The incorporation of water in high-pressure minerals is essential for the water cycle within the interiors of terrestrial planets. Majoritic garnet, a major component in the mantles of Earth and Mars, plays a significant role in this context. In this study, we use first-principles simulations to explore water incorporation mechanisms in MgSiO3-majorite, which is a key end-member of majoritic garnet, at conditions up to 2,000 K and 20 GPa. By dealing with the relationship between chemical potential and the Gibbs free energy changes for the reactions at equilibrium conditions, we determine the ratios of the seven potential hydrous defects. Our results reveal that the Si2 and Si3 defects, which are of the hydrogarnet-type, dominate water incorporation in MgSiO3-majorite. In addition, we evaluate the effects of these hydrous defects on seismic wave velocities. The presence of Si2 and Si3 defects, with an expected water concentration of similar to 700 ppm, has a small effect on both P-wave and S-wave velocities. Nevertheless, the influence of water on lateral variations in the seismic wave velocities of MgSiO3-majorite, which is opposite to that found for ringwoodite, offers a potential tool for investigating compositional heterogeneities in hydrated regions of planetary mantles.

Seismic observations show the Earth's inner core has significant and unexplained variation in seismic attenuation with position, depth and direction. Interpreting these observations is difficult without knowledge of the visco- or anelastic dissipation processes active in iron under inner core conditions. Here, a previously unconsidered attenuation mechanism is observed in zinc, a low pressure analog of hcp-iron, during small strain sinusoidal deformation experiments. The experiments were performed in a deformation-DIA combined with X-radiography, at seismic frequencies (similar to 0.003-0.1 Hz), high pressure and temperatures up to similar to 80% of melting temperature. Significant dissipation (0.077 <= Q(-1)(omega) <= 0.488) is observed along with frequency dependent softening of zinc's Young's modulus and an extremely small activation energy for creep (<= 7 kJ mol(-1)). In addition, during sinusoidal deformation the original microstructure is replaced by one with a reduced dislocation density and small, uniform, grain size. This combination of behavior collectively reflects a mode of deformation called "internal stress superplasticity"; this deformation mechanism is unique to anisotropic materials and activated by cyclic loading generating large internal stresses. Here we observe a new form of internal stress superplasticity, which we name as "elastic strain mismatch superplasticity." In it the large stresses are caused by the compressional anisotropy. If this mechanism is also active in hcp-iron and the Earth's inner-core it will be a contributor to inner-core observed seismic attenuation and constrain the maximum inner-core grain-size to less than or similar to 10 km.

The D″ region of the lower mantle, which lies just above the core–mantle boundary, is distinct from the bulk of the lower mantle in that it exhibits complex seismic heterogeneity and seismic anisotropy. Seismic anisotropy in this region is likely to be largely due to the deformation-induced texture (crystallographic preferred orientation) development of the constituent mineral phases. Thus, seismic anisotropy can provide a marker for deformation processes occurring in this dynamic region of the Earth. Post-perovskite-structured (Mg,Fe)SiO3 is believed to be the dominant mineral phase in many regions of the D”. As such, understanding deformation mechanisms and texture development in post-perovskite is important for the interpretation of observed seismic anisotropy. Here, we report on high-pressure diamond anvil cell deformation experiments on NaMgF3 neighborite (perovskite structure) and post-perovskite. During deformation, neighborite develops a 100 texture, as has been previously observed, both in NaMgF3 and MgSiO3 perovskite. Upon transformation to the post-perovskite phase, an initial texture of {130} at high angles to compression is observed, indicating that the {100} planes of perovskite become the ~{130} planes of post-perovskite. Further compression results in the development of a shoulder towards (001) in the inverse pole figure. Plasticity modeling using the elasto-viscoplastic self-consistent code shows this texture evolution to be most consistent with deformation on (001)[100] with some contribution of glide on (100)[010] and (001)<110> in NaMgF3 post-perovskite. The transformation and deformation mechanisms observed in this study in the NaMgF3 system are consistent with the behavior generally observed in other perovskite–post-perovskite systems, including the MgSiO3 system. This shows that NaMgF3 is a good analog for the mantle bridgmanite and MgSiO3 post-perovskite.

