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Water bound to various hydrous minerals can be transported deep into the mantle by slab subduction. Serpentine is one of most important hydrous minerals in the crust and shallow upper mantle. A partially serpentinized slab mantle limits the amount of water that can enter deep into the mantle. The partitioning of water between hydrous minerals and nominally anhydrous minerals (NAMs) is a crucial factor in controlling the physical properties and dynamics of slabs. Recent experiments on water partitioning have revealed that water strongly partitions to coexisting hydrous minerals. NAMs, such as olivine and its high-pressure polymorphs, have limited water content in water-undersaturated wet slabs. Metastable olivine wedges are not a feature of dry slabs, but can be of wet slabs that are not saturated with water. The transformation kinetics of the dry state, which generates deep-focus earthquakes and produces significant deformation in the slab, can work under wet slabs. Water bound to hydrous minerals is transported by the slab to the mantle transition zone and lower mantle. Hydrous minerals in stagnant slabs over 660 km depth release water as the slab warms, producing locally hydrated mantle transition zones and dense water-bearing magmas at the base of the upper mantle, and generating intraplate volcanism, which are referred to as the big mantle wedge model. Seismic scatterers are observed in the lower mantle at depths from 700 to 1900 km. These scatterers may be caused by water release at the top of the lower mantle by dehydration of hydrous minerals such as dense hydrous magnesium silicates. The shear instability due to the second order phase transformation from stishovite to CaCl2-type phase in hydrous aluminous SiO2 also causes the depth variation of seismic scatterers in the lower mantle. The high-pressure polymorphs of aluminous SiO2 contain a large amount of water more than 1 wt%, which can be important water carriers under lower mantle conditions. [ABSTRACT FROM AUTHOR]

Abstract Water bound to various hydrous minerals can be transported deep into the mantle by slab subduction. Serpentine is one of most important hydrous minerals in the crust and shallow upper mantle. A partially serpentinized slab mantle limits the amount of water that can enter deep into the mantle. The partitioning of water between hydrous minerals and nominally anhydrous minerals (NAMs) is a crucial factor in controlling the physical properties and dynamics of slabs. Recent experiments on water partitioning have revealed that water strongly partitions to coexisting hydrous minerals. NAMs, such as olivine and its high-pressure polymorphs, have limited water content in water-undersaturated wet slabs. Metastable olivine wedges are not a feature of dry slabs, but can be of wet slabs that are not saturated with water. The transformation kinetics of the dry state, which generates deep-focus earthquakes and produces significant deformation in the slab, can work under wet slabs. Water bound to hydrous minerals is transported by the slab to the mantle transition zone and lower mantle. Hydrous minerals in stagnant slabs over 660 km depth release water as the slab warms, producing locally hydrated mantle transition zones and dense water-bearing magmas at the base of the upper mantle, and generating intraplate volcanism, which are referred to as the big mantle wedge model. Seismic scatterers are observed in the lower mantle at depths from 700 to 1900 km. These scatterers may be caused by water release at the top of the lower mantle by dehydration of hydrous minerals such as dense hydrous magnesium silicates. The shear instability due to the second order phase transformation from stishovite to CaCl2-type phase in hydrous aluminous SiO2 also causes the depth variation of seismic scatterers in the lower mantle. The high-pressure polymorphs of aluminous SiO2 contain a large amount of water more than 1 wt%, which can be important water carriers under lower mantle conditions.

