Physics and Chemistry of Minerals最新文献

Bütschliite, K₂Ca(CO₃)₂, occurring as inclusions in mantle minerals, is regarded as one of the key phases to understand phase relationships of dense potassium carbonates and thus to evaluate their potential role within the Earth’s deep carbon cycle. Accordingly, the high-pressure behavior of synthetic bütschliite has been investigated by in-situ single-crystal X-ray diffraction under isothermal compression up to 20 GPa at T = 298 K. The compression mechanism before and after the trigonal-to-monoclinic (R-3m to C2/m) phase transition at ∼6 GPa, found previously, is characterized in terms of the evolution of the cation polyhedra and carbonate groups. On this basis, the modulation of the axial compression is interpreted, and the contribution of the cation polyhedra into the bulk compression is estimated. The refined compressibility of the monoclinic phase (K₀ = 44(2) GPa) fits to the trend of the carbonate bulk modulus versus average non-carbon cation radius. The analysis of the obtained and literature structural data suggests the distortion of a large cation polyhedron to be an effective tool to strengthen the carbonate structure at high pressure. On the other hand, the observed symmetrization of the cation polyhedra in trigonal bütschliite is apparently a crucial factor of its stabilization at high pressure upon the temperature rise observed previously. The structural crystallography provided in this study supports the enhanced stability of trigonal bütschliite at high P, T conditions and its significance of being considered as a constituent of the inclusions in deep minerals.

K-Ca double carbonates recently identified in inclusions in diamonds, as well as associated alkali-carbonate melts can play an important role in the deep carbon cycle. We studied pressure-induced changes in the crystal structure of high-pressure α-K₂Ca₃(CO₃)₄ phase up to 20 GPa using synchrotron single-crystal x-ray diffraction in diamond anvil cell. At ~ 7 GPa at room temperature the orthorhombic P2₁2₁2₁ phase of α-K₂Ca₃(CO₃)₄ undergoes displacive phase transition into monoclinic P112₁ phase. Despite the phase transition, PV-curve does not demonstrate any irregularities so that both phases can be described by the same 4th order Birch-Murnaghan equation of state with V₀ = 1072.5(3) ų, K₀ = 51.1(8) GPa, K’₀=3.7(3), K’’₀=0.12(6).

In this work, the self-made chrysotile fiber membrane (CFM) and raw chrysotile fiber (CF) were calcined in air from 500 to 800 °C. The XRD pattern of CFM showed that the diffraction peak of chrysotile weakened when the temperature was from room temperature to 550 °C, and CFM had a shorter amorphous interval at 600–700 °C. While, no amorphous phase appeared in CF during calcination, and forsterite begined to appear at 650 °C. SEM images showed that CFM could still maintain the integrity of the network structure at 600–800 °C, while CF gradually melted into coarse fiber bundles with the increase of calcination temperature, and sintering traces appeared. After that,the kinetics of the dehydroxylation of chrysotile in CFM and CF was studied. The dehydroxylation of CFM is a one-step reaction, the calculated activation energy is 243.33 kJ mol⁻¹, which conforms to the two-dimensional ‘Valensi’ model with mechanism function G(α) = (1−α)ln(1−α) + α. The dehydroxylation of CF is divided into two stages, the activation energy are 222.87 kJ mol⁻¹ and 316.04 kJ mol⁻¹. The first stage of CF conforms to two-dimensional ‘Jander’ model (n = 2) with mechanism function G(α) = [1−(1−α)^1/2]², the second stage of CF conforms to the random nucleation and subsequent growth ‘Avrami-Erofeev’ model (n = 3/2) with mechanism function G(α) = [−ln(1−α)]^2/3.