The equations of state and band-gap closures for PbCl2 and SnCl2 were studied using both experimental and theoretical methods. We measured the volume of both materials to a maximum pressure of 70 GPa using synchrotron-based angle-dispersive powder x-ray diffraction. The lattice parameters for both compounds showed anomalous changes between 16-32 GPa, providing evidence of a phase transition from the cotunnite structure to the related Co2Si structure, in contrast to the postcotunnite structure as previously suggested. First-principles calculations confirm this finding and predict a second phase transition to a Co2Si-like structure between 75- 110 GPa in PbCl2 and 60-75 GPa in SnCl2. Band gaps were measured under compression to -70 GPa for PbCl2 and -66 GPa for SnCl2 and calculated up to 200 GPa for PbCl2 and 120 GPa for SnCl2. We find an excellent agreement between our experimental and theoretical results when using the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional, which suggests that this functional could reliably be used to calculate the band gap of similar AX2 compounds. Experimental and calculated band-gap results show discontinuous decreases in the band gap corresponding to phase changes to higher-coordinated crystal structures, giving insight into the relationship between interatomic geometry and metallicity.

The iron spin crossover in ferropericlase, the second most abundant mineral in Earth’s lower mantle, causes changes in a range of physical properties, including seismic wave velocities. Understanding the effect of temperature on the spin crossover is essential to detect its signature in seismic observations and constrain its occurrence in the mantle. Here, we report the first experimental results on the spin crossover-induced bulk modulus softening at high temperatures, derived directly from time-resolved x-ray diffraction measurements during continuous compression of (Mg0.8Fe0.2)O in a resistive-heated dynamic diamond-anvil cell. We present new theoretical calculations of the spin crossover at mantle temperatures benchmarked by the experiments. Based on our results, we create synthetic seismic tomography models to investigate the signature of the spin crossover in global seismic tomography. A tomographic filter is applied to allow for meaningful comparisons between the synthetic models and data-based seismic tomography models, like SP12RTS. A negative anomaly in the correlation between Vs variations and Vc variations (S-C correlation) is found to be the most suitable measure to detect the presence of the spin crossover in tomographic models. When including the effects of the spin crossover, the misfit between the synthetic model and SP12RTS is reduced by 63%, providing strong evidence for the presence of the spin crossover, and hence ferropericlase, in the lower mantle. Future improvement of seismic resolution may facilitate a detailed mapping of spin state using the S-C correlation, providing constraints on mantle temperatures, thanks to the temperature sensitivity of the spin crossover.

The elastic bulk modulus softening of (Mg,Fe)O ferropericlase across the iron spin crossover induces dramatic changes in its physical properties, including seismic P‐velocities and viscosity. Here, we performed compression of powders of (Mg 0.8‐0.9 Fe 0.2‐0.1 )O in a piezo‐driven dynamic Diamond Anvil Cell (dDAC) and derive the bulk modulus by differentiation of pressure and volume data, providing first data on the broadness of the elastic softening for ferropericlase with mantle‐relevant compositions. We complement our experimental results with theoretical calculations that extend previous studies by considering multiple random configurations of iron, and going beyond treating high‐ and low‐spin iron as an ideal solution. Both experiments and computations show a broad and asymmetric softening of the bulk modulus, and suggest that the softening is sensitive to the distribution of iron in the ferropericlase structure. Our high‐temperature calculations show that mixed‐spin (Mg,Fe)O dominates the lower mantle at all depths below 1,000 km. In contrast to most previous works, we find that ferropericlase will not exist in pure low‐spin state along a typical mantle geotherm. Based on our model, the physical properties of ferropericlase will show significant lateral variation at depths below 1,400 km, with the strongest effects expected between 2,000 and 2,600 km.