We study the interplay of the thermal structure within subducted oceanic slabs together with the stability fields of hydrous phases that control slab dehydration and the amount of water transported into the deeper mantle. We implement different published thermodynamic data for phase A and Mg‐sursassite into a model of 56 subduction zones and evaluate the effect of these phases on the global water budget. We modeled vertical fluid fluxes within the slab such that dehydration‐derived fluid was allowed to react with fluid‐undersaturated rocks in other parts of the slab. The effect of Mg‐sursassite on the global water budget is limited, because the Clapeyron slope of this dehydration reaction is steeper than most pressure‐temperature trajectories in subduction zones. Two sets of published thermodynamic data for phase A yield significantly different values for the amount of deeply subducted water, ranging between 8 × 108 Tg/Ma and 1.4 × 109 Tg/Ma. In some subduction zones, the differences span several orders of magnitude. The absolute modeled amount of deeply subducted water strongly depends on the depth and intensity of slab mantle hydration, but a comparison of modeled and experimental data indicates that the thermodynamic dataset that yielded higher values is more reliable and should be implemented in future thermodynamic models. Our results show that the stable phases around the choke point as well as the slope and position of the phase A‐out reaction influence the deep water release from the slab, but the slope and position of the phase A dehydration reaction mainly control the recycling of water into the deep mantle. Plain Language Summary: Water on our planet is constantly exchanged between the oceans and the interior of the Earth. This happens at subduction zones where hydrated oceanic crust is sinking into the Earth's mantle, thus bringing water to depths exceeding 300 km. We quantify this amount of water that is globally recycled into the deeper Earth in subduction zones through computer models that combine the temperature structure of the subducted rocks with the stability of hydrous phases. Knowledge about the global water budget is important to quantify the Earth's total water amount, to understand the water release from the subducting oceanic plate, and to interpret sea level changes in Earth's history. We compared thermodynamic datasets for two minerals, namely Mg‐sursassite and phase A, in the oceanic plate that can transfer large amounts of water into the deeper Earth. Our thermodynamic models show that the stability of phase A in the down going oceanic plate controls much of the recycled water in most subduction zones. Key Points: Mg‐sursassite has a limited influence on the global amount of subducted waterMg‐sursassite can transfer water to depths of up to 250 km and lead to deep dehydration and water release from the slabThe Clapeyron slope of the phase A dehydration reaction is sub‐parallel to many slab P‐T paths and mainly controls the deep water subduction [ABSTRACT FROM AUTHOR]

Abstract We study the interplay of the thermal structure within subducted oceanic slabs together with the stability fields of hydrous phases that control slab dehydration and the amount of water transported into the deeper mantle. We implement different published thermodynamic data for phase A and Mg‐sursassite into a model of 56 subduction zones and evaluate the effect of these phases on the global water budget. We modeled vertical fluid fluxes within the slab such that dehydration‐derived fluid was allowed to react with fluid‐undersaturated rocks in other parts of the slab. The effect of Mg‐sursassite on the global water budget is limited, because the Clapeyron slope of this dehydration reaction is steeper than most pressure‐temperature trajectories in subduction zones. Two sets of published thermodynamic data for phase A yield significantly different values for the amount of deeply subducted water, ranging between 8 × 108 Tg/Ma and 1.4 × 109 Tg/Ma. In some subduction zones, the differences span several orders of magnitude. The absolute modeled amount of deeply subducted water strongly depends on the depth and intensity of slab mantle hydration, but a comparison of modeled and experimental data indicates that the thermodynamic dataset that yielded higher values is more reliable and should be implemented in future thermodynamic models. Our results show that the stable phases around the choke point as well as the slope and position of the phase A‐out reaction influence the deep water release from the slab, but the slope and position of the phase A dehydration reaction mainly control the recycling of water into the deep mantle.

Due to its large thermal stability, Al-phase D, the (Al,Fe3+)2SiO6H2 member of the dense hydrous magnesium silicate (DHMS) phase D, may survive along hot subduction geotherms or even at ambient mantle temperatures in the Earth's transition zone and lower mantle, therefore potentially playing a major role as a water reservoir and carrier in the Earth's interior. We have investigated the crystal structure and high-pressure behavior of Fe-bearing Al-phase D with a composition of Al1.53(2)Fe0.22(1) Si0.86(1)O6H3.33(9) by means of single-crystal X-ray diffraction. While the structure of pure Al-phase D (Al2SiO6H2) has space group P63/mcm and consists of equally populated and half-occupied (Al,Si)O6 octahedra, Fe-incorporation in Al-phase D seems to induce partial ordering of the cations over the octahedral sites, resulting in a change of the space group from P63/mcm to P6322 and in well-resolved diffuse scattering streaks observed in X-ray images. The evolution of the unit-cell volume of Fe-bearing Al-phase D between room pressure and 38 GPa, determined by means of synchrotron X-ray diffraction in a diamond anvil cell, is well described by a third-order Birch-Murnaghan equation of state having an isothermal bulk modulus KT0 = 166.3(15) GPa and first pressure derivative K′T0 = 4.46(12). Above 38 GPa, a change in the compression behavior is observed, likely related to the high-to-low spin crossover of octahedrally coordinated Fe3+. The evolution of the unit-cell volume across the spin crossover was modeled using a recently proposed formalism based on crystal-field theory, which shows that the spin crossover region extends from approximately 30 to 65 GPa. Given the absence of abrupt changes in the compression mechanism of Fe-bearing Al-phase D before the spin crossover, we show that the strength of H-bonds and likely their symmetrization do not greatly afect the elastic properties of phase D solid solutions, independently of their compositions. [ABSTRACT FROM AUTHOR]

