. Computations based on the THB1 empirical potential set for olivine show that the stable core configurations of the screw dislocations correspond to a dissociation in {110} planes involving collinear partial dislocations. As a consequence, glide appears to be favorable in {110} planes at low temperature. This study also highlights the difference between dislocation glide mechanism in {110} versus (010) or (100) for which glide is expected to occur through a locking–unlocking mechanism.

Transient shrinkage behavior of wustite-centered binary/ternary/quaternary/quinary-component oxide packed beds in a reducing atmosphere up to 1773 K

Wustite (FeO)-centered multicomponent oxides play an important role in the ironmaking process, and a complete understanding of their high temperature behaviors is of great importance for process optimization to achieve high efficiency and low emissions during industrial production. In this work, the transient shrinkages of FeO-centered multicomponent oxide packed beds are quantitatively determined in a reducing atmosphere up to 1773 K, and the effects of the interactions between the oxides on the shrinkage rate (SR) are qualitatively evaluated. The results show that although mixing CaO with FeO increases the SR to 0.42%/K below 1173 K, further mixing with SiO2 or Al2O3 significantly limits this enhancement effect due to the formation of an olivine or spinel phase. However, in the subsequent stage, the SR increases to as high as 0.44%/K after CO is injected. The interaction between FeO and MgO leads to an SR of greater than 0.20%/K at lower temperatures, but it causes a decrease in the SR from 0.33%/K to 0.16%/K between 1173 K and 1273 K. Meanwhile, adding SiO2 slows the reduction reaction, and the SR correspondingly decreases further to 0.04%/K. On the other hand, the interaction between CaO and Al2O3 takes precedence over the interaction between SiO2 and MgO and dominates the shrinkage process in the quinary-component case, and the preferentially formed CaAl2O4 spinel phase hinders the formation of the Mg2SiO4 olivine phase.

On the glide of [100] dislocation and the origin of ‘pencil glide’ in Mg2SiO4 olivine

ABSTRACTOlivine with chemical composition (Mg,Fe)2SiO4 is a silicate which is supposed to strongly constrain the flow of the Earth’s upper mantle under thermal convection. Its mechanical properties are thus of primary importance. Slip systems in olivine involve two types of dislocations with [100] and [001] Burgers vectors. In this study, we report atomistic modelling of screw dislocations with [100] Burgers vector and their intrinsic properties. We show that the [100] screw dislocation core exhibits several configurations corresponding to spreading in different planes and different relative stabilities. At low pressure, we identify a clear tendency for core spreading in (010). At higher pressure, relevant for the Earth’s mantle, we show that pressure promotes a change in core configuration with spreading into equivalent {021} planes. Based on the systematic investigation of the minimum energy path between the different configurations, we show that the variability of core structures allows complex glide paths which has been described at the macroscopic level as ‘pencil glide’. Our results suggest that the pencil glide is more efficient at high pressure.

Metastable structural transformations and pressure-induced amorphization in natural (Mg,Fe)2SiO4olivine under static compression: A Raman spectroscopic study

Raman spectroscopic data were obtained for (Mg,Fe)2SiO4 samples during compression to 57 GPa. Single crystals of San Carlos olivine compressed hydrostatically above 41 GPa showed appearance of a new “defect” peak in the 820–840 cm−1 region associated with SiOSi linkages appearing between adjacent Embedded Image tetrahedra to result in five- or sixfold-coordinated silicate species. Appearance of this feature is accompanied by a broad amorphous background. The changes occur at lower pressure than metastable crystalline transitions of end-member Mg2SiO4 forsterite (Fo-I) into Fo-II and Fo-III phases described recently. We complemented our experimental study using density functional theory (DFT) calculations and anisotropic ion molecular dynamics (AIMD) simulations to study the Raman spectra and vibrational density of states (VDOS) of metastably compressed Mg2SiO4 olivine, Fo-II and Fo-III, and quenched melts at high and low pressures. By 54 GPa all sharp crystalline peaks disappeared from observed Raman spectra indicating complete pressure-induced amorphization (PIA). The amorphous (Mg,Fe)2SiO4 spectrum contains Si-O stretching bands at lower wavenumber than expected for Embedded Image indicating high coordination of the silicate units. The amorphous spectrum persisted on decompression to ambient conditions but with evidence for reappearance of tetrahedrally coordinated units. Non-hydrostatic compression of polycrystalline olivine samples showed similar appearance of the defect feature and broad amorphous features between 43–44 GPa. Both increased in intensity as the sample was left at pressure overnight but they disappeared during decompression below 17 GPa with recovery of the starting olivine Raman signature. A hydrated San Carlos olivine sample containing 75–150 ppm OH was also studied. Significant broadening of the Embedded Image stretching peaks was observed above 43 GPa but without immediate appearance of the defect or broad amorphous features. However, both of these characteristics emerged after leaving the sample at 47 GPa overnight followed by complete amorphization that occurred upon subsequent pressurization to 54 GPa. During decompression the high-density amorphous spectrum was retained to 3 GPa but on final pressure release a spectrum similar to thermally quenched low-pressure olivine glass containing isolated Embedded Image groups was obtained. Leaving this sample overnight resulted in recrystallization of olivine. Our experimental data provide new insights into the metastable structural transformations and relaxation behavior of olivine samples including material recovered from meteorites and laboratory shock experiments.

