Physics and Chemistry of Minerals最新文献

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.

Marialite (Na₃Al₃Si₉O₂₄·NaCl) represents a key end-member of the scapolite mineral group because it has the potential for revealing the chloride content of the paleofluid from which it formed. Here we provide measurements of the basic thermophysical properties of synthetic marialite which do not currently exist and which complement similar data for calcium-carbonate-bearing scapolites. Synthetic marialite was made from reagent oxides and NaCl treated at 1050 °C and 1.7 GPa for 48–120 h. Average unit-cell dimensions for synthetic marialite at 298 K and 1 atm are a_o = 12.038 ± 0.002 Å, c_o = 7.539 ± 0.004 Å, and V_o = 1092.6 ± 0.8 ų, with a molar volume of 328.99 ± 0.24 cm³/mole. Thermal expansion measurements were made at 1 atm from 298–1105 K and showed that a increases while c decreases with an overall increase in volume upon heating. Compressibility measurements were made at room temperature in a diamond-anvil cell using 4:1 methanol: ethanol pressure medium in transmission mode at the Cornell High Energy Synchrotron Source facility with pressures ranging from 1 atm to 9.6 GPa. The a dimension is more compressible than c up to ~ 5 GPa, beyond which there is noticeable softening along the c axis. Equation of state modeling was done on the combined pressure–temperature-volume data using a Tait equation of state yielding bulk modulus and thermal expansion values for K_o, K’, and α of 51.0 ± 2.0 GPa, 6.68 ± 0.83, and 2.75 ± 0.17 × 10^–5/K, respectively. Compared with other scapolite data in the literature, the marialite (Na₃Al₃Si₉O₂₄·NaCl)-meionite (Ca₃Al₆Si₆O₂₄·CaCO₃) join behaves similarly to the albite-anorthite plagioclase join, with end-member marialite having the highest thermal expansion and lowest bulk modulus along the compositional join.

The phase diagram of the ZnO-SnO₂ system at 800–1600 °C was experimentally investigated using the classical equilibration/quenching method and differential thermal analysis (DTA) followed by X-ray diffraction (XRD) phase analysis and electron probe micro-analysis (EPMA). Sealed platinum capsules were employed to prevent the evaporation of ZnO and SnO₂ in the experiments. Based on new experimental phase diagram data and all available data in literatures, the binary ZnO-SnO₂, SnO₂-TiO₂, and ZrO₂-TiO₂ and the ternary ZnO-SnO₂-TiO₂ system was thermodynamically optimized using the CALculation of PHAse Diagram (CALPHAD) method to prepare a set of Gibbs energies of all phases within the binary systems which can be utilized to predict unknown phase equilibria and thermodynamic properties in the system.

The common wisdom that volume decreases with pressure and increases with temperature is analyzed in terms of Hillert nonequilibrium thermodynamics in the present work. It is shown that the derivative of volume to pressure in a stable system is always negative, i.e., volume decreases with the increase of pressure, when all other natural variables of the system are kept constant. This originates from the stability requirement that the conjugate variables, such as volume and negative pressure, must change in the same direction in a stable system. Consequently, since volume and temperature are not conjugate variables, they do not have to change in the same direction and thus do change in opposite directions in both natural and man-made systems. It is shown that the decrease of volume with the increase of temperature, commonly referred as negative thermal expansion (NTE) in the literature, originates from the statistical competitions of configurations in the system when the volumes of metastable configurations are smaller than that of the ground-state configuration. It is demonstrated that the zentropy theory can concisely explain and accurately predict NTE based on the density functional theory without fitting parameters.

In this study, the thermodynamic parameters of the Mn–Si–O system were re-evaluated using the CALPHAD approach. Available experimental data on phase equilibria were taken into account and thermodynamic properties such as heat capacity, standard entropy and standard enthalpy were reproduced within uncertainties. Three ternary compounds are found to be stable in the Mn–Si–O system: rhodonite (MnSiO(_3)), braunite (Mn(_7)SiO(_{12})) and tephroite (Mn(_2)SiO(_4)). Braunite was modeled by CEF, while tephroite and rhodonite were modeled as stoichiometric compounds. Two-sublattice partially ionic liquid model was used to describe the liquid phase. The braunite phase exhibits a homogeneity range and can dissolve Mn(_2)O(_3) in some extension. Phase diagrams for the MnO–SiO(_2) system in the presence of metallic Mn and the MnO(_x)–SiO(_2) system in air were calculated and showed good agreement with existing literature data. The thermodynamic parameters were evaluated to describe the experimental data over the entire compositional range of the system.