Oxyhydroxides like δ‐(Al,Fe)OOH may stabilize "water" in Mid‐Ocean Ridge Basalt (MORB) subducted into the Earth's lower mantle. The single‐crystal elasticity of δ‐(Al,Fe)OOH has not been experimentally constrained, hampering an accurate evaluation of the seismic detectability of this high‐pressure solid solution, and the presence of "water," in the deep Earth. Here, we report the first experimental single‐crystal elasticity results of δ‐(Al0.97Fe0.03)OOH measured by X‐ray diffraction and Brillouin spectroscopy. We use our results to compute seismic properties of hydrous and anhydrous MORB at pressures and temperatures expected in slabs at shallow lower mantle conditions. We show that hydrous MORB is less dense than anhydrous MORB, but has faster aggregate seismic velocities. This suggests that hydration in MORB has an effect on velocities opposite to that observed in other lithologies, and further indicates that hydration of MORB increases the seismic contrast to the background mantle. Plain Language Summary: Water can be delivered into the Earth's mantle via subduction of hydrous phases, playing an active role in global‐scale geological processes. Hence, tracking hydrous phases during subduction is pivotal to understand the geological evolution of our planet. At lower mantle depths, oxyhydroxides such as δ‐(Al,Fe)OOH are the primary hosts of water in subducted oceanic crust. Here, we investigated the single‐crystal elasticity of δ‐(Al0.97Fe0.03)OOH at pressures consistent with subduction in the upper mantle and transition zone. Our experimental results were used to evaluate the impact that hydration has on the physical properties of oceanic crust subducted into the lower mantle. For this purpose, we modeled density and aggregate properties of hydrous and anhydrous oceanic crust subducted into the lower mantle. Our modeling shows that hydration induces a reduction in the density of the modeled lithologies, but increases their aggregate velocities. Our results suggest that hydration may influence the subducting behavior of oceanic crust as well as their detectability through seismology. Key Points: Brillouin spectroscopy and X‐ray diffraction provide first constraints on the single‐crystal elasticity of δ‐(Al,Fe)OOH at mantle pressuresHydration decreases density but increases aggregate velocities of MORB in the shallow lower mantleHydration of MORB increases the seismic velocity contrast with the background mantle [ABSTRACT FROM AUTHOR]

In the past three years, Dirac half-metals (DHMs) have attracted considerable attention and become a high-profile topic in spintronics becuase of their excellent physical properties such as 100% spin polarization and massless Dirac fermions. Two-dimensional DHMs proposed recently have not yet been experimentally synthesized and thus remain theoretical. As a result, their characteristics cannot be experimentally confirmed. In addition, many theoretically predicted Dirac materials have only a single cone, resulting in a nonlinear electromagnetic response with insufficient intensity and inadequate transport carrier efficiency near the Fermi level. Therefore, after several attempts, we have focused on a novel class of DHMs with multiple Dirac crossings to address the above limitations. In particular, we direct our attention to three-dimensional bulk materials. In this study, the discovery via first principles of an experimentally synthesized DHM LaNiO3 with many Dirac cones and complete spin polarization near the Fermi level is reported. It is also shown that the crystal structures of these materials are strongly correlated with their physical properties. The results indicate that many rhombohedral materials with the general formula LnNiO3 (Ln = La, Ce, Nd, Pm, Gd, Tb, Dy, Ho, Er, Lu) in the space group R3c are potential DHMs with multiple Dirac cones.

