Physics and Chemistry of Minerals最新文献

Due to experimental challenges and computational complexities, limited research has explored high-temperature and high-pressure conditions on mineral vibrations. This study employs the quasi-harmonic approximation (QHA) and density functional theory (DFT) to investigate the impact of temperature and pressure on the structural properties and infrared and Raman vibrational modes of forsterite. The computational process involves determining lattice parameters, optimizing the internal crystal structure, and calculating IR and Raman spectra at various temperature and pressure values, both separately and combined. Results highlight significant anisotropy in forsterite, with the b-axis being most sensitive to temperature and pressure, followed by the c-axis, while the a-axis exhibits greater stiffness. The positions of vibrational modes typically shift to higher frequencies with increasing pressure (average shift of 2.70 ± 1.30 cm⁻¹/GPa) or to lower frequencies with increasing temperature (average shift of − 0.017 ± 0.018 cm⁻¹/K). Modes associated with SiO₄ stretching and bending are less affected by temperature or pressure than translational and rotational modes. A brief investigation into isotope and chemical substitution, as well as cation distribution, in the solid solution (Mg, Fe)₂SiO₄ reveals lower wavenumbers in fayalite modes compared to forsterite modes, attributed to the heavier Fe mass and larger cell parameters. This study establishes a methodology for computing vibrational frequencies under simultaneous temperature and pressure and emphasizes the significant impact of various factors on vibrational modes. Caution is advised when using vibrational modes for identifying compositions within solid solutions.

Studying the anisotropic physical properties of hexagonal closed-packed (hcp) iron is essential for understanding the properties of the Earth’s inner core related to the preferred orientation of the inner core materials suggested by seismic observations. Investigating the anisotropic physical properties of hcp iron requires (1) the synthesis of hcp iron samples that exhibit several distinctive types of strong lattice preferred orientation (LPO) and (2) the quantitative LPO analysis of the samples. Here, we report the distinctive LPO of hcp iron produced from single-crystal body-centered cubic (bcc) iron compressed along three different crystallographic orientations ([100], [110], and [111]) in a diamond anvil cell based on synchrotron multiangle X-ray diffraction measurements up to 80 GPa and 300 K. The orientation relationships between hcp iron and bcc iron are consistent with the Burgers orientation relationship with variant selection. We show that the present method is a way to synthesize hcp iron with strong and characteristic LPO, which is beneficial for experimentally evaluating the anisotropic physical properties of hcp iron.

CaAl₁₂O₁₉, which can incorporate Ti as both Ti³⁺ and Ti⁴⁺ (charge coupled substitution with Mg²⁺), is one of the first minerals to condense from a gas of solar composition and is used as a ceramic. It is variously known as hibonite, calcium hexaluminate (CaO.6Al₂O₃), and CA₆. The lattice parameters and unit cell volumes of Ti-substituted hibonite (P6₃/mmc) with the formulae CaAl_11.8Ti³⁺_0.2O₁₉ and CaAl_9.8Ti³⁺_0.54Mg_0.83Ti⁴⁺_0.83O₁₉ were determined as a function of temperature from ~ 10 to 275 K by neutron powder diffraction. The thermal expansion is highly anisotropic with the expansion in c a factor of ~ 5 greater than that in a. The change in a is approximately equal for the two compounds whereas the change in c is almost 50% larger for CaAl_11.8Ti³⁺_0.2O₁₉. CaAl_11.8Ti³⁺_0.2O₁₉ also exhibits negative thermal expansion between 10 and 70 K. The change in unit cell volume with temperature of both compositions is well described by a two term Einstein expression. The large change in c is consistent with substitution of Ti onto the M2 and M4 sites of the R-block structural unit.