Nucleation on Heating/Loading Path and Its Effects on the Evaluation of Nucleation Rate

AbstractWe investigate the nucleation on heating/loading paths and its effects on the olivine phase transformation and the determinations of nucleation rate in in‐situ experiments by the numerical simulation based on the kinetic parameters from low temperature experiments in of Ni2SiO4 olivine phase transformation[1]. Results show that the high temperature experiments, of which samples are first loaded and then heated to the experimental conditions after annealing, are greatly affected by the nucleation during heating. The predicted nucleation rates based on these experiments are obviously overrated. Especially in the case that most of experiments have high target temperatures, the predicted nucleation rates will significantly deviate from their true values, which will have influences on the evaluations of olivine grain size and the rheological structures of subduction zones. Although there are many growth data of Mg2SiO4 olivine and a rough evaluation on the nucleation rate of (Mg0.89Fe0.11)2SiO4 olivine based on the feature of grain impingement, only Ref.[2] showed some nucleation rates of Mg2SiO4 olivine from in‐situ high temperature experiments, in which the sample is finally heated to the target experimental condition. The results in present study suggest that the experiments with low target temperature or those reached their experimental condition finally by loading are needed to derive the correct nucleation parameters of mantle olivine.

Pressure-induced slip-system transition in forsterite: Single-crystal rheological properties at mantle pressure and temperature

Deformation experiments were carried out in a Deformation-DIA high-pressure apparatus (D-DIA) on oriented Mg2SiO4 olivine (Fo100) single crystals, at pressure (P) ranging from 2.1 to 7.5 GPa, in the temperature (T) range 1373-1677 K, and in dry conditions. These experiments were designed to investigate the effect of pressure on olivine dislocation slip-system activities, responsible for the lattice-preferred orientations observed in the upper mantle. Two compression directions were tested, promoting either [100] slip alone or [001] slip alone in (010) crystallographic plane. Constant applied stress ({sigma}) and specimen strain rates (Formula) were monitored in situ using time-resolved X-ray synchrotron diffraction and radiography, respectively. Transmission electron microscopy (TEM) investigation of the run products reveals that dislocation creep assisted by dislocation climb and cross slip was responsible for sample deformation. A slip transition with increasing pressure, from a dominant [100]-slip to a dominant [001]-slip, is documented. Extrapolation of the obtained rheological laws to upper-mantle P, T, and {sigma} conditions, suggests that [001]-slip activity becomes comparable to [100]-slip activity in the deep upper mantle, while [001] slip is mostly dominant in subduction zones. These results provide alternative explanations for the seismic anisotropy attenuation observed in the upper mantle, and for the 'puzzling' seismic-anisotropy anomaliesmore » commonly observed in subduction zones.« less

The vibrational frequencies of forsterite Mg2SiO4: an all-electron ab initio study with the CRYSTAL code

The vibrational spectrum of Mg2SiO4 olivine was calculated at the Γ point by using the periodic ab initio CRYSTAL program. An all electron localized Gaussian-type basis set and the B3LYP Hamiltonian were employed. The full set of frequencies (35 IR active, 36 Raman active, 10 “silent” modes) was computed and compared to experimental data from different sources (four for IR and four for Raman). A generally good agreement is observed with experiment (the mean absolute difference ranging from 7 to 10 cm−1 for the various sets), when some of the experimental frequencies, whose attribution is uncertain or appears to be affected by large errors, are not taken into account. A small number of observed peaks are not consistent with calculated frequencies, and a few theoretical peaks do not correspond to measured values. The implications are discussed in detail. The full set of modes are characterized using different tools, namely isotopic substitution, direct inspection of the eigenvectors and graphical representation, so as to obtain a consistent mode assignment.