The elastic properties of Mg2.12(2)Fe0.21(2)Ni0.01Si1.15(1)O6H2.67(8) phase E single crystals with Fe3+/ΣFe = 0.25(3) have been determined by Brillouin spectroscopy at ambient conditions. We find that that the elasticity of iron-bearing phase E is described by the six independent stiffness tensor components (all in units of GPa): C11 = 192.2(6), C12 = 56.4(8), C13 = 43.5(8), C14 = –4.3(3), C33 = 192.1(7), C44 = 46.4(3). The Voigt-Reuss-Hill averages of bulk and shear moduli are 95.9(4) and 59.6(2) GPa, respectively. The aggregate velocities of iron-bearing phase E are vP = 7.60(2) and vS = 4.43(1) km/s, markedly lower than those of major mantle minerals at ambient conditions. Modeling based on our results suggests that the presence of iron-bearing phase E may reduce the sound wave velocities in upper mantle and transition zone rocks, making it a possible target for future seismological investigations aiming to map hydration in subducting slabs. [ABSTRACT FROM AUTHOR]

Phase H (MgSiO4H2) is considered an important carrier of water into the lower mantle by the subduction of slabs. This phase has been reported to decompose into H2O ice VII and MgSiO3 bridgmanite under pressure. However, the dissociation phase boundary under the mantle pressure and temperature conditions has not been determined thus far. In this work, the dissociation phase boundary of phase H is determined by the calculation of Gibbs free energy of H2O ice VII. The stability field of phase H is found to be significantly extended from 52 to 62 GPa by the inclusion of zero‐point vibrational energy. Phase H decomposes into MgSiO3 bridgmanite and H2O ice VII at approximately 60 GPa (at ∼1000 K). This result indicates that the transportation of water by dense hydrous magnesium silicates may be terminated at a depth of approximately 1,500 km in the middle of the lower mantle in a pure Mg‐endmember composition. Key Points: The dissociation phase boundary of phase H MgSiO4H2 is determined by calculation of Gibbs free energy of ice VIIPhase H decomposes into MgSiO3 bridgmanite and H2O ice VII at approximately 60 GPaThe transportation of water by DHMSs may be terminated at a depth of approximately 1,500‐km depth in a pure Mg‐endmember composition [ABSTRACT FROM AUTHOR]

Phase H (MgSiO4H2) is considered an important carrier of water into the lower mantle by the subduction of slabs. This phase has been reported to decompose into H2O ice VII and MgSiO3 bridgmanite under pressure. However, the dissociation phase boundary under the mantle pressure and temperature conditions has not been determined thus far. In this work, the dissociation phase boundary of phase H is determined by the calculation of Gibbs free energy of H2O ice VII. The stability field of phase H is found to be significantly extended from 52 to 62 GPa by the inclusion of zero‐point vibrational energy. Phase H decomposes into MgSiO3 bridgmanite and H2O ice VII at approximately 60 GPa (at ∼1000 K). This result indicates that the transportation of water by dense hydrous magnesium silicates may be terminated at a depth of approximately 1,500 km in the middle of the lower mantle in a pure Mg‐endmember composition. Key Points: The dissociation phase boundary of phase H MgSiO4H2 is determined by calculation of Gibbs free energy of ice VIIPhase H decomposes into MgSiO3 bridgmanite and H2O ice VII at approximately 60 GPaThe transportation of water by DHMSs may be terminated at a depth of approximately 1,500‐km depth in a pure Mg‐endmember composition [ABSTRACT FROM AUTHOR]

In the subducting oceanic lithosphere, a significant part of the seismicity is triggered in the mantle, especially along the upper and lower Wadati-Benioff planes. Several studies have investigated the potential involvement of dehydration reactions in the triggering mechanism of mantle earthquakes. Recent experimental results reveal that, under subduction conditions, mechanical instabilities nucleate in strong stable mineral aggregates during the destabilization of minor amounts of antigorite, i.e. the high-temperature serpentine, through a stress transfer, without any fluid overpressure. Here I confront these laboratory results to seismological observations. On one hand, most of the natural hydrous magnesium silicates seem to be known, with experimentally deduced stability limits up to 7 GPa, at least, available as relatively accurate estimates. On the other hand, recent achievements in thermal structure of subduction zones combined with precise hypocentre relocation give access to pressure and temperature conditions at earthquakes hypocentres. A series of P-T diagrams summarizes the stability limit of minerals that may be part of natural peridotites or serpentinites with variable compositions at pressures from 0.5 to 6 GPa and temperatures from 200 to 950 °C, and compares it with seismicity. Both hydrous and anhydrous phases are considered. A myriad of minor metamorphic reactions could participate in a transformation-driven stress transfer, even if the stability limits of serpentine minerals seem to correlate with most of the observed seismicity. • Minor mineral phases play a major role in the triggering mechanism of mantle earthquakes. • A myriad of metamorphic reactions may participate in transformation-driven stress transfers within the subducting mantle. • Mantle seismicity within slabs is far from fitting the ideal dehydration curve of antigorite in a P-T diagram. • Considering variable paragenesis, chemistry and H2O saturation, serpentine stability limits correlate with most seismicity. • The lower Wadati-Benioff plane is due to both the dehydration reactions and the hydration limit of the subducting plate. • The upper plane of cold subducting mantle could be due to H2O transfer from low-grade serpentine minerals to antigorite. • The sharp seismicity increase at 90 km, 3 GPa, beneath Japan and Chile, would indicate H2O transfer to HP hydrous phases. [ABSTRACT FROM AUTHOR]

