Physics and Chemistry of Minerals最新文献

Troilite was synthesized and its Mössbauer spectra in the temperature range 90 ÷ 295 K were obtained. The equilibrium iron isotope fractionation factors (β-factors) for troilite were estimated from the temperature shift (TS) in the Mössbauer spectra. The TS was described by the Debye model, and the Mössbauer temperature (θ_M) was calculated. It is shown that the quantum component of TS, at temperatures above ~ 0.6θ_M, does not exceed the statistical error of the measurements. The use of experimental results at these temperatures leads to significant errors in the estimation of θ_M. Based on Mössbauer data at temperatures below 0.6θ_M (from 90 to 190 K), θ_M = 319 K was found. The temperature dependence of the iron β-factor for troilite, calculated from this value of θ_M, is as follows: ⁵⁷Fe/⁵⁴Fe 10³lnβ = 0.42388x − 0.51351 × 10⁻³x² + 0.96769 × 10⁻⁶x³; x = 10⁶/T² where T is the absolute temperature. The Mössbauer temperature dependence of the iron β-factor for troilite agrees well with the results of its estimation by nuclear resonance inelastic X-ray scattering on ⁵⁷Fe nuclei. The same approach was applied to assess the iron β-factors for aegirine. Previously obtained θ_M = 540 K for aegirine was corrected down to θ_M = 479 K using Mössbauer data at temperatures below 0.6θ_M. The temperature dependence of the iron β-factor for aegirine: ⁵⁷Fe/⁵⁴Fe 10³lnβ = 0.95573x − 2.6105 × 10⁻³x² 11.09185 × 10⁻⁶x³ matches with that from the first principal calculations. This resolves the contradiction between Mössbauer-derived and first principle calculated iron β-factors for aegirine.

The temperature dependence of the unit-cell parameters of synthetic Na(Mg_0.5Si_0.5)Si₂O₆ clinopyroxene and Na₂MgSi₅O₁₂ garnet were measured, at atmospheric pressure, using X-ray powder diffraction, from 40 K up to their decomposition temperatures (673 K and 793 K respectively). At 300 K the cell parameters were found to be: (i) a = 9.4073(3) Å, b = 8.6487(3) Å, c = 5.2685(2) Å, β = 108.113(2)°, V = 407.41(3) ų and ρ = 3.286 g/cm³, for Na(Mg_0.5Si_0.5)Si₂O₆ pyroxene in space-group P2/n with Z = 4 and (ii) a = 11.4155(7) Å, V = 1487.61(5) ų and ρ = 3.561 g/cm³ for Na₂MgSi₅O₁₂ garnet in space-group Ia(overline{3})d with Z = 8. The thermal expansion tensors have been calculated and analysis of the lattice parameters in terms of a Grüneisen-Debye model is used to estimate the Debye temperature and the incompressibility of the two materials at ambient pressure.

High-pressure Raman spectra of Al-rich phase D (Mg_0.93Al_0.70Si_1.29O₆H_2.88) and pure-Mg phase D (Mg_1.03Si_1.71O₆H_3.05) were measured up to 20 GPa in diamond-anvil cells using argon as a pressure medium. The results show that the intensity of the major 777 cm^− 1 band in the Raman spectra of the pure-Mg phase D exhibits a significant intensity reduction within the 18–20 GPa range during compression. However, this band displays a highly linear shift in the Raman spectra of the Al-rich phase D without notable decrease in intensity in the same pressure range. This implies that the pressure stability of the M2 octahedra in the Al-rich phase D is higher than that in the pure-Mg phase D due to the substitution of Al³⁺ for Si⁴⁺. The major OH band at about 2900 cm^− 1 in the Raman spectra of the pure-Mg phase D sample shifts continuously toward higher frequencies with increasing pressure due to the pressure-induced transition from straight H bonds to bent ones. Whereas, this transition occurs at pressures above 10 GPa in the Al-rich phase D, indicating that Al³⁺ substitution in the crystal structure of phase D can also alter the high-pressure response of hydroxyl ion.

