Physics and Chemistry of Minerals最新文献

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.

The Cr³⁺ luminescence spectra of zoisite and kyanite, two geologically important minerals, were studied up to 40 and 20 GPa, respectively, in various pressure media. Cr³⁺ substitutes into the octahedral aluminum sites in both minerals and the R-line luminescence is a particularly sensitive site-specific probe of the octahedral Al site. Unlike many previous studies where Cr³⁺ luminescence was utilized, both these minerals have multiple highly distorted octahedral sites resulting in very large splitting of their R-lines, ~ 300 cm⁻¹ in zoisite and ~ 360 cm⁻¹ in kyanite (for reference, ruby is 29 cm⁻¹). For zoisite, the R-line splitting increases as pressure increases and more than triples from its ambient value by 40 GPa, while the R-line splitting in kyanite from the M1 and M2 sites does not change when compressed in a Ne pressure medium up to 20 GPa. We do not observe evidence of any phase transitions in either zoisite or kyanite across the pressure range of these new luminescence measurements. We present some high-pressure luminescence results where kyanite was known to be bridged between the diamond anvils and show how these spectra illustrate the different effect of uniaxial relative to hydrostatic stress on luminescence spectra.

GeoSoilEnviroCARS (GSECARS) is a comprehensive analytical laboratory for Earth and environmental science research using X-ray beams from the Advanced Photon Source, Argonne National Laboratory. State-of-the-art instruments are available for (1) high-pressure/high- or low-temperature diffraction, total scattering, and spectroscopy (Brillouin, Raman, and VIS-IR) using the laser heated diamond anvil cell (DAC); (2) high-pressure/high-temperature diffraction, scattering, and imaging as well as acoustic emission (AE) and ultrasonics using the large-volume press (LVP); (3) powder, single crystal, and surface/interface diffraction; (4) X-ray absorption fine structure spectroscopy; (5) X-ray fluorescence microprobe analysis; and (6) microtomography. Experiments are facilitated by senior level staff who collaborate on all aspects of the analytical work including experiment design, sample preparation, data collection, data interpretation, and publication preparation. Both technical and scientific synergies occur as a result of the intimate association of the various techniques and scientists experienced in the applications of synchrotron radiation to Earth, environmental, and planetary science problems. The facility includes state-of-the-art instrumentation designed and built in-house, including custom X-ray optics, online and offline laser-based systems, specialized sample environments and positioning systems, as well as pixel-array and multi-crystal energy dispersive X-ray detectors, which are available to be shared among the experimental stations.

Agates from Yozgat province are appreciated on the gem market for their white and purple-blue banded colours. In this study, we present a detailed investigation aimed at the identification of the atomic and structural origin of this peculiar colouration of chalcedony. X-ray diffraction and Raman spectroscopy revealed the presence of fine grains of quartz and moganite with a preferential accumulation of the latter in the blue bands. Near-infrared diffuse absorption spectra show overtones of hydroxyls vibrations at 1425, 1900, and 2250 nm. In the visible, the broad absorption at about 500 nm, as well as its behaviour at low temperatures, is compatible with the optical activity of iron impurities in quartz matrices, such as that observed in amethysts. Peak intensities and shapes are very similar for spectra collected in blue and white bands. Accordingly, trace-element composition from laser ablation inductively coupled mass spectrometry confirmed that the two regions have similar Fe content. The perceived changes in band colours are indeed originated by differences in microstructural arrangement and size of the grains visualised through scanning electron microscopy. White and blue stripes have grains of about 5 µm and 300 nm in size, respectively, resulting in an accentuated scattering component for the white bands. Therefore, the unique purple-blue shades typical of Yozgat agates are a combination of iron-related colour centres and scattering effect.

Adding hydrogen to forsterite strongly increases the diffusion rate of Mg, but the reason for this is unclear. As Mg diffusion in forsterite can influence its electrical conductivity, understanding this process is important. In this study we use density functional theory to predict the diffusivity of H-bearing Mg vacancies and we find that they are around 1000 times slower than H-free Mg vacancies. H-bearing Mg vacancies are many orders of magnitude more concentrated than H-free Mg vacancies, however, and diffusion is a combination of diffusivity and defect concentration. Overall, the presence of hydrated Mg vacancies is predicted to cause a large (multiple orders of magnitude) increase in both diffusion rate and diffusional anisotropy with a strong preference for diffusion in the [001] direction predicted. In models of experimental data, the effect of water concentration on diffusion is often described by a constant best-fitting exponent. Our results suggest that this exponent will vary between 0.5 and 1.6 across common experimental conditions with pressure decreasing and temperature increasing this exponent. These results suggest that Mg diffusion in forsterite could vary considerably throughout upper mantle conditions in ways that cannot be captured with a simple single-exponent model. Comparisons to measures of hydrogen diffusivity suggest that the diffusion of hydrated Mg vacancies also controls the diffusion of hydrogen in (iron-free) forsterite and that our conclusions above also apply to hydrogen diffusion rates and anisotropy. We also find that cation diffusivity likely cannot explain experimental measurements of the effect of water on electrical conductivity in olivine.