Physics and Chemistry of Minerals最新文献

The single-crystal elastic moduli of three natural aragonites, CaCO₃, have been determined at ambient conditions by Brillouin spectroscopy. The samples contain different amounts of Sr ranging from 0.1 mol% to 1.5 mol% and cover the majority of the compositional range of natural aragonites. Brillouin spectroscopy can resolve changes in elasticity produced by small differences in the Sr content in aragonite, however, these changes are at the limit of the resolving power of our measurements. Our results are in good agreement with the full tensor of a Sr-bearing natural aragonite determined by Brillouin spectroscopy and with the tensor obtained by DFT calculations. The decrease we measured of the aggregate adiabatic bulk modulus, (K_S), with increasing Sr content is qualitatively in good agreement with the softening observed in previously measured isothermal bulk modulus, (K_T), of synthetic CaCO₃-SrCO₃ solid solutions and with the value of (K_S) determined for SrCO₃. Our study provides the first full tensor on nominally pure (end-member) aragonite and places contraints on its dependence on Ca/Sr substitutions at levels observed in natural samples. Furthermore, we compare the elastic tensor of aragonite and calcite, the two main CaCO₃ polymorphs observed at ambient conditions, and describe their differences based on the arrangement of the polyhedral structural units of the two polymorphs. In particular, the different arrangement of the Ca-O polyhedra leads to a smaller compressibility of aragonite along the c-axis, higher compressibilities along the a- and b-axes and an overall lower bulk modulus ((-)9.7%) with respect to that of the less dense calcite.

Quartz is one of the most abundant minerals in the Earth crust and therefore quartz inclusions in garnet are of great interest for elastic geobarometry, an approach that exploits the elastic properties of the mineral pair to back-calculate the conditions of inclusion entrapment. However, the high-temperature behavior of quartz inclusions close to the (alpha)–(beta) transition boundary has not been studied experimentally. We have therefore performed in situ high-temperature Raman spectroscopy on a quartz-in-garnet system up to 1000 K, and have also collected an improved reference data set for the temperature dependence of the Raman scattering of free quartz. Our results show that the (alpha)-to-(beta) phase transition is hindered by the stress imposed by the host on the quartz inclusion, resulting in a thermosalient effect of the whole host-inclusion system or a mechanical cracking of the host mineral.

The pioneering hydrothermal synthesis of the compound Be₂[(Si_1−xGe_x)O₄] with a phenakite structure (the size of individual crystals up to 1 mm) was carried out in acidic alkaline-containing fluoride solutions at a temperature of 625 °C and a pressure of ~ 150 MPa. Uniform (x = 0, 0.80 and 1) and zonal (x = 0.04 − 0.025) Be₂[(Si_1−xGe_x)O₄] crystals synthesized in the Li-containing mineralizer were obtained. The possibility of formation of intermediate compounds under hydrothermal conditions remains an open question. In the Na-containing mineralizer, only Be₂SiO₄ crystallizes due to the formation of insoluble sodium germanates. The fading of formation of Be₂[(Si_1−xGe_x)O₄] was determined with the use of technique of temperature-induced zoning and can be explained by the fact that newly formed crystals screen the surface of the initial BeO. The instantaneous growth rates of the prismatic faces of Ge-substituted phenakite crystals, decreasing from 18 microns/day to 2, were determined using the technique of temperature-induced zonality. The crystal structures of Be₂[(Si_1−xGe_x)O₄] samples with x = 0, 0.80 and 1 were refined by direct X-ray diffraction methods, and the linear dependence of the unit cell parameters and bond lengths on the germanium content has been quantitively described. First Raman spectroscopy study of Be₂[(Si_1−xGe_x)O₄] on zonal crystals indicated the linear shift of vibration bands in Raman spectra to a lower frequencies with an increase in germanium concentration (x up to 0.25). A new Raman band of Ge–O stretching vibrations at ~ 1115 cm⁻¹, which is not common for natural and synthetic germanium-free phenakites, was observed.

Fe–Ni–S–Si alloy is considered to be one of the plausible candidates of Mercury core material. Elastic properties of Fe–Ni–S–Si liquid are important to reveal the density profile of the Mercury core. In this study, we measured the P-wave velocity (V_P) of Fe–Ni–S–Si (Fe₇₃Ni₁₀S₁₀Si₇, Fe₇₂Ni₁₀S₅Si₁₃, and Fe₆₇Ni₁₀S₁₀Si₁₃) liquids up to 17 GPa and 2000 K to study the effects of pressure, temperature, and multiple light elements (S and Si) on the V_P and elastic properties.

