The seismic discontinuity around 520 km is believed to be caused by the phase transition from wadsleyite to ringwoodite, the dominant minerals in the mantle transition zone (MTZ). Both wadsleyite and ...ringwoodite can contain more than 1.0 wt% water at MTZ's conditions, but it is not well known how water affects the wadsleyite-ringwoodite transition. Here we investigated water partitioning between wadsleyite and ringwoodite and the water effect on this phase boundary using first-principles calculations. Our results show that the presence of water will shift the phase boundary to higher pressures, and the width of the two-phase coexistence domain in the Mg2SiO4-H2O system is insignificant at mid-MTZ conditions. For the (Mg0.9Fe0.1)2SiO4 system, the incorporation of 1.0 wt% water can narrow the effective width of two-phase coexistence by two-thirds. Together with elastic data, we find that velocity and impedance contrasts are only mildly changed by the water partitioning. We suggest that compared to the anhydrous condition, the presence of 1.0 wt% water will increase velocity gradients across the wadsleyite-ringwoodite transition by threefold, enhancing the detectability of the 520 km discontinuity.
Direct valorization of methane to its alcohol derivative remains a great challenge. Photocatalysis arises as a promising green strategy which could exploit hydroxyl radical (·OH) to accomplish ...methane activation. However, both the excessive ·OH from direct H
O oxidation and the neglect of methane activation on the material would cause deep mineralization. Here we introduce Cu species into polymeric carbon nitride (PCN), accomplishing photocatalytic anaerobic methane conversion for the first time with an ethanol productivity of 106 μmol g
h
. Cu modified PCN could manage generation and in situ decomposition of H
O
to produce ·OH, of which Cu species are also active sites for methane adsorption and activation. These features avoid excess ·OH for overoxidation and facilitate methane conversion. Moreover, a hypothetic mechanism through a methane-methanol-ethanol pathway is proposed, emphasizing the synergy of Cu species and the adjacent C atom in PCN for obtaining C
product.
Equilibrium Ti isotope fractionation factors among major Ti-bearing minerals are critical for understanding Ti isotope fractionation during magmatic processes. We use the first-principles ...calculations based on the density functional theory (DFT) to obtain Ti isotope reduced partition function ratios (103lnβ of 49Ti/47Ti) in a series of important Ti-bearing minerals, including Ti-doped clinopyroxene, orthopyroxene, olivine, and pyrope, geikielite-ilmenite solid solutions, and rutile. There is a large variation in our calculated 103lnβ, which are linearly correlated to their Ti force constants, a parameter related to the average TiO bond length and the Ti valence state. Among all studied minerals, silicates with Ti4+ occupying the tetrahedral Si site have the highest 103lnβ, and rutile has the lowest 103lnβ. The valence state also significantly controls the 103lnβ. Typically, Ti3+-doped silicates have lower 103lnβ than those of Ti4+-doped silicates. At the natural abundance levels, the 103lnβ of Ti4+Si-doped and Ti3+Mg-doped (Ti3+ occupying the Mg site) silicate minerals show no concentration effect. That is, their 103lnβ do not vary with their Ti4+ and Ti3+ contents, respectively. In contrast, the 103lnβ of geikielite-ilmenite solutions significantly decrease with increasing Fe/(Fe + Mg) ratio.
Our calculations predict no significant Ti isotope fractionation among Ti4+Si-doped clinopyroxene, orthopyroxene, olivine, and pyrope (<0.08 % at 1200 K), whereas the 103lnα between geikielite-ilmenite solutions and Ti4+Si-doped clinopyroxene ranges from ∼−0.67‰ to −0.49‰ at 1200 K, supporting the hypothesis that FeTi oxides are important fractionating Ti isotopes during magma differentiation. Finally, the large equilibrium Ti isotope fractionation between geikielite-ilmenite solutions and clinopyroxene suggests that Ti isotopes can be used as a thermometer with precision comparable to that of elemental geothermometer.