The infrared spectroscopic properties of selected defects involving one proton and one nearby M³⁺ (M = Al, Cr, Fe) substitution in orthoenstatite are investigated by first-principles calculations. Based on the theoretical results, the absorption bands experimentally observed on synthetic samples with high crystalline quality and low doping levels can be assigned to specific defect configurations. Most of them correspond to Mg vacancies at M2 sites locally compensated by one proton and one M³⁺ cation at a nearby M1 site. This confirms that the M³⁺ + H⁺ = 2 Mg²⁺ exchange mechanism is the dominant hydrogen incorporation mechanism at the lowest concentration levels in doped enstatite. At higher concentration levels, more complex incorporation mechanisms could become dominant in Al-bearing samples.

Platinum-iron (Pt-Fe) alloys have long served as oxygen fugacity sensors in high-temperature experiments investigating Earth and planetary interiors, relying on the equilibrium between Fe within the alloy and FeO in coexisting oxides or silicates. Despite their significance, studies on intermediate compositions remain limited. This investigation focuses on compressibility of Fe₁₈Pt₈₂ up to (sim) 40 GPa at ambient temperature and explores the pressure-dependent characteristics of the oxygen fugacity relationship. In-situ X-ray diffraction measurements confirm the stability of the fcc phase in Fe₁₈Pt₈₂ across the pressure range. The fit to the compression data by the third-order Birch–Murnaghan equation of state results in ({V}_{0}=59.14 pm 0.08)ų, ({K}_{0}=266 pm 13) GPa, and ({K}_{0}^{prime}=4.7 pm 0.7). The differences between this fit and the Vinet and Kunc equations of state fall within the range of uncertainty. Comparing results with reported data for other Pt-Fe alloys reveals a nearly linear trend between volume and the Fe content in Pt-Fe alloys at ambient pressure. Unlike more iron-rich alloys, the excess volume of mixing of Fe₁₈Pt₈₂ ((sim) 0.21 cm³/mol) remains nearly constant across the examined pressure range. Estimates of the excess Gibbs free energy suggest diminishing non-ideal contributions to thermodynamic activities as pressure increases.

Low impurity content is crucial for graphite applications and microcrystalline graphite is an important candidate material. In this study, natural microcrystalline graphite, with a fixed carbon content of 76.65%, was purified by an alkaline autoclave-acid leaching method. The effects of the mole ratio of NaOH to Si and Al in graphite, the liquid–solid ratio of NaOH solution and graphite, alkali autoclave temperature and reaction time on the purity of microcrystalline graphite were studied. Results showed that the dissolution and phase transformation of non-carbon impurities were closely related to the purification process. During the alkali autoclave stage, complete dissolution of quartz was observed. The Si–O tetrahedra and Al–O octahedra structures in aluminosilicate minerals were damaged and [Al (OH)₄]⁻, [H₂SiO₄]²⁻ and [SiO₂ (OH)₃]⁻ were released. The soluble silicate and aluminate ions underwent recrystallization, producing cancrinite and sodalite that could be dissolved by acid leaching, resulting in purified microcrystalline graphite. The purity of microcrystalline graphite was further improved due to the autoclave treatment allowed NaOH solution to penetrate into the cracks of microcrystalline graphite aggregates under high pressure. In addition, the acid solution could enter the micropores left by alkali etching to dissolve the residual impurities. The fixed carbon content of microcrystalline graphite could be increased to 99.9% through the alkaline autoclave-acid leaching method.

Graphical abstract

The causes of colour in beryl have been a research topic for decades. For some varieties, such as emerald (green, coloured by Cr³⁺ and/or V³⁺), the main cause of colour is substitutions by metal atoms within the framework. However, the causes for the yellow and blue colours in heliodor, golden beryl and aquamarine are still debated. It is generally agreed that Fe ions are responsible for the colour, but there are differing conclusions about the valence states of these ions, the occupied positions and the colour-inducing processes involved. The colour of aquamarine is commonly attributed to intervalence charge transfer (IVCT) between Fe³⁺ and Fe²⁺. Various combinations of sites have been proposed to host the Fe ions engaging in this IVCT. Here we present a new approach to address the topic of colour generation: atomic resolution scanning transmission electron microscopy (STEM). For the first time, atomic resolution images of a beryl (natural aquamarine) are presented in the three crystallographic directions [0001], [1-210] and [1-100]. Ions are clearly resolved in the channels. From the ratio of channel occupation and the correlation of the atoms per formula unit (apfu) calculations we conclude that Fe resides in the framework, not in the channels. The projections in the [1-210] direction directly show that the cavity channel site 2a is occupied, most likely by Cs, in agreement with recent results in the literature.

