Physics and Chemistry of Minerals最新文献

We performed molecular dynamics simulations based on density functional theory to systematically investigate the Fe–Ni–C system including (1) pure Fe and Ni; (2) binary Fe–Ni, Fe–C, and Ni–C; and (3) ternary Fe–Ni–C liquid compositions at 3000 K and three simulation volumes corresponding to pressure (P) up to 83 GPa. Liquid structural properties, including coordination numbers, are analyzed using partial radial distribution functions. Self-diffusion coefficients are determined based on the atomic trajectories and the asymptotic slope of the time-dependent mean-square displacement. The results indicate that the average interatomic distance between two Fe atoms (r_Fe–Fe) decreases with P and is sensitive to Ni (X_Ni) and C (X_C) concentration, although the effects are opposite: r_Fe–Fe decreases with increasing X_Ni, but increases with increasing X_C. Average r_Fe–C and r_Ni–C values also decrease with increasing X_Ni and generally remain constant between the two lowest P points, corresponding to a coordination change of carbon from ~ 6.8 to ~ 8.0, and then decrease with additional P once the coordination change is complete. Carbon clustering occurs in both binary (especially Ni–C) and ternary compositions with short-range r_C-C values (~ 1.29 to ~ 1.57 Å), typical for r_C-C in diamond and graphite. The self-diffusion results are generally consistent with high-P diffusion data extrapolated from experiments conducted at lower temperature (T). A subset of additional simulations was conducted at 1675 and 2350 K to estimate the effect of T on diffusion, yielding an activation enthalpy of ~ 53 kJ/mol and activation volume of ~ 0.5 cm³/mol.

One of the most extensively investigated properties of Fe is electrical resistivity (ρ) which features several well-established temperature (T) dependent characteristics. Based on systematic data for Fe and Fe–Si alloys and guidance from the 1 atm diagram, the T-X (composition X) phase diagram can be obtained at constant pressure (P) by plotting maps of constant ρ contours, proposed herein to be called ‘iso-ohms’, maps of constant δρ/δT, as well as maps of variations of exponent x in power law fits to ρ(T^x). Electrical resistivity measurements of Fe-(≤ 17wt%)Si alloys are available at 3–5 GPa and up to liquid temperatures. This study provides a new approach to mapping high P–T structural phase diagrams in the Fe-rich region of the Fe–Si system at moderate pressures and may be used for other compositions. Maps of iso-ohms suggest the α phase is the less resistive phase followed by α₂, α₁, η + α₁, and ε + α₁. Above ~ 11 wt% Si, the ε + α₁ displays a higher ρ than the liquid phase. This is discussed in terms of s and d electron scattering. Maps of constant δρ/δT emphasize the magnetic transition and boundary between the ε + α₁ and η + α₁ phases, while variations of exponent x outline the α₂ region. The Fe–Si system is of interest to the cores of the moon and Mercury.

Using the evolutionary crystal structure prediction algorithm USPEX, we showed that at pressures of the Earth’s lower mantle (24-136 GPa) CaAl₂O₄ is the only stable calcium aluminate. At pressures above 7.0 GPa it has the CaFe₂O₄-type structure with space group Pnma. This phase is one of the prime candidate aluminous phases in the lower mantle of the Earth. We show that at low pressures 5CaO • 3Al₂O₃ (C₅A₃) with space group Cmc2₁, CaAl₄O₇ (with space group C2/c) and CaAl₂O₄ (space group P2₁/m) are stable at pressures of up to 2.1, 1.8 and 7.0 GPa, respectively. The previously unknown structure of the orthorhombic "CA-III" phase is also found in our calculations. This phase is metastable at 0 K and has a layered structure with space group P2₁2₁2.

The thermal behaviour of an uvite from San Piero in Campo (Elba Island, Italy) was investigated at room pressure through in situ high-temperature powder X-ray diffraction (PXRD), until the breakdown conditions were reached. The variation of uvite structural parameters (unit-cell parameters and mean bond distances) was monitored together with site occupancies and we observed the thermally induced Fe oxidation process counterbalanced by (OH)⁻ deprotonation, which starts at 450 °C and is completed at 650 °C. The uvite breakdown reaction occurs between 800 and 900 °C. The breakdown products were identified at room temperature by PXRD and the breakdown reaction can be described as follows: tourmaline → indialite + yuanfuliite + plagioclase + “boron-mullite” phase + hematite.

