Physics and Chemistry of Minerals最新文献

Compressibility and structural evolution of K-cymrite, hexagonal high-pressure KAlSi₃O₈·H₂O, has been studied up to 18 GPa using synchrotron single crystal X-ray diffraction in Ne pressure medium. K-cymrite retains its original symmetry P6/mmm up to a pressure of 7.3 GPa. As the pressure increases from 7.3 to 8.5 GPa, the weak satellite reflections appear on diffraction patterns and remain up to maximum applied pressure of 18 GPa indicating incommensurate modulation. However, main reflections can be still indexed in hexagonal cell and structure successfully solved in initial P6/mmm group. After pressure release, K-cymrite reverts to initial non-modulated single-crystal state. The parameters of third-order Birch-Murnaghan equation of state for K-cymrite are V₀ = 190.45(12) ų, K₀ = 56.5(7) GPa and K₀’ = 3.2(12), with bulk modulus notably deviating from earlier result (K₀ = 45(2) GPa and K₀’ = 1.3(10)) obtained in vaseline media.

High-pressure and high-temperature Raman spectroscopic measurements of synthetic liebenbergite and Ni₂SiO₄ spinel have been conducted up to 22 GPa and 700 ℃, respectively. Isothermal and isobaric mode Grüneisen parameters were calculated based on the observed Raman modes. The intrinsic anharmonicities of liebenbergite and Ni₂SiO₄ spinel were also evaluated. The changes of the asymmetric SiO₄ stretching band of Ni₂SiO₄ spinel in frequency are irreversible under decompression, indicating a potential pressure-induced modification in the crystal structure at elevated pressures. The values of isothermal mode Grüneisen parameters show that the SiO₄ internal vibrations in Ni-rich olivines are more sensitive to the variations of pressure. For spinel-group minerals, the SiO₄ internal vibrations can be less sensitive to the pressure change due to nickel incorporation. In contrast, according to the values of isobaric mode Grüneisen parameters, nickel increases the sensitivity of these vibrations to the variations of temperature. In addition, nickel has distinctive effects on the intrinsic anharmonicities of different vibration modes in both olivine and spinel-group minerals, and therefore alter the thermodynamic properties of their crystal structures.

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.