Thermodynamic descriptions and experimentally verified phase relations in the FeS-MgS-CaS system are important both for steelmaking and for natural reduced systems. Experimental and thermodynamic data for such oxygen-poor systems are sparse due to the difficulty of conducting experiments under conditions at which these sulfides are stable. In this study, phase relationships were determined for FeS-MgS at 1170–1550 °C, for FeS-CaS at 1025–1600 °C, for MgS-CaS at 900–1500 °C and for FeS-MgS-CaS at 1050 and 1360 °C. Experiments were performed in evacuated silica glass tubes with excess Fe⁰ to favour troilite (FeS) rather than pyrrhotite (Fe_1–xS) for the FeS-rich phase. Textural interpretations and measured compositions indicate that the FeS-CaS system melts eutectically at 1063 ± 3 °C at 7 ± 1 mol% CaS. The FeS-MgS system is also modelled to be eutectic (at 1180 and 2.5 mol% MgS), yet, experimentally, its eutectic or peritectic character could not be unequivocally determined. This system’s liquidus has a higher dT/dX than previously reported. The MgS-CaS system was found to have a symmetric miscibility gap that closes at 1210 °C. Differences to the outcome of previous experimental studies can be explained by the presence of troilite rather than pyrrhotite in our experiments when Fe-rich solid solution coexists with liquid or solid solution. The experimental data are fit by a thermodynamic model that reproduces the experimentally determined phase relations, and is capable of predicting melting phase relations for the FeS-MgS-CaS ternary.

This work systematically investigates the thermodynamic stability in M-BTC metal organic frameworks, where M = Y, Eu, or La and BTC = (1,3,5-benzenetricarboxylate) linker. Enthalpies of formation obtained from calorimetric measurements of Y(BTC)·5.43(H₂O), Eu(BTC)·5.82(H₂O) and La(BTC)·4.85(H₂O) enable determination of the energetic landscape for metal substitution (Y, Eu, and La) in M-BTC materials. The enthalpies of formation from linker plus metal of La-BTC, Eu-BTC, and Y-BTC are − 3219.3 ± 3.4, 3.9 ± 2.0 and 713.3 ± 3.0 kJ mol^− 1_, respectively. The highly endothermic enthalpy of formation of Y(BTC)·5.43(H₂O) reflects a thermodynamic penalty for a change in the coordination environment of Y metal atoms in the BTC framework compared to Y₂O₃. The high thermodynamic stability of the M-BTC framework employing La metal confirms greater stabilization from the use of larger metal atoms in frameworks with oxygen-based linkers. The results from thermodynamic analysis suggest water is a stabilizing agent. Thus, the choice of metal atom and presence of guest water molecules can enthalpically stabilize the M-BTC materials by as much as ~ 3932 kJ mol^− 1. More broadly, the results indicate complex interplay among choice of metal, water content, and thermodynamic stability in M-BTC frameworks.

Laser-heated diamond anvil cell (LH-DAC) is needed to investigate melting properties of deep planetary interiors. Interpretation of the melting behavior is however challenging because extreme temperature gradients are inevitable. In this work, we investigate how the peak temperature at the center of the laser spot, from sample solidus to 1000 K above (ΔT), affects the chemical relations between melt and solid residue. We investigate the melting behavior of two possible mantle compositions, pyrolite and chondritic type material, at pressures corresponding to depths of ~ 1000 km and higher (40–73 GPa). Recovered samples are characterized at nanoscale spatial resolution using electron microscopy. Samples tend to show that chemical composition of melts and bridgmanite-melt relations vary largely with peak temperature. With increasing ΔT, the (Mg,Fe) exchange coefficient (K_Fe-Mg^Bg/melt) decreases from 0.29 to 0.11, and SiO₂ contents in melt ([SiO₂]^melt) from 43 to 18 wt%. In addition, we observe that the higher ΔT, the more the liquid is depleted in bridgmanitic-type composition. These experimental features are contrary with those expected from the known melting diagram of typical mantle material. Instead, they are well explained by considering fast solidification of bridgmanite (Bg) at the edge of the molten zone, in disequilibrium conditions. The sample prepared at solidus temperature and for short duration presents a central melt pool of Ca-bearing melt in close contact with Bg and ferropericlase. The degree of partial melting is coherently estimated to 18(2) wt% by two independent observations. This corresponds to pseudo-eutectic conditions where only the third mineral, davemaoite, is exhausted. For a pressure of 40.5 GPa, K_Fe-Mg^Bg/melt and [SiO₂]^melt are found to be 0.29 and 43 wt%, respectively, in good agreement with multi-anvil press experiments. Altogether, this work shows that erroneous solid–liquid chemical relations can arise from samples synthesized at temperatures well above solidus in the LH-DAC.

This study investigates the fabrication of CuO and Cu₂O nanostructures by a simple solution-based method followed by thermal annealing. Pure CuO was obtained at 180 °C, while higher temperatures yielded Cu₂O/CuO heterostructures. The pure CuO film exhibited the highest photocurrent density (140.05 mA/cm²) and a notable dark current density (60.27 mA/cm²). Although the heterostructures showed lower photocurrents, they also demonstrated significant dark currents (9.03–16.27 mA/cm²) across different annealing temperatures. These findings suggest a promising approach for developing effective CuO-based photoelectrodes with both excellent photocatalytic capabilities and a potential for dark current generation.