Rare earth elements are helping drive the global transition towards a greener economy. However, the way in which they are produced is far from being considered green. One of the major obstacles to developing greener production methods and the design of novel processes and materials involving rare earth elements is the limited thermodynamic data available. In the present work, we apply a suite of methods to estimate the enthalpy of formation of several rare earth compounds, including a new method based on a linear relationship, established by the authors. Experimental values of the enthalpy of formation of LnCl3, LnOCl, LnPO4, Ln2O2S, Ln2O2CO3 and NaLnO2 were collated and used to assess the accuracy of the different methods, which were then used to predict values for compounds for which no data exists. It is shown that Mostafa et al.'s group contribution method and the linear relationship proposed by the authors give the lowest mean absolute error (<9%). The volume based thermodynamics (VBT) method yields estimates with absolute mean errors below 16.0% for LnPO4 and Ln2O2S, but above 26.0% for other compounds. Correction of the VBT method using an improved estimate of the Madelung energy for the calculation of the lattice enthalpy decreases the absolute mean error below 12.0% for all compounds except LnPO4. These complementary methods provide options for calculating the enthalpy of formation of rare earth compounds, depending on the experimental data available and desired accuracy.

The Earth’s inner core exhibits significant anisotropy in both seismic velocity and attenuation as well as hemispherical and depth variations. These observations point to an inner core that is both complex and dynamic. However, interpretation of these observations without knowledge of the attenuation processes active in the inner-core is difficult. To address this we have used zinc, as a low-pressure analogue of the hexagonal close pack (hcp) structured iron that forms the inner core, to provide first-order constraints on the anelasticity of hcp metals at seismic frequencies and high temperatures. A D-DIA apparatus was combined with X-radiography were utilised to measure the anelastic response of zinc up to a homologous temperature (T/Tm) of ≈ 0.8. To analyse the data we developed an improved image processing method that reduces systematic errors and gives up to 3 orders of magnitude improvement in strain measurement precision. Using this algorithm, in the frequency range 0.1 to 0.003Hz, significant dissipation and softening of zinc’s Young’s modulus is observed. The softening occurs in the absence of significant impurities or a fluid phase and appears to be caused by, or related to, recrystallisation of the samples in response to the stress. The recrystallisation results in a steady-state grain-size and low dislocation density. The predicted reduction in shear wave speed is 2-3 times greater than that of for compressional waves, which is consistent with anelasticity playing a significant role in the seismic velocity of the inner core. Therefore anelastic effects in hcp iron must be considered in the interpretation of inner-core.

As the major component of garnet, the second most abundant phase in Earth’s transition zone, MgSiO3-majorite (Mgmj) may be an important reservoir of water. Previous works at ambient conditions show that water is incorporated in Mgmj by substituting Si with H in the Si-O tetrahedra. Due to the challenges of experiments and simulations, there are still very limited data on hydrous MgSiO3-majorite at simultaneously high temperatures and pressures. In this study, we have carried out extensive first principles calculations to determine the properties of hydrous MgSiO3-majorite up to 2000 K and 40 GPa. We systematically considered all possible incorporation mechanisms with seven substitutional reactions. By solving the equations of equilibrium constants of the reactions, we obtained the ratios of various polyhedral substitutions and their temperature and pressure dependences. Our results show that the occupations of water (hydrogen) in Mgmj change mainly with temperature and are generally pressure-independent. Almost all of hydrogen occupy in the Si-O tetrahedra at 300 K, but the ratio of Mg-O dodecahedra increases remarkably with temperature and is the highest at 2000 K, which suggests that water in Mgmj under mantle conditions would be different from that observed at ambient conditions and implies the importance of in-situ measurements of high-TP experiments. We have also calculated the elastic velocities and anisotropies of two types of hydrous Mgmj which caused by substitutions of Mg and Si. Our results show that ~1 wt% water in Si vacancy would decrease wave velocities of Mgmj by ~3% in both VP and VS, while the same amount of water in Mg vacancy would more effectively decrease the velocities by ~6%. In addition, water in Mg vacancy would noticeably improve the seismic anisotropy and the dlnVs/dlnVP of Mgmj while water in Si vacancy shows much smaller effects.

Constraining the flux of carbon in and out of the interior of the Earth due to long-term geological processes is important, because of the influence that it has on climate change. On timescales of billions of years, host minerals such as carbonate phases could play a significant role in the global carbon cycle, transporting carbon into the lower mantle as a component of subducting slabs. We use density functional theory based calculations to study the high-pressure, high-temperature phase stability of Mg 1-x Fe x CO 3 . Our results show that iron-rich phases, where carbon is in tetrahedral coordination, are only stable at lower mantle conditions due to their magnetic entropy, which is also responsible for the unusual shape of their phase boundary. Low-pressure carbonate phases are found to be highly anisotropic, but high-pressure carbonate phases are not, which has important implications for their seismic detectability. Our work confirms that future discussions of the global carbon cycle should include the deep Earth.

