Physics and Chemistry of Minerals最新文献

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.

The ultimate mechanical properties of MgSiO₃ orthoenstatite (OEN), as characterized here by the ideal strengths, have been calculated under tensile and shear loadings using first-principles calculations. Both ideal tensile strength (ITS) and shear strength (ISS) are computed by applying homogeneous strain increments along high-symmetry directions ([100], [010], and [001]) and low index shear planes ((100), (010), and (001)) of the orthorhombic lattice. We show that the ultimate mechanical properties of OEN are highly anisotropic during tensile loading, with ITS ranging from 4.5 GPa along [001] to 8.7 GPa along [100], and quite isotropic during the shear loading with ISS ranging from 7.4 to 8.9 GPa. During tensile test along [100] and [001], a modified structure close to OEN has been found. This modified structure is more stable than OEN under stress (or strain). We have characterized its elastic and ultimate properties under tensile loading. With ITS ranging from 7.6 GPa along [010] to 25.6 GPa along [001], this modified structure appears to be very anisotropic with exceptional strength along [001].

A methodology has been developed, detailing the theory and workflow, for applying the double-difference relocation method to acoustic emission (AE) event location in high-pressure/high-temperature deformation experiments in the multi-anvil apparatus. The process is predicated on the fact that events originating from a common source region will traverse similar ray paths from the source to the receiver and display similar waveforms in seismograms. This implies their travel-time difference results only from their spatial offset and any velocity heterogeneity along the ray path is negated. To demonstrate the efficacy of this approach we applied it to a transformational faulting experiment on the isostructural olivine analogue Mg₂GeO₄ under controlled deformation at 2.5 GPa and 700 °C while simultaneously monitoring stress, strain, and acoustic activity. Waveforms from all 1456 AE events were cross-correlated to measure differential arrival times and construct multiplet groups of similar events. In total, 110 multiplets were identified whose size is dominated by two large groups containing 272 and 202 events. Relocation of these two multiplets using the double-difference method significantly reduces event separation and improves location uncertainty by more than an order of magnitude when compared to absolute location techniques whose uncertainty rivals that of the sample size. In particular, event locations of the two largest multiplets reveal two dense clusters whose spatial geometry closely mirrors that of macroscopic faulting displayed in computerized tomography images of the recovered sample. In this way, we are able to link specific faults with their associated AE events, which would otherwise not be possible using traditional absolute location methods.

Experiments in which two identical polycrystalline ice Ih specimens are simultaneously subjected to the same time–temperature history while one of the specimens is actively deformed via grain size-sensitive (GSS) creep demonstrate distinctly different microstructural evolution: for particular ranges of starting grain size and differential stress, grains do not grow in the deforming specimen. Ice Ih specimens having initial, uniform grain sizes in the range d = 6–63 μm were tested in pairs that were subjected to identical time–temperature conditions (durations t = 4–12 days; T = 240 K) but of which only one was subjected to differential stress (σ₁ = 0.25–1.85 MPa; σ₃ = 0). Comparing specimens within a pair, for those with coarser initial grain size, the deformed specimens exhibit suppressed or no grain growth. Our results are interpreted from the perspective of nonequilibrium thermodynamics, specifically comparing the energy dissipation rates associated with both grain growth and plastic flow: if the rate of energy dissipation associated with flow exceeds that of grain growth, the grains will not grow. An examination of the limited database on GSS flow and grain growth in silicates conforms to our analysis. The results are applied to the question of the mechanical evolution of terrestrial glaciers and to the ice-rich shells of the outer satellites.

The equation of the state of a natural elbaite sample has been investigated at room temperature and up to 21.1 GPa for the first time using in situ synchrotron X-ray diffraction in this study. No phase transition is observed on elbaite over the experimental pressure range. The pressure–volume data were fitted by the third-order Birch-Murnaghan equation of state (EoS) with the zero-pressure unit-cell volume V₀ = 1540.7 (6) ų, the zero-pressure bulk modulus K_T0 = 114.7 (7) GPa, and its pressure derivative K'_T0 = 4.2 (1), while obtained V₀ = 1540.1 (4) ų and K_T0 = 116.4 (4) GPa when fixed K'_T0 = 4. Furthermore, the axial compressional behavior of elbaite was also fitted with a linearized third-order Birch-Murnaghan EoS, the obtained axial moduli for a-axis and c-axis are K_a0 = 201 (4) GPa and K_c0 = 60 (1) GPa, respectively. The axial compressibilities of a-axis and c-axis are β_a = 1.66 × 10^–3 GPa⁻¹ and β_c = 5.56 × 10^–3 GPa⁻¹ with an anisotropic ratio of β_a: β_c = 0.30: 1.00, which shows an intense axial compression anisotropy. The potential influencing factors on the bulk moduli and the anisotropic linear compressibilities of tourmalines were further discussed.

The high-pressure (HP) behavior of natural stellerite |Ca_4.00Na_0.16 (H₂O)₃₂| [Al_8.16Si_27.84O₇₂] has been studied by single-crystal X-ray diffraction using a diamond-anvil cell under pressures up to 4.5 GPa, with a 4:1 ethanol:water mixture and paraffin as pressure-transmitting media. The changes in the structure of stellerite at high pressures, especially the STI framework deformation, are similar to those in Na-rich stilbite |Ca_4.00Na_1.47 (H₂O)₃₀| [Al_9.47Si_26.53O₇₂]. Both stilbite and stellerite udergo pressure-induced hydration, in which H₂O molecules first occupy partly vacant sites and then the initially vacant positions. Some difference in the behavior of the two minerals is due to the presence of Na⁺ cations in stilbite. Sodium occupies positions in the 10-membered ring and prevents H₂O molecules from penetrating near the ring center. Meanwhile, both stellerite and stilbite can fill the initially vacant sites in the 8-membered ring at high pressures. The pressure-induced changes, including the reduction of H₂O sites in the cation coordination and a total number of H₂O molecules, are less significant in Na-bearing stilbite than in stellerite.

Fission tracks are used for geologic age-dating and for the reconstruction of thermal histories of Earth’s upper crust. However, there remains a gap in the understanding between the atomic-scale annealing mechanism of latent (unetched) fission tracks and the observations of etched tracks at the micrometer scale. This is because the structure of latent fission tracks is lost during the leaching process. We have conducted the first comparison of the thermal-annealing behavior of latent and etched tracks in epidote, using transmission electron microscopy (TEM) and optical microscopy, respectively. For high-resolution TEM observations, we used ion tracks instead of fission tracks to control the density of tracks, and we demonstrated that latent ion tracks are amorphous in epidote. The reduction in diameters of latent tracks is insignificant after thermal annealing at 800 °C for 24 h, indicating that the track diameter does not appear to change substantially until the final stage of annealing. The optical observations show that the parallel etched ion-induced tracks have a lower track density reduction rate at or below 500 °C and an accelerated reduction rate above 500 °C as compared with randomly oriented fission tracks. However, the two types of tracks display a comparable thermal-annealing behavior as evidenced by the fit of the two curves for the normalized density of ion and fission tracks as a function of temperature with the same equation but with different fitting parameters. Thus, ion-induced tracks can be used to simulate fission tracks in epidote and provide a basis for understanding the annealing of latent and etched tracks.