Magnetic and structure transitions of Mn_3–xFe_xO₄ solid solutions under extreme conditions are clarified by neutron time-of-flight scattering diffraction and X-ray Mössbauer measurement. The ferrimagnetic-to-paramagnetic transition temperature (100 °C) of Mn₂FeO₄ spinel is different from the tetragonal-to-cubic structure transition temperature (180 °C). The structure transition temperature decreases with increasing pressure. The transition is not coupled with the magnetic transition. Synchrotron X-ray Mössbauer experiments have revealed the pressure effects on the distribution of Fe²⁺ and Fe³⁺ at the tetrahedral and octahedral sites in the spinel structure. Ferrimagnetic MnFe₂O₄ and Mn₂FeO₄ spinels show sextet spectral features with hyperfine structure elicited by internal magnetic fields. Cubic MnFe₂O₄ spinel and tetragonal Mn₂FeO₄ transform to high-pressure orthorhombic postspinel phase above pressures of 18.4 GPa and 14.0 GPa, respectively. The transition pressure decreases with increasing Mn content. The postspinel phase has a paramagnetic property. Mn₂O₁₀ dimers of two octahedra are linked via common edge in three dimentional direction. The occupancy of Fe²⁺ in the tatrahedral site is decreased with increasig pressure, indicating more oredered structure. Consequently, the inverse parameter of the spinel structure is increased with increasing pressure. The magnetic structure refinements clarify the paramagnetic and ferrimagnetic structure of MnFe₂O₄ and Mn₂FeO₄ spinel as a function of pressure. The magnetic moment is ordered between A and B sites with the anti-parallel distribution along the b axis. The nuclear tetragonal structure (a_N, a_N, c_N) has the ferrimagnetic structure but the orthorhombic magnetic structure has the ferrimagnetic structure with the lattice constants (a_M, b_M, c_M). The magnetic moment is ordered between A and B sites with the anti-parallel distribution along the b_M axis.

In this work, we studied the hydrothermal agates from the Neogene–Quaternary volcanic district of Allumiere-Tolfa, north-west of Rome (Latium, Italy) using a combination of micro-textural, spectroscopic, and geochemical data. The examined sample consists of (1) an outer cristobalite layer deposited during the early stages of growth, (2) a sequence of chalcedonic bands (including i.e., length-fast, zebraic, and minor length-slow chalcedony) with variable moganite content (up to ca. 48 wt%), (3) an inner layer of terminated hyaline quartz crystals. The textures of the various SiO₂ phases and their trace element content (Al, Li, B, Ti, Ga, Ge, As), as well as the presence of mineral inclusions (i.e., Fe-oxides and sulfates), is the result of physicochemical fluctuations of SiO₂-bearing fluids. Positive correlation between Al and Li, low Al/Li ratio, and low Ti in hyaline quartz points to low-temperature hydrothermal environment. Local enrichment of B and As in chalcedony-rich layers are attributed to pH fluctuations. Analysis of the FT-IR spectra in the principal OH-stretching region (2750–3750 cm⁻¹) shows that the silanol and molecular water signals are directly proportional. Strikingly, combined Raman and FT-IR spectroscopy on the chalcedonic bands reveals an anticorrelation between the moganite content and total water (SiOH + molH₂O) signal. The moganite content is compatible with magmatic-hydrothermal sulfate/alkaline fluids at a temperature of 100–200 °C, whereas the boron-rich chalcedony can be favored by neutral/acidic conditions. The final Bambauer quartz growth lamellae testifies diluted SiO₂-bearing solutions at lower temperature. These findings suggest a genetic scenario dominated by pH fluctuations in the circulating hydrothermal fluid.

The exhalation copper oxychloride vanadates attract increasing interest in the fields of both physics and chemistry. Based on the results of HT X-ray diffraction study of synthetic analogs of averievite (1) and yaroshevskite (2) and products of their thermal decomposition in air within the temperature range from 25 °C to 800 °C, it was found that 1 is stable up to 500 °C, and 2 is stable up to 480 °C. Both copper oxychloride vanadates expand anisotropically, but exhibit completely different thermal expansion patterns. 1 demonstrates an expansion in the direction perpendicular to the [O₂Cu₅]⁶⁺ layers, but inside the layer, the expansion is isotropic. The thermal expansion of 2 is much more anisotropic. The compression direction α₃₃ is close to the c axis, along which the structure tends to align the chains [O₂Cu₆]⁸⁺ into positions they would occupy in the layers [O₂Cu₅]⁶⁺ of the kagome type which exist in averievite. Meanwhile, the expansion direction α₁₁ is close to the a axis, along which the [O₂Cu₆]⁸⁺ chains shift tending to arrange as fragments of [O₂Cu₅]⁶⁺ layers. The thermal decomposition proceeds with loss of chlorine (most likely, both via hydrolysis/oxidation and evaporation of copper halides) and formation of copper vanadates.