Constraining the flux of carbon in and out of the interior of the Earth due to long-term geological processes is important, because of the influence that it has on climate change. On timescales of billions of years, host minerals such as carbonate phases could play a significant role in the global carbon cycle, transporting carbon into the lower mantle as a component of subducting slabs. We use density functional theory based calculations to study the high-pressure, high-temperature phase stability of Mg1-xFexCO3. Our results show that iron-rich phases, where carbon is in tetrahedral coordination, are only stable at lower mantle conditions due to their magnetic entropy, which is also responsible for the unusual shape of their phase boundary. Low-pressure carbonate phases are found to be highly anisotropic, but high-pressure carbonate phases are not, which has important implications for their seismic detectability. Our work confirms that future discussions of the global carbon cycle should include the deep Earth. (C) 2019 The Authors. Published by Elsevier B.V.

As the major component of garnet, the second most abundant phase in Earth's transition zone, MgSiO3-majorite plays a fundamental role in controlling the state and dynamics of Earth's mantle. However, due to challenges of experiments and simulations, there are still very limited data on the elastic properties of MgSiO3-majorite at simultaneously high temperatures and pressures. In this study, we have carried out extensive first principles calculations to determine the thermoelastic properties of MgSiO3-majorite up to 2000 K and 40 GPa. We find that the elastic constants of MgSiO3-majorite change significantly over the temperature and pressure range studied, with noticeable non-linearities in their pressure dependences. The seismic anisotropy of MgSiO3-majorite is high and generally increases with pressure. It is much higher than that of the other end-members of garnet and ringwoodite, which makes it the most anisotropic mineral in assemblages expected in the lower transition zone. Based on our calculated elastic moduli and with careful elimination of systematic errors, we establish a third-order Birch-Murnaghan-Mie-Gruneisen model for MgSiO3-majorite with the parameters: V-0 = 114.1 cm(3)/mol, K-0 = 163.6 GPa, G(0) = 86.4 GPa, K-0' = 4.44, G(0)' = 1.16, gamma(0) = 1.08, q(0) = 0.48, eta(S0) = 0.76, and theta(0) = 822.5 K. Integrating our results into a thermodynamic model able to predict the properties of mantle assemblages, we find that a pyrolite composition produces velocities that agree with the seismic model AK135 in the upper transition zone. In the lower transition zone, a pyrolite composition fits well with some specific local observations, but a mechanical mixture with 18% basalt and 82% harzburgite is in better agreement with the global seismic model PREM. The much larger abundance of MgSiO3-majorite in the garnet phase of harzburgite suggests that the anisotropy in the lower transition zone may not be negligible and would be observable at least in the heterogeneous zones near subducting slabs.

A better understanding of the solution chemistry of the lanthanide (Ln) salts in water would have wide ranging implications in materials processing, waste management, element tracing, medicine and many more fields. This is particularly true for minerals processing, given governmental concerns about lanthanide security of supply and the drive to identify environmentally sustainable processing routes. Despite much effort, even in simple systems, the mechanisms and thermodynamics of Ln(III) association with small anions remain unclear. In the present study, molecular dynamics (MD), using a newly developed force field, provide new insights into LnCl(3)(aq) solutions. The force field accurately reproduces the structure and dynamics of Nd3+, Gd3+ and Er3+ in water when compared to calculations using density functional theory (DFT). Adaptive-bias MD simulations show that the mechanisms for ion pairing change from dissociative to associative exchange depending upon cation size. Thermodynamics of association reveal that whereas ion pairing is favourable, the equilibrium distribution of species at low concentration is dominated by weakly bound solvent-shared and solvent-separated ion pairs, rather than contact ion pairs, reconciling a number of contrasting observations of Ln(III)-Cl association in the literature. In addition, we show that the thermodynamic stabilities of a range of inner sphere and outer sphere LnClx(3-x)+ coordination complexes are comparable and that the kinetics of anion binding to cations may control solution speciation distributions beyond ion pairs. The techniques adopted in this work provide a framework with which to investigate more complex solution chemistries of cations in water.