Enstatite (Mg₂Si₂O₆) is a member of the pyroxene group and an important mineral in the lower crust and upper mantle. Enstatite has three phases at ambient pressure: protoenstatite, orthoenstatite, and clinoenstatite. Previously, the polymorphic transformation of pyroxene has been characterized using bulk techniques such as X-ray diffraction of powders. Given that rocks are crystal aggregates, it is important to use aggregates to understand phase transformations. We therefore conducted grain growth and deformation experiments using aggregates of enstatite to investigate phase transformations. Grain growth experiments were conducted at temperatures (T) of 1345 and 1360 °C under a vacuum of ≈ 10 Pa using an alumina tube furnace. Deformation experiments were conducted at T = 1310 °C and room pressure, a strain rate of ≈ 10^–4 s^–1, and a resulting stress of ≈ 150 MPa. The samples were analyzed using a scanning electron microscope, electron backscatter diffraction (EBSD), and X-ray diffraction. The results indicate that the grain size affects the transformation from protoenstatite to clinoenstatite, whereas deformation by diffusion creep does not. The EBSD analyses show that the volume fraction of clinoenstatite increases with increasing grain size. The samples underwent diffusion creep during the deformation experiments, and there were no distinct microstructural differences between deformed and undeformed samples. The EBSD analyses show that the transformed clinoenstatite has a characteristic twin structure with a misorientation angle of 180° and a rotation axis of [100] or [001]. Grain sizes become smaller during the phase transformation, even if the mechanism can be characterized as a second-order transformation.

Bütschliite, K₂Ca(CO₃)₂, occurring as inclusions in mantle minerals, is regarded as one of the key phases to understand phase relationships of dense potassium carbonates and thus to evaluate their potential role within the Earth’s deep carbon cycle. Accordingly, the high-pressure behavior of synthetic bütschliite has been investigated by in-situ single-crystal X-ray diffraction under isothermal compression up to 20 GPa at T = 298 K. The compression mechanism before and after the trigonal-to-monoclinic (R-3m to C2/m) phase transition at ∼6 GPa, found previously, is characterized in terms of the evolution of the cation polyhedra and carbonate groups. On this basis, the modulation of the axial compression is interpreted, and the contribution of the cation polyhedra into the bulk compression is estimated. The refined compressibility of the monoclinic phase (K₀ = 44(2) GPa) fits to the trend of the carbonate bulk modulus versus average non-carbon cation radius. The analysis of the obtained and literature structural data suggests the distortion of a large cation polyhedron to be an effective tool to strengthen the carbonate structure at high pressure. On the other hand, the observed symmetrization of the cation polyhedra in trigonal bütschliite is apparently a crucial factor of its stabilization at high pressure upon the temperature rise observed previously. The structural crystallography provided in this study supports the enhanced stability of trigonal bütschliite at high P, T conditions and its significance of being considered as a constituent of the inclusions in deep minerals.

K-Ca double carbonates recently identified in inclusions in diamonds, as well as associated alkali-carbonate melts can play an important role in the deep carbon cycle. We studied pressure-induced changes in the crystal structure of high-pressure α-K₂Ca₃(CO₃)₄ phase up to 20 GPa using synchrotron single-crystal x-ray diffraction in diamond anvil cell. At ~ 7 GPa at room temperature the orthorhombic P2₁2₁2₁ phase of α-K₂Ca₃(CO₃)₄ undergoes displacive phase transition into monoclinic P112₁ phase. Despite the phase transition, PV-curve does not demonstrate any irregularities so that both phases can be described by the same 4th order Birch-Murnaghan equation of state with V₀ = 1072.5(3) ų, K₀ = 51.1(8) GPa, K’₀=3.7(3), K’’₀=0.12(6).

In this work, the self-made chrysotile fiber membrane (CFM) and raw chrysotile fiber (CF) were calcined in air from 500 to 800 °C. The XRD pattern of CFM showed that the diffraction peak of chrysotile weakened when the temperature was from room temperature to 550 °C, and CFM had a shorter amorphous interval at 600–700 °C. While, no amorphous phase appeared in CF during calcination, and forsterite begined to appear at 650 °C. SEM images showed that CFM could still maintain the integrity of the network structure at 600–800 °C, while CF gradually melted into coarse fiber bundles with the increase of calcination temperature, and sintering traces appeared. After that,the kinetics of the dehydroxylation of chrysotile in CFM and CF was studied. The dehydroxylation of CFM is a one-step reaction, the calculated activation energy is 243.33 kJ mol⁻¹, which conforms to the two-dimensional ‘Valensi’ model with mechanism function G(α) = (1−α)ln(1−α) + α. The dehydroxylation of CF is divided into two stages, the activation energy are 222.87 kJ mol⁻¹ and 316.04 kJ mol⁻¹. The first stage of CF conforms to two-dimensional ‘Jander’ model (n = 2) with mechanism function G(α) = [1−(1−α)^1/2]², the second stage of CF conforms to the random nucleation and subsequent growth ‘Avrami-Erofeev’ model (n = 3/2) with mechanism function G(α) = [−ln(1−α)]^2/3.