The interest of the scientific community on the role of Dense Hydrous Magnesium Silicates (DHMSs) as geochemical reservoir is constantly increasing, particularly for boron (B) which is widely considered as one of the best proxies to understand geological processes involving serpentinized materials. In particular, the genesis of blue B-bearing diamonds in the lower mantle has been proposed to be related to the deep recycling of serpentinized materials through DHMSs. However, it is still unknown whether DHMSs are capable to incorporate and transport significant amounts of B to great depth. In this Letter, we attempt to fill this knowledge gap by providing the results of a pilot experiment aimed to investigate the B content in Phase-A, which is the first DHMS mineral replacing antigorite along a prograde cold subduction path. By using a Walker-type Multi Anvil module, we synthetized Phase-A at 700 °C and 7.0 GPa starting from a stoichiometric composition modelled in the MSH system (2 SiO 2 : 7 MgO) and adding 1000 ppm of B as ultrapure boric acid (H 3 BO 3) in H 2 O plus trace of NH 4 OH. The experimental product has been characterized through electron microprobe, single-crystal X-ray diffraction, and laser ablation ICP-MS. Structural analyses confirmed the crystallization of the Phase-A and chemical analyses show B contents of ca. 200 ppm that are structurally incorporated in the Phase-A, thus suggesting its role as viable B source at depth. Our preliminary results extend the current knowledge of the B cycle and open new perspectives to better disclose the deep recycling of elements at the origin of the geochemical heterogeneity of the Earth's mantle. • Phase-A is synthetized at 7.0 GPa and 700 °C. • First report of B incorporation within Phase-A. • Phase-A can be a carrier of B in the Earth's mantle. [ABSTRACT FROM AUTHOR]

The 10 Å phase, Mg3Si4O10(OH)2·H2O, is a high-pressure hydrous phyllosilicate considered as an important link in the succession of hydrous phases transporting water into the deep mantle. In this study, in situ synchrotron X-ray diffraction combined with external heating diamond-anvil cell was used to determine limits of the 10 Å phase stability at pressures above 7 GPa. A reaction "10 Å phase + H2O → hydroxide-perovskite (3.65 Å phase) + stishovite" at about 10 GPa was found to be a high-pressure boundary of the 10 Å phase stability field. A dehydration temperature of the 10 Å phase "10 Å phase → enstatite + stishovite + H2O" decreases with pressure from 690 °C at 7 GPa to 450 °C at 10 GPa; a nonvariant point where 10 Å phase, hydroxide-perovskite and enstatite coexist in the presence of stishovite and hydrous fluid was found near 10 GPa and 450 °C. [ABSTRACT FROM AUTHOR]

The plasticity of the dense hydrous magnesium silicate (DHMS) phase A, a key hydrous mineral within cold subduction zones, was investigated by two complementary approaches: high-pressure deformation experiments and computational methods. The deformation experiments were carried out at 11 GPa, 400 and 580 °C, with in situ measurements of stress, strain and lattice preferred orientations (LPO). Based on viscoplastic self-consistent modeling (VPSC) of the observed LPO, the deformation mechanisms at 580 °C are consistent with glide on the (0 0 0 1) basal and ( 0 1 1 ¯ 0 ) prismatic planes. At 400 °C the deformation mechanisms involve glide on ( 2 ¯ 1 1 0 ) prismatic, (0 0 0 1) basal and { 1 1 2 ¯ 1 } pyramidal planes. Both give flow stresses of 2.5–3 GPa at strain rates of 2–4 × 10 −5 s −1 . We use the Peierls–Nabarro–Galerkin (PNG) approach, relying on first-principles calculations of generalized stacking fault (γ-surface), and model the core structure of potential dislocations in basal and prismatic planes. The computations show multiple dissociations of the 1 3 [ 2 1 ¯ 1 ¯ 0 ] and [ 0 1 1 ¯ 0 ] dislocations (〈 a 〉 and 〈 b 〉 dislocations) in the basal plane, which is compatible with the ubiquity of basal slip in the experiments. The γ-surface calculations also suggest 1 3 [ 2 1 ¯ 1 ¯ 3 ] and [ 0 1 ¯ 1 1 ] dislocations (〈 a + c 〉 or 〈 c − b 〉 directions) in prismatic and pyramidal planes, which is also consistent with the experimental data. Phase A has a higher flow strength than olivine. When forming at depths from the dehydration of weak and highly anisotropic hydrated ultramafic rocks, phase A may not maintain the mechanical softening antigorite can provide. The seismic properties calculated for moderately deformed aggregates suggest that S-wave seismic anisotropy of phase A-bearing rocks is lower than hydrous subduction zone lithologies such as serpentinites and blueschists. [ABSTRACT FROM AUTHOR]

