Phase relations in Cr3S4 and the substituted system Cr3S4–x
Se
x
are studied to determine the influence of chemical substitutions on the thermoelectric properties. In addition to the expected ...equilibrium phase crystallizing in the monoclinic space group I2/m, some samples exhibit a defect phase with Cr2S3‐like structure. The defect phase can be observed in a few samples prior to sintering, with the majority being phase‐pure Cr3S4. The defect phase can, however, be introduced in phase‐pure samples through in situ heating. It can be proven that the defect phase has an influence on the thermoelectric properties, by lowering the electrical and thermal conductivity, while increasing the Seebeck coefficient. Substitution in the anion lattice of Cr3S4 with Se lowers the thermal conductivity. The improvement is mainly achieved through a reduction of the electronic contribution to the thermal conductivity, leading to total values as low as 1.6 Wm−1 K−1 for the substituted system in comparison to the pristine material 2.3 Wm−1 K−1.
Herein, phase relations in Cr3S4 and the substituted system Cr3S4–x
Se
x
are studied. A defect phase occurs that exhibits a Cr2S3‐like crystal structure but retains the stoichiometry of Cr3S4. Electrical and thermal conductivities are affected and both are lowered by the inclusion of the defect phase. Anion substitution with Se suppresses the defect phase and lowers the thermal conductivity.
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The exposure of mantle peridotite to water at crustal levels leads to a cascade of interconnected dissolution-precipitation and reduction-oxidation reactions-a process referred to as ...serpentinization. These reactions have major implications for microbial life through the provision of hydrogen (H
). To simulate incipient serpentinization under well-constrained conditions, we reacted centimeter-sized pieces of uncrushed harzburgite with chemically modified seawater at 300 °C and 35 MPa for ca. 1.5 yr (13 441 h), monitored changes in fluid chemistry over time, and examined the secondary mineralogy at the termination of the experiment. Approximately 4 mol% of the protolith underwent alteration forming serpentine, accessory magnetite, chlorite, and traces of calcite and heazlewoodite. Alteration textures bear remarkable similarities to those found in partially serpentinized abyssal peridotites. Neither brucite nor talc precipitated during the experiment. Given that the starting material contained ~4 times more olivine than orthopyroxene on a molar basis, mass balance requires that dissolution of orthopyroxene was significantly faster than dissolution of olivine. Coupled mass transfer of dissolved Si, Mg, and H
between olivine and orthopyroxene reaction fronts was driven by steep activity gradients and facilitated the precipitation of serpentine. Hydrogen was released in significant amounts throughout the entire experiment; however, the H
release rate decreased with time. Serpentinization consumed water but did not release significant amounts of dissolved species (other than H
) suggesting that incipient hydration reactions involved a volume increase of ~40%. The reduced access of water to fresh olivine surfaces due to filling of fractures and coating of primary minerals with alteration products led to decreased rates of serpentinization and H
release. While this concept might seem at odds with completely serpentinized seafloor peridotites, reaction-driven fracturing offers an intriguing solution to the seemingly self-limiting nature of serpentinization. Indeed, the reacted sample revealed several textural features diagnostic of incipient reaction-driven fracturing. We conclude that fracturing must have far reaching impacts on the rates of serpentinization and H
release in peridotite-hosted hydrothermal systems.
Liquidus phase relations were experimentally studied in model kimberlite melts at 6–12GPa. Four starting materials were used with different CO2 and H2O contents but almost identical proportions of ...other components (~35wt.% SiO2, 3wt.% Al2O3, 10wt.% FeO, 33wt.% MgO, 15wt.% CaO, 0.3wt.% Na2O, and 1wt.% K2O on a 100% volatile-free basis). The liquidus phase of the CO2 richest mixture (33wt.% CO2 and no H2O) is coesite at P>6–8GPa and low-Ca pyroxene at lower pressures. In an H2O-free mixture with lower CO2 content (18wt.%) low-Ca pyroxene is the main liquidus phase and coesite was never observed in the experimental products. The addition of water (mixtures with 10wt.% H2O+9wt.% CO2 and 12wt.% H2O+5wt.% CO2) depresses the liquidus temperature and expands the crystallization field of olivine. At high pressures (>8GPa), garnet crystallization dominates near-liquidus phase relations. Based on the experimental data, possible conditions of the simultaneous saturation of kimberlite melt with olivine+low-Ca pyroxene±garnet were estimated. High-Ca pyroxene was never found in the near-liquidus assemblages despite the high-CaO compositions of the starting mixtures. It was shown that the supposed primary kimberlite melt can be in equilibrium with the garnet harzburgite assemblage at 6–8GPa and variable volatile contents, from ~15wt.% CO2 under anhydrous conditions to 20wt.% H2O and low CO2. These conditions correspond to possible parameters of kimberlite magma generation by partial melting of carbonated garnet harzburgite in the lithospheric mantle. The primary CO2 content could be higher if the kimberlite magma was poorer in SiO2 (more ‘carbonatitic’). The maximum CO2 content is limited by the appearance of magnesite on the liquidus at 20–22wt.%, which is much lower than CO2 solubility in kimberlite melt at 7–8GPa (>30wt.%). The content of H2O in primary kimberlite magmas depends on the bulk H2O content in the source, because no hydrous phases are stable near the kimberlite liquidus.