The V_P of Fe–Ni–S–Si liquids are less sensitive to temperature. The effect of pressure on the V_P are close to that of liquid Fe and smaller than those of Fe–Ni–S and Fe–Ni–Si liquids. Obtained elastic properties are K_S0 = 99.1(9.4) GPa, K_S’ = 3.8(0.1) and ρ₀ =6.48 g/cm³ for S-rich Fe₇₃Ni₁₀S₁₀Si₇ liquid and K_S0 = 112.1(1.5) GPa, K_S’ = 4.0(0.1) and ρ₀=6.64 g/cm³ for Si-rich Fe₇₂Ni₁₀S₅Si₁₃ liquid. The V_P of Fe–Ni–S–Si liquids locate in between those of Fe–Ni–S and Fe–Ni–Si liquids. This suggests that the effect of multiple light element (S and Si) on the V_P is suppressed and cancel out the effects of single light elements (S and Si) on the V_P. The effect of composition on the EOS in the Fe–Ni–S–Si system is indispensable to estimate the core composition combined with the geodesy data of upcoming Mercury mission.

In the first two papers of this series [Behrens, Phys Chem Minerals 48:8, 2021a; Behrens, Phys Chem Minerals 48:27, 2021b], incorporation of hydrogen in the feldspar structure, partitioning of hydrogen between feldspars and gases/fluids and self-diffusion of hydrogen in feldspars have been discussed, with particular focus on sanidine. Here, the results of reactions between sanidine containing strongly bonded hydrogen defects and (Na,K)Cl are presented. Experiments were performed at ambient pressure at temperatures of 605–1000 °C, and hydrogen profiles were measured by IR microspectroscopy. Profiles can be interpreted by an incomplete dehydrogenation at the crystal surface or a strong concentration dependence of hydrogen diffusivity. Both are consistent with hydrogen located on interstitial sites and difficult to substitute by the larger alkali ions. Chemical diffusivities of hydrogen derived from fitting of the profiles or Boltzmann–Matano analysis are similar to self-diffusivities determined by D/H exchange experiments. Activation energies are also comparable. Comparison to sodium and potassium diffusion data for sanidine (Wilangowski et al. in Defect Diffus Forum 363: 79–84, 2015; Hergemöller et al. in Phys Chem Minerals 44:345–351, 2017) supports a mechanism of proton diffusion charge-compensated by Na⁺ diffusion for hydrogen removal in the sanidines under dry conditions.

Turquoise is a well-known gemstone that has been used in artefacts across many cultures throughout history. However, due to its porosity it is often treated to enhance its color and beauty. One appreciated treatment is the patented Zachery process, although its details remain publicly undisclosed. Previous studies indicated that only a high K content distinguishes Zachery-treated from natural turquoises. In this study, natural and Zachery-treated turquoise samples from the famous Kingman mine, Arizona, USA, were analysed by means a multi-methodological approach, including standard gemological testing, electron microprobe (EMPA), scanning electron microscope with energy dispersive spectrometer (SEM–EDS) and X-ray diffraction (XRD), Fourier-Transform InfraRed (FTIR), non-destructive External Reflection-Fourier-Transform InfraRed (ER-FTIR) spectroscopy and X-ray computed microtomography (μCT). The results revealed new chemical–mineralogical and microstructural features that distinguish the Zackery-treated from the natural turquoise: higher specific gravity and lower porosity, associated with high and uneven concentrations of Cu, K and Na, the occurrence of tenorite (CuO), the presence and extension of reaction edges in the entire volume are distinctive of treated samples. Moreover, Cu-rich seeds and feldspar crystals may be interpreted as additional components used during the treatment. The hypothesis is that the Zachery treatment induces the re-crystallization of a new turquoise-like phase, which differs from the natural one from a chemical and microstructural point of view.

The thermal transport properties of minerals at high temperature and high pressure are important for understanding the internal evolution and dynamic processes of the Earth. Here, we carry out a detailed study on the lattice thermal conductivities (({kappa }_text {latt})) of (text {Mg}_2text {SiO}_4) under upper mantle and transition zone conditions by anharmonic lattice dynamics method. The calculations show that the ({kappa }_text {latt}) of (text {Mg}_2text {SiO}_4) increase with the phase transitions, which agree with the previous measurements and are attributed to the increase of lifetime and group velocity under extreme conditions. The ({kappa }_text {latt}) of (text {Mg}_2text {SiO}_4) along the geotherm shows a 64(%) jump at 410 ({textrm{km}}) and 71(%) jump at 520 (textrm{km}). The anisotropy in the ({kappa }_text {latt}) of olivine and wadsleyite decreases with increasing pressure. The present findings offer a fundamental knowledge of the ({kappa }_text {latt}) of (text {Mg}_2text {SiO}_4) under extreme conditions, which are crucially important for understanding the thermal transport processes in the Earth.