Phase transformations in natural serpentine and chlorite have been studied at 25–50 GPa and 900–1280 °C by in-situ X-ray diffraction measurements using the multianvil apparatus with sintered diamond anvils. Antigorite was found to transform to a phase assemblage including phase H, at pressures above about 35–40 GPa corresponding to those in the upper part of the lower mantle. The zero pressure bulk modulus K 0 of phase H was determined to be 160 GPa, assuming its pressure derivative K ′ = 4. The thermal stability of phase H is significantly enhanced by a solid solution with a δ-AlOOH component, suggesting that aluminous phase H in cold slabs would deliver a certain amount of water into the deepest part of the lower mantle. [ABSTRACT FROM AUTHOR]

Electrical conductivities of dense hydrous magnesium silicates (DHMS), phase A, super-hydrous phase B (SuB) and phase D, were measured by means of impedance spectroscopy in the frequency range of 10−1–106 Hz at temperatures up to 775, 700 and 700K and pressures of 10, 18 and 22GPa, respectively. For all phases, the increase in electrical conductivity (σ) with temperature follows the Arrhenian formula: . The pre-exponential factors (σ 0) and activation enthalpies (ΔH) of phase A, SuB and phase D yield values of 7.28±0.82S/m and 0.77±0.01eV, 292±48S/m and 0.83±0.01eV and 1342±154 and 0.75±0.01eV, respectively. Higher pressure DHMS phases show higher conductivity values. The electrical conductivities of phase D and super hydrous phase B are about two and one orders of magnitude higher than that of phase A in the same temperature range, respectively. Although the proton conduction is considered to be a dominant mechanism, there is no clear relationship between water content and conductivity. Rather the conductivity of DHMS phase is closely related to the distance. The conductivity–depth profiles for a cold subduction zone were constructed based on the phase proportion predicted in the descending slab. The results show distinctly lower conductivity values than those geophysically observed beneath the northeastern China and the Philippine Sea, where the cold slab stagnates in the mantle transition zone. Consequently, the DHMS phases themselves cannot be a main contributor to enhance the conductivity in the stagnant slab. Dehydration of the stagnant slab would strongly enhance the conductivities in the transition zone beneath northeastern China and Philippine Sea. [ABSTRACT FROM AUTHOR]

First-principles calculations based on density-functional theory (DFT) and low-T IR spectroscopy were performed to gain more insight into the structure of the so-called 3.65 Å phase, a high-pressure phase of the composition MgSi(OH)6. DFT-calculations predict a monoclinic symmetry with ordered sixfold-coordinated Mg and Si and six unique hydrogen sites as the most stable structure. Adapting the structural parameters of the DFT-determined lowest-energy configuration and assuming (MgSi)-ordering, a new Rietveld refinement of the powder XRD pattern of the 3.65 Å phase was performed, which resulted in excellent refinement statistics and successful assignment of X-ray reflections that were unassigned in former structural models with orthorhombic symmetry. A configuration with ordered Mg and Si at the octahedral positions causes a small monoclinic distortion of the network of strongly tilted octahedra and thus leads to space group P21. The structural refinement yields the following unit-cell parameters: a = 5.1131(3), b = 5.1898(3), c = 7.3303(4) Å, β = 90.03(1)°, V = 194.52(2) ų, space group: P21, Z = 2, p = 2.637 g/cm³. The structure of the 3.65 Å phase can be considered as a modified A-site defective perovskite with a unique network of corner-sharing alternating Mg(OH)6 and Si(OH)6 octahedra and is probably related to the structure of stottite group minerals. Low-T IR spectroscopy confirms the presence of 6 different H-positions in the proposed structure. Measured IR-spectra and computed spectra compare favorably, which further supports the computed structure as the correct model for the 3.65 Å phase. [ABSTRACT FROM AUTHOR]