High-Pressure Collaborative Access Team (HPCAT) is a synchrotron-based facility located at the Advanced Photon Source (APS). With four online experimental stations and various offline capabilities, HPCAT is focused on providing synchrotron x-ray capabilities for high pressure and temperature research and supporting a broad user community. Overall, the array of online/offline capabilities is described, including some of the recent developments for remote user support and the concomitant impact of the current pandemic. General overview of work done at HPCAT and with a focus on some of the minerals relevant work and supporting capabilities is also discussed. With the impending APS-Upgrade (APS-U), there is a considerable effort within HPCAT to improve and add capabilities. These are summarized briefly for each of the end-stations.

The equations of state of MgSiO₃-pyroxenes (low-pressure clinoenstatite, orthoenstatite and high-pressure clinoenstatite) are constructed using a thermodynamic model based on the Helmholtz free energy and optimization of known experimental measurements and calculated data for these minerals. The obtained equations of state allow us to calculate a full set of thermodynamic and thermoelastic properties as depending on T–P or T–V parameters. We offer open working MS Excel spreadsheets for calculations, which are a convenient tool for solving various user’s tasks. The phase relations in the MgSiO₃ system are calculated based on the estimated Gibbs energy for studied MgSiO₃-pyroxenes and clarify other calculated data at pressures up to 12 GPa and temperatures up to 2000 K. The obtained orthoenstatite → high-pressure clinoenstatite phase boundary corresponds to the following equation P(GPa) = 0.0021 × T(K) + 4.2. The triple point of equilibrium is determined at 1100 K and 6.5 GPa. Isotropic compressional (P) and shear (S) wave velocities of orthoenstatite and high-pressure clinoenstatite at different pressures are calculated based on the obtained equations of state. The calculated jumps of P- and S-wave velocities of orthoenstatite → high-pressure clinoenstatite phase transition at a pressure of ~ 9 GPa are 0.7 and 5.1%, respectively. The calculated jump of the density of this phase transition at a pressure of 8 GPa, which corresponds to the depth of ~ 250 km, is 2.9%. These results are used to discuss the location of the seismic X-discontinuity at the depths of 250–340 km, which is associated with phase boundaries in enstatite.

Here, we investigated high-pressure behaviors of four end-members of K-Na-Ca-Mg alkali-bearing double carbonates (K₂Mg(CO₃)₂, K₂Ca(CO₃)₂, Na₂Mg(CO₃)₂, and Na₂Ca(CO₃)₂) using first-principles calculations up to ~ 25 GPa. For K₂Mg, K₂Ca, and Na₂Mg double carbonates, the transitions from rhombohedral structures (R (stackrel{mathrm{-}}{3}) m or R (stackrel{mathrm{-}}{3})) to monoclinic (C2/m) or triclinic (P (stackrel{mathrm{-}}{1})) structures are predicted. While for Na₂Ca(CO₃)₂, the P2₁ca structure remains stable across the calculated pressure range. But the high-pressure behavior of Na₂Ca double carbonate has changed over 8 GPa: the b-axis becomes more compressible than a-axis; [CO₃] –I groups tilt out of the a-b plane upon compression and reverse the direction of rotation at 8 GPa. The parameters for the equations of state of these minerals and their high-pressure phases were all theoretically determined. The predicted transformation is driven by the differences in the compressibility of structural units. The K⁺ and Na⁺ coordination polyhedra are more compressible in the structure, compared with the high axial rigidity of C–O bonds in the [CO₃] triangle along the a-b plane. Our results provide projections of the high-pressure behaviors of trigonal double carbonates, in part by helping to clarify the relation among the average metallic ionic radius (R_avg), the bulk modulus (K₀), and the transition pressure (P_T). The transition pressure (P_T) is anticorrelated to the average metallic ionic radius (R_avg), and a larger R_avg results in a lower bulk modulus (K₀) for the trigonal double carbonates. Furthermore, alkali-bearing double carbonates found as inclusions in the natural diamond may indicate a hydrous parental medium composition and a deeper genesis mechanism.