The infrared spectroscopic properties of selected defects involving one proton and one nearby M³⁺ (M = Al, Cr, Fe) substitution in orthoenstatite are investigated by first-principles calculations. Based on the theoretical results, the absorption bands experimentally observed on synthetic samples with high crystalline quality and low doping levels can be assigned to specific defect configurations. Most of them correspond to Mg vacancies at M2 sites locally compensated by one proton and one M³⁺ cation at a nearby M1 site. This confirms that the M³⁺ + H⁺ = 2 Mg²⁺ exchange mechanism is the dominant hydrogen incorporation mechanism at the lowest concentration levels in doped enstatite. At higher concentration levels, more complex incorporation mechanisms could become dominant in Al-bearing samples.

Platinum-iron (Pt-Fe) alloys have long served as oxygen fugacity sensors in high-temperature experiments investigating Earth and planetary interiors, relying on the equilibrium between Fe within the alloy and FeO in coexisting oxides or silicates. Despite their significance, studies on intermediate compositions remain limited. This investigation focuses on compressibility of Fe₁₈Pt₈₂ up to (sim) 40 GPa at ambient temperature and explores the pressure-dependent characteristics of the oxygen fugacity relationship. In-situ X-ray diffraction measurements confirm the stability of the fcc phase in Fe₁₈Pt₈₂ across the pressure range. The fit to the compression data by the third-order Birch–Murnaghan equation of state results in ({V}_{0}=59.14 pm 0.08)ų, ({K}_{0}=266 pm 13) GPa, and ({K}_{0}^{prime}=4.7 pm 0.7). The differences between this fit and the Vinet and Kunc equations of state fall within the range of uncertainty. Comparing results with reported data for other Pt-Fe alloys reveals a nearly linear trend between volume and the Fe content in Pt-Fe alloys at ambient pressure. Unlike more iron-rich alloys, the excess volume of mixing of Fe₁₈Pt₈₂ ((sim) 0.21 cm³/mol) remains nearly constant across the examined pressure range. Estimates of the excess Gibbs free energy suggest diminishing non-ideal contributions to thermodynamic activities as pressure increases.

Low impurity content is crucial for graphite applications and microcrystalline graphite is an important candidate material. In this study, natural microcrystalline graphite, with a fixed carbon content of 76.65%, was purified by an alkaline autoclave-acid leaching method. The effects of the mole ratio of NaOH to Si and Al in graphite, the liquid–solid ratio of NaOH solution and graphite, alkali autoclave temperature and reaction time on the purity of microcrystalline graphite were studied. Results showed that the dissolution and phase transformation of non-carbon impurities were closely related to the purification process. During the alkali autoclave stage, complete dissolution of quartz was observed. The Si–O tetrahedra and Al–O octahedra structures in aluminosilicate minerals were damaged and [Al (OH)₄]⁻, [H₂SiO₄]²⁻ and [SiO₂ (OH)₃]⁻ were released. The soluble silicate and aluminate ions underwent recrystallization, producing cancrinite and sodalite that could be dissolved by acid leaching, resulting in purified microcrystalline graphite. The purity of microcrystalline graphite was further improved due to the autoclave treatment allowed NaOH solution to penetrate into the cracks of microcrystalline graphite aggregates under high pressure. In addition, the acid solution could enter the micropores left by alkali etching to dissolve the residual impurities. The fixed carbon content of microcrystalline graphite could be increased to 99.9% through the alkaline autoclave-acid leaching method.

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