The 3.65 Å phase, a member of the family of dense hydrous magnesium silicates (DHMS) in the system MgO-SiO2-H2O, was synthesized in a 77 h multi-anvil press experiment at conditions of 10 GPa and 425 °C by using a gel of composition MgSiO3 plus water in excess as starting materials. From our multi-methodical study including SEM, TEM, EMP, IR, and Raman analysis, we determined the composition of the 3.65 Å phase to be MgSi(OH)6. Powder XRD combined with Rietveld refinement revealed the 3.65 Å phase to be isostructural with δ-Al(OH)3. The 3.65 Å phase can be described as a hydrous A-site vacant perovskite with probably long-range random distribution of Si and Mg at octahedral sites. Locally, some ordering of Mg and Si might exist, as indicated from the spectroscopic measurements. The 3.65 Å phase represents the second DHMS with exclusively sixfold-coordinated Si, the other being phase D. The 3.65 Å phase is stable at pressures above about 9.0 GPa and temperatures below 500 °C. This limited P-T stability together with its high water content makes the 3.65 Å phase an unrealistic mantle component. If at all, it might only occur under hydrous conditions in the coldest parts of deeply and extremely fast subducted old oceanic slabs. [ABSTRACT FROM AUTHOR]

A thermal equation of state of phase D with a Mg1.02Si1.73H3.03O6 composition has been derived from in situ X-ray diffraction experiments using synchrotron radiation and multianvil apparatus with sintered diamond anvils at pressures up to 46 GPa and temperatures up to 1300 K. The refined unit-cell parameters at ambient conditions are: a=4.7643(7) Å, c=4.346(2) Å, and V0=85.43(4)Å3 and the zero-pressure density of ρ0=3.357(2) g/cm3 (Z=1; formula weight=172.70 g/mol). At room temperature, the unit-cell parameters show anisotropic compression behavior up to ∼35 GPa, where the c-axis is more compressible than the a-axis. Above 35 GPa, however, the c-axis becomes less compressible than the a-axis. This change of the axial compression behavior tends to start at higher pressure with increasing temperature. The change of the linear compressibilities may be related to the hydrogen-bond symmetrization in phase D, as suggested by recent first-principles calculations. [ABSTRACT FROM AUTHOR]

Molecular dynamics (MD) modeling of the 10-Å phase, Mg3Si4O10(OH)2·xH2O, with x=2/3, 1.0 and 2.0 shows complex structural changes with pressure, temperature and water content and provides new insight into the structures and stabilization of these phases under subduction zone conditions. The structure(s) of this phase and its role as a reservoir of water in the mantle have been controversial, and these calculations provide specific predictions that can be tested by in situ diffraction studies. At ambient conditions, the computed structures of talc (x=0) and the 10-Å phases with x=2/3 and 1.0 are stable over the 350-ps period of the MD simulations. Under these conditions, the 10-Å phases show phlogopite-like layer stacking in good agreement with previously published structures based on powder X-ray diffraction data for samples quenched from high-pressure and high-temperature experiments. The calculations show that the 10-Å phase with x=2.0 is unstable at ambient conditions. The computed structures at P=5.5 GPa and T=750 K, well within the known stability field of the 10-Å phase, change significantly with water content, reflecting changing H-bonding configurations. For x=2/3, the layer stacking is talc-like, and for x=1.0, it is phlogopite-like. The calculations show that transformation between these two stackings occurs readily, and that the talc-like stacking for the x=2/3 composition is unlikely to be quenchable to ambient conditions. For x=2.0, the layer stacking at P=5.5 GPa and T=750 K is different than any previously proposed structure for a 10-Å phase. In this structure, the neighboring basal oxygens of adjacent magnesium silicate layers are displaced by b/3 (about 3 Å) resulting in the Si atoms of one siloxane sheet being located above the center of the six-member ring across the interlayer. The water molecules are located 1.2 Å above the center of all six-member rings and accept H-bonds from the OH groups located below the rings. The b/3-displaced structure does not readily transform to either the talc-like or phlogopite-like structure, because neither of these stackings can accommodate two water molecules per formula unit. There is likely to be a compositional discontinuity and phase transition between the b/3-displaced phase and the phase with phlogopite-like stacking. The simulations reported here are the first to use the recently developed CLAYFF force field to calculate mineral structures at elevated pressures and temperatures. [Copyright &y& Elsevier]