Perovskite-type oxides BaZr_1–xY_xO_3−x/2 (x = 0.1, 0.2) were synthesized and their enthalpy increments were measured by means of high-temperature drop calorimetry in the temperature range of (373–1273) K in air. The data obtained were used for estimating the high-temperature thermodynamic functions (constant pressure heat capacity and entropy increments) of the zirconates BaZr_1–xY_xO_3−x/2 (x = 0.1, 0.2). They were found to be only weakly dependent on the concentration of Y-dopant. Thermal expansion coefficient of zirconates BaZr_1–xY_xO_3−x/2 (x = 0.1, 0.2) was successfully estimated by Grüneisen equation. Also, Neumann-Kopp rule was shown to be inapplicable for accurate estimation of heat capacities of the studied oxides. Thermodynamic analysis showed that BaZr_1–xY_xO_3−x/2 (x = 0.1, 0.2) oxides are prone to chemical interaction with CO₂ at typical working temperatures of proton-conducting solid oxide fuel cells. Some possibilities to overcome this issue have been discussed.

The temperature dependence of the infrared absorption spectra of two Verneuil-grown corundum samples is investigated in the OH stretching range. The spectra display three main bands at 3184, 3232 and 3309 cm^− 1, belonging to the so-called “3309 cm^− 1 series”, as well as two additional bands at 3163 and 3278 cm^− 1 previously reported in some synthetic corundum samples. The anharmonic behavior of the observed bands is analyzed using the pure dephasing model of Persson and Ryberg and depends on the local geometry of the OH defects, which are all associated with Al vacancies. The unexpected increase with temperature in the absorbance of a weak band at 3209 cm^− 1 supports a revised interpretation of both the 3209 and 3232 cm^− 1 bands. These two bands are interpreted as resulting from the low-temperature equilibrium between two Ti-associated OH defects, enabled by the possibility of hydrogen hopping within the Al vacancy. The temperature-dependent properties of the 3278 cm^− 1 band are similar to those of the other Al-vacancy related defects and a comparison with the theoretical properties of selected OH defects suggests that this band corresponds to the association of the H atom with a non-dissociated Al Frenkel pair. Finally, the properties of the band at 3163 cm^− 1 are consistent with its previously proposed association with Si for Al substitution in corundum.

Studies conducted in NaF-containing hydrothermal fluids have shown that the oxide compounds Sb⁵⁺ are unstable at 800 °C, Р_total = 200 MPa and fO₂ (fH₂) specified by Co–CoO and Ni–NiO buffers interact with the Pt material of the ampoule, forming antimony intermetallics with platinum on the inner surface of the ampoule. The formation of the following intermetallics was established through the analysis of data obtained from studies conducted on an electronic microscope: Pt_90.3±0.8Sb_9.7 (~ Pt₁₀Sb), Pt_82.8±1.3Sb_17.2 (~ Pt₅Sb) and Pt_69.2±4.4Sb_30.8. Pt₁₀Sb compound which was obtained on the inner surface of the Pt ampoule is the limiting solid solution of antimony in platinum at 800 °C. It exhibits a cubic crystal system (Fmoverline{3}m) with a lattice constant of a = 3.943(3) Å and forms an underdeveloped surface < 111>. Pt₅Sb compound, presumably hexagonal P6/mmm crystal system with unit cell parameters a = b = 4.56(4), c = 4.229(2) Å, α = β = 90°, γ = 120°, forms a thin film (≤ 10 μm) on the Pt surface and appears to be a metastable phase. The intermetallic compound of Pt₆₉Sb₃₁ is a rapidly cooled melt of appropriate composition.

A mechanism for deep penetration of Sb into the walls of the Pt ampoule is proposed.