Photocatalytic water splitting is the most promising process to convert solar energy into high purity chemical fuel (hydrogen), which has received significant attention in recent years. Only several ...photocatalysts have been reported in the literature for pure water splitting under visible light. Herein we report for the first time quantum sized BiVO4 can decompose pure water into H2 and O2 simultaneously under simulated solar light irradiation without any cocatalysts or sacrificial reagents. By electrochemical measurement, we demonstrate that the significantly different photocatalytic activity of the quantum sized BiVO4 arises from the negative shift of conduction band edge by a quantum confinement effect and a decreased overpotential for water reduction. Although the generated H2 and O2 are nonstoichiometric in the present study, these findings establish the great potential of using quantum sized BiVO4 photocatalyst and solar energy for overall water splitting.
The shape‐controlled synthesis of nano‐ and microstructured materials has opened up new possibilities to improve their physical and chemical properties. In this work, new types of Bi2WO6 with complex ...morphologies, namely, flowerlike, tyre‐ and helixlike, and platelike shapes, have been controllably synthesized by a facile hydrothermal process. The benefits of the present work also stem from the first report on the transformation of Bi2WO6 from three‐dimensional (3D) flowerlike superstructures to 2D platelike structures, and on the formation of tyre‐ and helixlike Bi2WO6 superstructures. UV/Vis absorption spectra show that the optical properties of Bi2WO6 samples are relevant to their size and shape. More importantly, the photocatalytic activities of Bi2WO6 nano‐ and microstructures are strongly dependent on their shape, size, and structure for the degradation of Rhodamine B (RhB) under visible‐light irradiation. The reasons for the differences in the photocatalytic activities of these Bi2WO6 nano‐ and microstructures are further investigated.
Shaping up: Bi2WO6 samples with complex morphologies of flower‐ (A), tyre‐ (B), helix‐ (C), and platelike (D) shapes (see image), are controllably synthesized by a facile hydrothermal process. All of the synthesized Bi2WO6 nano‐ and microstructures exhibit interesting shape‐associated optical properties and visible‐light‐driven photocatalytic activity.
The K concentration effects on K-O bond length and the reduced partition function ratios of 41K/39K in alkali feldspars have been explored using the density functional theory (DFT) method. In alkali ...feldspars, the average K-O bond length increases with increasing K content, measured as K/(K + Na) molar ratio, ranging from 2.724 Å in alkali feldspar with a K/(K + Na) of 1/16 to 2.880 Å in microcline (K/(K + Na) = 1). Our results show large K concentration effect on the calculated reduced partition function ratio. For example, 103ln41K/39Kαfeldspar-microcline between alkali feldspar with a K/(K + Na) of 1/16 and microcline are 2.21‰ at 300 K and 0.42‰ at 700 K, which are comparable to the 41K/39K variation observed in natural samples. Furthermore, isotope fractionation, 103ln41K/39Kαfeldspar-microcline, is negatively linearly correlated with the average K-O bond length in alkali feldspars. Therefore, the concentration effect on K isotope fractionations needs to be considered in the applications of K isotopes in the fields of geochemistry and cosmochemistry, such as the formation of the lunar anorthositic crust and the evolution of the Earth’s crust.
Velocity and density jumps across the 410-km seismic discontinuity generally indicate olivine contents of ∼30 to 50 vol.% on the basis of the elastic properties of anhydrous olivine and wadsleyite, ...which is considerably less than the ∼60% olivine in the widely accepted pyrolite model for the upper mantle. A possible explanation for this discrepancy is that water dissolved in olivine and wadsleyite affects their elastic properties in ways that can reconcile the pyrolitic model with seismic observations. In order to more fully constrain the olivine content of the upper mantle near the 410-km discontinuity, and to place constraints on the mantle water content at this depth, we determined the full elasticity of hydrous wadsleyite at the P-T conditions of the discontinuity based on density functional theory calculations. Together with previous determinations for the effect of water on olivine elasticity, we simultaneously modeled the density and seismic velocity jumps (Δρ, ΔVP, ΔVS) across the olivine-wadsleyite transition. Our models allow for several scenarios that can well reproduce the density and seismic velocity jumps across the 410-km discontinuity when compared to globally averaged seismic models. When the water content of olivine and wadsleyite is assumed to be equal as in a simple binary system, our modeling indicates a best fit for low water contents (<0.1 wt.%) with an olivine proportion of ∼50%, suggesting a relatively dry, non-pyrolitic mantle at depths of the 410-km discontinuity. However, our modeling can be reconciled with a pyrolitic mantle if the water content in wadsleyite is ∼0.9 wt.% and that in olivine is at its storage capacity of ∼500-1500 ppm. The result would be consistent with a hydrous melt phase produced at depths just above the phase transition.