We conducted high-pressure phase equilibrium experiments in the systems MgSiO3 with 15 wt% H2O and Mg2SiO4 with 5 wt% and 11 wt% H2O at 20 ∼ 27 GPa. Based on the phase relations in these systems, together with the previous works on the related systems, we have clarified the stability relations of dense hydrous magnesium silicates in the system MgO-SiO2-H2O in the pressure range from 10 to 27 GPa. The results show that the stability field of phase G, which is identical to phase D and phase F, expands with increasing water contents. Water stored in serpentine in the descending cold slabs is transported into depths greater than 200 km, where serpentine decomposes to a mixture of phase A, enstatite, and fluid. Reaction sequences of the hydrous phases which appear at higher pressures vary with water content. In the slabs with a water content less than about 2 wt%, phase A carries water to a depth of 450 km. Hydrous wadsleyite, hydrous ringwoodite, and ilmenite are the main water reservoirs in the transition zone from 450 to 660 km. Superhydrous phase B is the water reservoir in the uppermost part of the lower mantle from 670 to 800 km, whereas phase G appears in the lower mantle only at depths greater than 800 km. In cold slabs with local water enrichment greater than 2 wt%, the following hydrous phases appear with increasing depths; phase A to 450 km, phase A and phase G from 450 km to 550 km, brucite, superhydrous phase B, and phase G from 550 km to 800 km, and phase G at depths greater than 800 km. [ABSTRACT FROM AUTHOR]

In the past three years, Dirac half-metals (DHMs) have attracted considerable attention and become a high-profile topic in spintronics becuase of their excellent physical properties such as 100% spin polarization and massless Dirac fermions. Two-dimensional DHMs proposed recently have not yet been experimentally synthesized and thus remain theoretical. As a result, their characteristics cannot be experimentally confirmed. In addition, many theoretically predicted Dirac materials have only a single cone, resulting in a nonlinear electromagnetic response with insufficient intensity and inadequate transport carrier efficiency near the Fermi level. Therefore, after several attempts, we have focused on a novel class of DHMs with multiple Dirac crossings to address the above limitations. In particular, we direct our attention to three-dimensional bulk materials. In this study, the discovery via first principles of an experimentally synthesized DHM LaNiO 3 with many Dirac cones and complete spin polarization near the Fermi level is reported. It is also shown that the crystal structures of these materials are strongly correlated with their physical properties. The results indicate that many rhombohedral materials with the general formula LnNiO 3 (Ln = La, Ce, Nd, Pm, Gd, Tb, Dy, Ho, Er, Lu) in the space group R 3 c are potential DHMs with multiple Dirac cones., (© Wang et al. 2019.)

The dense hydrous-magnesium silicate phase D, which has the ideal formula MgSi2H2O6, may be an important link in a chain of hydrous phases that carry H2O in the ultramafic portions of subducting lithosphere, into the Earth's lower mantle. We have synthesized a new Al-rich form of phase D, containing up to 50 wt% Al2O3, using a multi-anvil device at ~1300 °C and 25 GPa. The phase, with the formula Mg0.2Fe0.15Al1.8H1.8SiO6, was initially produced in a bulk composition designed to synthesize Al- and Fe-rich magnesium silicate perovskite with a composition similar to that produced in experiments on mid-ocean ridge basalt bulk compositions at lower mantle conditions. Further experiments using a starting mixture based on the composition of this Al-rich phase resulted in the synthesis of 60-70 µm long single crystals at similar conditions. The recovered crystals were slightly richer in H2O (Mg0.2Fe0.12Al1.5Si0.92H3.1O6) and their unit-cell parameters were similar to those of MgSi2H2O6 phase D. A refinement of the crystal structure was carried out in the P3̅1m space group and revealed a more disordered cation distribution than magnesium silicate phase D. All cation-oxygen distances are similar, suggesting a high degree of Si/Al disorder. Although the stability field of this new variant of phase D is yet to be determined, this phase may be an important host for H2O within portions of subducted oceanic crust in the lower mantle. [ABSTRACT FROM AUTHOR]