Graphical abstract

Bergslagite, Ca₂Be₂As₂O₈(OH)₂, is one of the only three known berylloarsenate minerals and is a member of the gadolinite supergroup. To date, very little is known about the thermal behavior of beryllium compounds and not much more about arsenates, while the thermal behavior of berylloarsenates (both natural and synthetic) has not been previously studied at all. In this work, the low and high-temperature behavior and thermal stability of bergslagite were studied in situ using single-crystal X-ray diffraction. Besides, its Raman spectrum was obtained and compared to the calculated one. Bergslagite does not undergo a phase transition in the temperature range − 173 to 700 °C, whereas it amorphizes at higher temperatures. The TO₄-based (T = Be, As) framework remains stable, while the CaO₆(OH)₂ polyhedra are slightly expanding. The volume thermal expansion coefficient (32 × 10^− 6 °C^− 1) is comparable with borosilicate / beryllophosphate analogues (30–35 × 10^− 6 °C^− 1). The low thermal stability of bergslagite can be associated with the vacant octahedral position, which is occupied by divalent cations in more thermally stable beryllosilicate analogues.

Calcite, CaCO₃, has been reported to exist in as many as seven different structural forms. The structure at room temperature and pressure (space group R(overline{3 })c, ‘Phase I’) was established by Bragg many years ago. A phase transition to a higher temperature phase (space group R(overline{3 })m, ‘Phase V’) was noted to occur at around 1240 K—this may proceed via an intermediate phase (space group again R(overline{3 })c, referred to as ‘Phase IV’). These phases differ primarily in the disposition of the CO₃ groups. Additional phases are found at higher pressures. We report a para-phase (parent phase, virtual prototype, aristotype) which assists in understanding the different phases, the phase transitions, and especially the domain structures and twin wall boundaries associated with these transitions. Molecular dynamics methods were used to study the temperature evolution of an isothermal-isobaric (NPT) ensemble of some 384,000 atoms. These computations reproduced the features of the known structures in R(overline{3 })c and R(overline{3 })m and then, at higher temperature, revealed a structure of the sodium chloride type (space group Fm(overline{3 })m) in which the entities were the Ca²⁺ cation and the CO₃²⁻ anion, this latter with effectively spherical symmetry. On this basis we have upon cooling a necessarily first order ferroelastic transition from cubic Fm(overline{3 })m to rhombohedral R(overline{3 })m, computed to occur at a simulated temperature of 1900 K, and a possibly continuous transition from the R(overline{3 })m to rhombohedral (on a doubled cell) R(overline{3 })c computed to occur at about 1525 K. The computations also allowed us to follow the domain structure and twin walls as a function of temperature, during both heating and cooling. The structure just below the R(overline{3 })m to R(overline{3 })c transition shows strong disorder in the orientation of the CO₃ groups, and this may be what is sometimes referred to as Phase IV. The domain structure just below the cubic to rhombohedral transition shows twinning of typical ferroelastic character. The doubling of the cell below the R(overline{3 })m to rhombohedral (on a doubled cell) R(overline{3 })c leads to a more complicated twin pattern. Indeed, the different structures can be identified from patterns of twinning. Differences between domain structures obtained on heating and cooling indicate extensive thermal metastabilities.

Grain boundaries in polycrystalline materials significantly affect their properties, such as ionic transport, corrosion, and chemical durability. The pyrochlore compound (Gd₂Ti₂O₇) is employed as a model for complex oxides and is known for its diverse applications, including nuclear waste immobilization. Density functional theory-based first-principles molecular dynamics simulations were performed at different temperatures on the hydrated grain boundary system. The results show extensive transformations within the grain boundaries among hydrous water species (OH⁻, H₂O, and H₃O⁺). The temperature dependence of self-diffusion coefficients follows Arrhenius behavior, with an activation energy of 35.9 kJ/mol for hydrogen and 46.3 kJ/mol for oxygen. The lifetime of OH⁻ is about three to four times longer than that of H₂O at temperatures from 800 to 2100 K, suggesting the greater stability of OH⁻ over H₂O, a unique characteristic of the grain boundaries. The estimated lifetime of the hydrous species decreases as the temperature increases, with an activation energy of 9.9 kJ/mol for OH⁻ and 13.4 kJ/mol for H₂O. While Gd₃⁺ is more mobile than Ti⁴⁺, both the Gd₃⁺ and Ti⁴⁺ cations are orders of magnitude less mobile than the water species. The results suggest that water species are much more mobile within grain boundaries than in the bulk crystal and have the potential to penetrate deep into polycrystalline materials through grain boundaries, leading to grain boundary degradation and dissolution. The different mobilities of cations in complex oxides can lead to leaching of certain cations and incongruent dissolution during the chemical weathering of Earth and industrial materials.