Mechanisms of the transformations between the ?, ? and ? polymorphs of Mg2SiO4 at 15 GPa

Data on the mechanisms of mantle phase transformations have come primarily from studies of analogue systems reacted experimentally at low pressures. In order to study transformation mechanisms in Mg2SiO4 at mantle pressures, forsterite (α) has been reacted in the stability field of β-phase, at 15 GPa and temperatures up to 900° C, using a multianvil split-sphere apparatus. Transmission electron microscope studies of samples reacted for times ranging from 0.25–5.0 h show that forsterite transforms to β-phase by an incoherent nucleation and growth mechanism involving nucleation on olivine grain boundaries. This mechanism and the resultant microstructures are very similar to those observed at much lower pressures in analogue systems (Mg2GeO4 and Ni2SiO4) as the result of the olivine to spinel (α→γ) transformation. Metastable spinel (γ) also forms from Mg2SiO4 olivine at 15 GPa, in addition to γ-phase, by the incoherent nucleation and growth mechanism. With time, the spinel progressively transforms to the stable β-phase. After 1 h, spinels exhibit a highly striated microstructure along {110}γ and electron diffraction patterns show streaking parallel to [110]γ which indicates a high degree of structural disorder. High resolution imaging shows that the streaking results from thin lamellae of β-phase intergrown with the spinel. The two phases have the orientation relationship [001]β//[001]γ and [010]β//[110]γ so that the quasi cubic-close-packed oxygen sublattices are continuous between both phases. These microstructures are similar to those observed in shocked meteorites and show that spinel transforms to β-phase by a martensitic (shear) mechanism. There is also evidence that the mechanism changes to one involving diffusion-controlled growth at conditions close to equilibrium.

Mechanism of the γ–β phase transformation of Mg2SiO4 at high temperature and pressure

THE transformation of Mg1.8Fe0.2SiO4 olivine first to the β-phase (modified spinel) and then to the γ-spinel phase occurs with increasing depth in the Earth's mantle as a result of the increasing pressure1,2. The mechanisms of these two transformations have an important influence on mantle rheology and may also be related to the origin of deep-focus earthquakes3–8. We report here the results of experiments on the phase transformation of Mg2SiO4 olivine at 15 GPa pressure in a multi-anvil cell. At this pressure and a temperature of 900 °C, early formed metastable γ-spinel transforms partially to the β-phase. The observed microstructures, which are similar to those in shocked meteorites9–11, show that the γ-to-β transformation can occur either by diffusion-controlled growth or by a martensitic (shearing) mechanism, depending on how far the pressure-temperature conditions deviate from their values at phase equilibrium. Our results suggest that the diffusion-controlled mechanism is most likely to operate at the β/γ phase boundary in the mantle, but a martensitic β-to-γ transformation might occur in subduction zones and could reduce the shear strength of the subducting slab

Why olivine transforms to spinel at high pressure

Because of its importance in understanding the static and dynamic properties of the Earth's mantle, the olivine to spinel structure transformation is one of the most frequently studied crystal structure changes. The volume change is considerable (∼7–10%, depending on the compound involved) but the reason for this is unclear. In most high-pressure transformations, the volume contraction is accompanied by an increase in the primary coordination number of the atoms. However, there is no such increase in the case of the transformation from the olivine to the spinel structure. Both are A2BX4 compounds (for example, Mg2SiO4 olivine and Al2MgO4 spinel) with cation coordinations AX6 (octahedral) and BX4 (tetrahedral) and anion coordinations XA3B (tetrahedral). Indeed the conventional description of both structures starts with approximately ‘close-packed’ (‘eutactic’1) anion arrays (hexagonal for olivine and cubic for spinel) with cations in one half of the octahedral interstices and one eighth of the tetrahedral interstices, suggesting that the volumes of each structure should be very similar for a given compound. Here we show that a less conventional, but more appropriate description of the structures resolves the volume problem and also sheds light on the crystal chemistry of these and related structures.

Electrostatic energies of polymorphs of M2SiO4 stoichiometry (M=Ni, Mg, Co, Fe and Mn).

Electrostatic energies of olivine, modified spinel and spinel with M2SiO4 stoichiometry (M=Ni, Mg, Co, Fe and Mn) and those of their constituent ions were calculated by the method of Bertaut, to obtain better understanding on each polymorph; the stability relations under high pressures and the effect of ionic size on the electrostatic stability.Stability relations of these polymorphs can be explained on the basis of proposed criteria; minerals with high repulsive energy and “simple” energetics only can survive under high pressures, where the “simple” energetics means that the electrostatic energies of constituent ions do not differ greatly from each other. Olivine does not have high repulsive energy, but involves a complex energetics. It cannot, therefore, survive under high pressures. Instability of modified spinel under very high pressures can be explained also in terms of its complex energetics. Spinel has high repulsive energy and does not involve a complex energetics. It is, therefore, stable under high pressures.Electrostatic energies plotted against ionic radius ratio rM⁄rSi reveal the effect of ionic size on crystal energetics as follows : Electrostatic energies of M ion, O ion and the whole crystal become higher, whereas that of Si ion becomes lower with an increase of ionic radius ratio. Electrostatic energy of Mg2SiO4 olivine is slightly different from the trend of those of transition metal olivines. A rapid increase in electrostatic instability of olivine structure is observed at the ionic radius of Mn.