► Near-liquidus phase relations of kimberlites with various CO2 and H2O contents. ► Increasing CO2 activity stabilizes SiO2-rich phases on the liquidus. ► Addition of H2O results in a moderate decease in liquidus temperature and reduction of pyroxene crystallization field. ► Primary kimberlite magmas contain 15–20wt.% CO2+H2O at CO2/H2O controlled by source composition.
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The availability of thermodynamic data for geologically relevant phases has made practical the calculation of stable phase relations throughout the mantle and crust of terrestrial planets. GeoPS ...(http://www.geops.org) is a program designed for this purpose in which both input and output are done through an intuitive graphical user interface. GeoPS provides a wide range of phase equilibrium calculations based on a novel Gibbs energy minimization algorithm. The algorithm provides for exceptionally robust and computationally efficient solution to the phase equilibrium problem by successive alternation between a linear programming step to identify stable phase compositions and a non‐linear programming step to refine the compositions estimated during the linear programming. Applications include calculation of various types of phase diagrams and path‐dependent phase fractionation. By combining an easy‐to‐use graphical user interface with a robust and efficient solver, GeoPS makes phase equilibrium modelling accessible to researchers and students with minimal training and provides a powerful tool for understanding natural phase relations and for planning experimental work.
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Zinc vapor pressures over binary Pt–Zn alloys were determined in the composition range from 63 to 84 at.% Zn for temperatures between 840 –1150 K using an isopiestic method. The phase boundaries of ...four different phase fields in this composition range could be fixed based on the experimental data. Thermodynamic activities and partial molar enthalpies of zinc within these phase fields were derived from the vapor pressure data and the composition dependence of the zinc activity was calculated for 1023 K. The integral Gibbs energy at 1273 K was derived as a function of composition by Gibbs–Duhem integration, combining the activity data of the present investigation with earlier results from the literature.
The phase relations of the
Fe
B–Ce
Fe
B (
= Nd, Pr) pseudobinary systems have been investigated by using X-ray powder diffraction, scanning electron microscopy equipped with energy dispersive X-ray ...spectroscopy and differential thermal analysis. The X-ray powder diffraction results show that continuous solid solutions were formed in this
Fe
B–Ce
Fe
B (
= Nd, Pr) pseudobinary systems, and all (
Ce)
Fe
B solid solutions belong to the tetragonal structure with space group
. With the increase in Ce content, the lattice parameters
,
, the peritectic reaction temperatures of the (
, Ce)
Fe
B (
= Nd, Pr) solid solutions decrease linearly, and the cell volume
varies in the form of the regression line. Based on the X-ray powder diffraction results and differential thermal analysis data, the quasi-binary phase diagrams of the
Fe
B–Ce
Fe
B (
= Nd, Pr) systems have been established.
The stability of CO
fluid in the Earth’s mantle is restricted by the carbonation of rock-forming minerals. Among those, the reaction with garnet is of particular interest because it constrains the ...stability of CO
fluid in eclogites, whose minerals have been found in the CO
-bearing diamonds. In this work, we determined the equilibrium boundary for the reaction Mg
Al
Si
(Prp) + 3CO
(fluid) = Al
SiO
(Ky) + 2SiO
(Coe/Qz) + 3MgCO
(Mgs) over the pressure interval 3–6 GPa using a multi-anvil press. Owing to the slow kinetics, the reaction was studied in both forward (left to right) and reverse (right to left) directions in experiments with durations extending up to 260 h. Our newly determined boundary is situated 3 GPa/950 ± 50 °C, 4.5 GPa/1150 °C, and 6 GPa/1350 ± 50 °C and has the equation
(GPa) = 0.0075 ×
(°C) – 4.125. The boundary crosses the graphite-to-diamond transition curve near 4.7 GPa and 1180 °C. Thus, the assemblage garnet + CO
fluid is stable in the diamond (Dia) stability field under
conditions of the continental geotherm with a heat flow of 41 mW/m
The phase relations of Al-bearing magnetite were investigated between 6–22 GPa and 1000–1550 °C using a multi-anvil apparatus. This study demonstrates that the spinel-structured phase persists up to ...~9–10 GPa at 1100–1400 °C irrespective of the amount of hercynite (FeAl
) component present (20, 40, or 60 mol%). At ~10 GPa, the assemblage Fe
(Al,Fe)
+ (Al,Fe)
forms and remains stable up to 16–20 GPa and 1200–1550 °C. Fe
(Al,Fe)
adopts the CaFe
-type structure with the
space group. At 18–22 GPa and
> 1300 °C the assemblage Fe
(Fe,Al)
+ (Al,Fe)
becomes stable. Fe
(Fe,Al)
is isostructural with Fe
, having the monoclinic structure of the
2/
space group. At
<1300 °C, Fe
(Fe,Al)
+ (Al,Fe)
gives way to the assemblage of a hp-Fe(Fe,Al)
+ (Al,Fe)
. This hp-Fe(Fe,Al)
phase is unquenchable; a defect-bearing spinel-structured phase was recovered instead, and it contained numerous lamellae parallel to {100} or {113} planes and notably less Al than the initial starting composition. While low-pressure spinel can have a complete solid solution between Fe
-Al, the post-spinel phases have only very limited Al solubility, with a maximum of ~0.1 cpfu Al in hp-Fe(Fe,Al)
, ~0.3 cpfu in Fe
(Fe,Al)
, and ~0.4 cpfu in Fe
(Fe,Al)
, respectively. As a result, the phase relations of Fe(Fe
Al
can also be applied to bulk compositions richer in Al with the only difference being that larger amounts of an (Al,Fe)
phase are present.