Fluorite is one of the most common minerals in the crust and is of widespread economic importance. It shows strong UV-excited luminescence, variously attributed to defects within the fluorite structure and lanthanide substitutions. We present here a detailed chemical characterisation of a suite of natural fluorite samples, chosen to represent the range of compositions observed in nature. We perform X-ray excited luminescence spectroscopy on the samples as a function of temperature (20–673 K) in the wavelength range 250–800 nm to provide insights into physical defects in the lattice and their interactions with lanthanide substituents in natural fluorite. Most broad bands in the UV are attributed to electronic defects in the fluorite lattice, whereas sharp emissions are attributed to intra-ion energy cascades in trivalent lanthanides. Lanthanides are accommodated in fluorite by substitution for Ca²⁺ coupled with interstitial F⁻, O²⁻ (substituting for F⁻) and a variety of electronic defect structures which provide local charge balance. The chondrite-normalised lanthanide profiles show that fluorite accommodates a greater proportion of heavy lanthanides (and Y) as the total Rare Earth Element (REE) concentration increases; whereas cell parameters decrease and then increase as substitution continues. Luminescence intensity also goes through a maximum and then decreases as a function of REE concentration. All three datasets are consistent with a model whereby lanthanides initially act as isolated centres, but, beyond a critical threshold (~ 1000 ppm), cluster into lanthanide-rich domains. Clustering results in shorter REE-O bond distances (favouring smaller heavier ions), a larger unit cell but more efficient energy transfer between lanthanides, thereby promoting non-radiative energy loss and a drop in the intensity of lanthanide emission.

Topaz is an important mineral formed in deeply subducted sediments and might be a major carrier of both H₂O and fluorine into the Earth’s interior. To better understand the seismic velocities and H₂O and fluorine recycling in subduction zones, we determined the thermal expansivity of a natural topaz (Al_1.93(1)Si_1.06(1)O₄(OH)_0.48(3)F_1.52(3), space group pbnm) up to 1073 K using high-temperature powder X-ray diffraction. No phase transition or decomposition was observed within the investigated temperature range. The volume thermal expansion coefficient is 2.24(1) × 10^–5 K⁻¹, and the ratio of the axial thermal expansion coefficients α₀(a):α₀(b):α₀(c) is 1.15:1:1.32 at 300 K. We also investigated its compressional (P) and shear (S) wave velocities up to 13.6 GPa at room temperature using ultrasonic interferometry in a multi-anvil apparatus. The adiabatic bulk modulus (K_s) and shear modulus (G) of topaz and their pressure derivatives are K_S0 = 151(1) GPa, K_S′ = 4.9(1), G₀ = 109.4(10) GPa, and G′ = 1.8(1), respectively, by fitting the velocities and density data to finite strain equations. The density and velocity profiles of the topaz were calculated under the upper mantle P–T conditions. Our results reveal that topaz is prone to subduction which drives H₂O and fluorine to migrate to the deep Earth. Meanwhile, topaz also has unusually high V_P and V_S, and low V_P/V_S ratio relative to common upper mantle phases and the preliminary reference Earth model (PREM, Dziewonski and Anderson, Phys Earth Planet Inter 25:297–356, 1981), which may be diagnostic seismic properties in subducted slabs.

The behaviors of aragonite (CaCO(_3)), strontianite (SrCO(_3)), cerussite (PbCO(_3)), and witherite (BaCO(_3)) at increasing pressure have been studied up to 6 GPa using density functional theory with plane waves. A parallelism of the orthorhombic carbonates with the closed-packed AsNi structure is considered in our analysis, being the CO(_3^{2-}) groups not centered in the interstice of the octahedron. The decomposition of the unit-cell volume into atomic contributions using the Quantum Theory of Atoms in Molecules has allowed the analysis of the bulk modulus in atomic contributions. The bulk, axes, interatomic distances, and atomic compressibilities are calculated. The largest compression is on the c crystallographic axis, and the c linear modulus has a linear function with the mineral bulk modulus ((K_0)). Many of the interatomic distances moduli of the alkaline earth (AE) carbonates show linear functions with the bulk modulus; however, the whole series (including cerussite) only gives linear functions when (K_0) is related either with the CC distances modulus or the modulus of the distances of the C to the faces of the octahedron perpendicular to c. These last distances are the projections of the Metal–Oxygen (MO) distances to the center of the octahedron. (K_{0AE}) carbonates also show linear functions with the atomic moduli of their cations. However, the whole series show a linear relation with the atomic modulus of C atoms. Therefore, the whole series highlight the importance of the C atoms and their interactions in the mechanism of compression of the orthorhombic carbonate series.