•Elasticity of hydrous wadsleyite at high P-T were obtained from ab initio method.•High-precision data was used to model 410-km discontinuity without extrapolation.•Hydrous MTZ can reconcile the long-standing discrepancy on olivine content.•Pyrolitic MTZ is globally hydrous at least at its top.
Calcium is a major element of the Earth, the Moon, terrestrial planets, and rocky meteorites. Here we present equilibrium Ca isotope fractionation factors of Ca-bearing minerals using the ...first-principles calculations based on density functional theory (DFT). The sequence of minerals from the isotopically heaviest to the lightest in Ca is forsterite > orthopyroxene (opx) > grossular ∼ pigeonite > diopside > anorthite > oldhamite. Overall, the equilibrium fractionation of Ca isotopes is mainly controlled by the average bond lengths. Although oldhamite is enriched in light Ca isotopes relative to silicate minerals in equilibrium, natural oldhamite of enstatite chondrites are isotopically heavier than coexisting silicate materials. This implies that enstatite chondrites oldhamites should have been formed during solar nebular gas condensation instead than during parent body processing.
Following previous models for crystallization of the Lunar Magma Ocean (LMO), we simulated Ca isotopic fractionation of the LMO based on our calculated equilibrium Ca isotope fractionation factors. It shows that the δ44/40Ca of the lunar anorthositic crust should be lower than the average of the bulk Moon by 0.09–0.11‰. Considering that the lunar mantle might have overturned and mixed after solidification of the LMO, we further predict that the lunar mantle should be isotopically heavier than the bulk Moon by 0.17–0.26‰ if the mantle was fully overturned, or only by 0.06–0.08‰ for the case of fully mixing. Therefore, we predict that the potential offset of Ca isotopic composition between the anorthositic crust and the lunar mantle can be used to test LMO evolution models.
•We calculate 103lnα of Ca isotopes among minerals using first-principles calculations.•The order of decreasing 103lnβ: ol → opx → grossular ∼ pigeonite → diopside → anorthite → oldhamite.•Oldhamite in enstatite-chondrites likely formed by solar nebular gas condensation.•Ca isotopes are fractionated between the anorthositic crust and lunar mantle.
Earth's habitability is closely tied to its late-stage accretion, during which impactors delivered the majority of life-essential volatiles. However, the nature of these final building blocks remains ...poorly constrained. Nickel (Ni) can be a useful tracer in characterizing this accretion as most Ni in the bulk silicate Earth (BSE) comes from the late-stage impactors. Here, we apply Ni stable isotope analysis to a large number of meteorites and terrestrial rocks, and find that the BSE has a lighter Ni isotopic composition compared to chondrites. Using first-principles calculations based on density functional theory, we show that core-mantle differentiation cannot produce the observed light Ni isotopic composition of the BSE. Rather, the sub-chondritic Ni isotopic signature was established during Earth's late-stage accretion, probably through the Moon-forming giant impact. We propose that a highly reduced sulfide-rich, Mercury-like body, whose mantle is characterized by light Ni isotopic composition, collided with and merged into the proto-Earth during the Moon-forming giant impact, producing the sub-chondritic Ni isotopic signature of the BSE, while delivering sulfur and probably other volatiles to the Earth.
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