Coexisting rhombohedral-structured phases demonstrate that the binary miscibility gap established at low pressure between hematite and corundum is still valid up to 20 GPa. Since iron oxides (e.g., magnetite) with variable Al contents are found in extraterrestrial rocks or as inclusions in diamond, constraints on their high-
stability might help unravel their formation conditions.
The first quantitative thermodynamic model of two-phase fluid induced by N2 and equilibrated with mineral assemblage in the system SiO2–TiO2–Al2O3–Fe2O3–MnO–MgO–CaO–Na2O–K2O–P2O5–H2O–CO2–N2 ...(STAFMMCNKPO–HCN) in the conditions of amphibolite facies rocks is proposed. Calculations are performed using the Gibbs free energy minimization technique. The fluid may consist of one phase, either aqueous solution (AS) or fluid with gas-like properties (FG), and also these two fluid phases (AS+FG) may co-exist, depending upon nitrogen content in the system. The values of total nitrogen at which AS co-exists with FG in the fluid were calculated at carbon saturation over a range in pressure and temperature of 3–7kb and 550–650°C. The model is capable of accurately describing various parameters of two fluid phases at each step of nitrogen increment in the system: density, partial pressure and mole fraction of constituents, pH, Eh, concentration of aqueous species. Increments of such an inert component as nitrogen cause the changes in the fluid resulting into subtle quantitative re-distribution of mineral phases in the assemblage plagioclase–biotite–quartz–ilmenite–garnet–apatite–graphite plus minor sillimanite and andalusite.
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Phase relations in the MgSiO3–MgTiO3 and Mg3Al2Si3O12–MgTiO3 systems were studied at 10–24 GPa and 1600 °C using a high-pressure Kawai-type multianvil apparatus. We investigated the full range of ...starting compositions for the enstatite-geikielite system to derive a P–X phase diagram and synthesize titanium-bearing phases, such as olivine/wadsleyite, rutile, pyroxene, MgTiSi2O7 weberite, bridgmanite and MST-bridgmanite in a wide pressure range. Olivine and pyroxene in run products are characterized by a low titanium content (<0.6 and <0.3 wt% TiO2, respectively) whereas the content of TiO2 in wadsleyite reaches 2 wt% at 12 GPa. The concentration of Ti in MgTiSi2O7 weberite decreases with pressure from 52 wt% TiO2 at 14 GPa to 43 wt% TiO2 at 18 GPa. Two perovskite-type structures (MgSiO3 bridgmanite and Mg(Si,Ti)O3 bridgmanite) were detected in the studied system. MgSiO3 bridgmanite (Brd) is formed at a pressure of >20 GPa and characterized by significant titanium solubility (up to 13 wt% TiO2 at 24 GPa). Mg(Si,Ti)O3 perovskite is formed at a pressure of >17 GPa. The concentration of TiO2 in this phase varies from 29 wt% to 49 wt%. It was found that addition of Ti to the system moves the boundaries of Ol/Wad phase transformations to lower pressures. Addition of Al to the starting material allows us to simulate the composition of natural Ti-rich garnets and bridgmanites. It is important to note that garnet in the Prp-Gkl system is stable throughout a wide pressure range (10–24 GPa). Al incorporation does not affect the distribution of titanium between two types of bridgmanite. It is shown that high contents of Ti stabilize bridgmanite-like compounds at considerably lower pressure than that at the lower mantle/transition zone boundary. Our experiments simulate the composition of natural Ti-rich primary garnet found in eclogite from the Sulu ultrahigh-pressure (UHP) terrane.
•P-X diagram constructed for En–Gkl join at 10–24 GPa simulates phase associations of Ti-rich lithology in the Earth’s mantle.•Ti stabilizes the bridgmanite-type structure to the lower pressure area.•Incorporation of Ti in bridgmanite results in an increase of the cell volume in comparison with that of pure MgSiO3-Brd.•Melting of peridotite under the P-T parameters of the lower mantle will produce high titanium phases in residue
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