The heat of mixing of some petrological relevant substitutions (i.e., Mg-Al, Si-Al, Mg-Ti, Mg-Ca, and Mg-Fe) was investigated systematically in silicates, titanates, tungstates, carbonates, oxides, hydroxides, and sulphates by density functional theory calculations (e.g., melilite, chlorite, biotite, brucite, cordierite, amphibole, talc, pseudobrookite, pyroxene, olivine, wadsleyite, ilmenite, MgWO₄, ringwoodite (spinel), perovskite, pyrope-grossular, magnesite-calcite, MgO-CaO, anhydrous and different hydrated MgSO₄). A specific substitution is characterised by different microscopic interaction energies in different minerals, e.g., the octahedral Mg-Al exchange on a single crystallographic site in pyroxene has a microscopic interaction energy that is more than twice compared to that in biotite. A comparative investigation of the heat of mixing using microscopic interaction energies on a single crystallographic site has the advantage that they are not influenced by cation ordering. They could be successfully correlated with the stiffnesses of the minerals, which in turn were scaled to the oxygen packing fraction, a parameter that is easily available for poorly investigated minerals. With this information, the interaction energies of a certain substitution can be transferred from minerals where they are well-known to mineral groups where they are less- or unknown. Using the cross-site terms and the microscopic interaction energies, the macroscopic interaction energies of the coupled substitution, e.g., Mg + Si = Al + Al, of biotite and pyroxene were calculated, which are, however, affected by cation ordering and different degrees of local charge balance, for which appropriate models are necessary.

The crystal chemistry of inderite, a hydrous borate with known ideal formula MgB₃O₃(OH)₅·5H₂O from the Kramer deposit, was re-investigated by electron probe micro-analysis in wavelength dispersive mode, laser ablation-(multi collector-)inductively coupled plasma-mass spectrometry and single-crystal neutron diffraction. The chemical data prove that the real composition of the investigated inderite is substantially identical to the ideal one, with insignificant content of potential isomorphic substituents, so that, excluding B, inderite does not contain any other industrially-relevant element (e.g., Li concentration is lower than 2.5 wt ppm, Be or REE lower than 0.1 wt ppm). The average δ¹¹B_NIST951 value of ca. − 7 ‰ lies within the range of values in which the source of boron is ascribable to terrestrial reservoirs (e.g., hydrothermal brines), rather than to marine ones. Neutron structure refinements, at both 280 and 10 K, confirm that the building units of the structure of inderite consist of: two BO₂(OH)₂ tetrahedra (B-ion in sp³ electronic configuration) and one BO₂(OH) triangle (B-ion in sp² electronic configuration), linked by corner-sharing to form a (soroborate) B₃O₃(OH)₅ ring, and a Mg-octahedron Mg(OH)₂(OH₂)₄. The B₃O₃(OH)₅ ring and the Mg-octahedron are connected, by corner-sharing, to form an isolated Mg(H₂O)₄B₃O₃(OH)₅ (molecular) cluster. The tri-dimensional edifice of inderite is therefore built by heteropolyhedral Mg(H₂O)₄B₃O₃(OH)₅ clusters mutually connected by H-bonds, mediated by the zeolitic (“interstitial”) H₂O molecules lying between the clusters, so that the correct form of the chemical formula of inderite is Mg[B₃O₃(OH)₅](H₂O)₄·H₂O, rather than MgB₃O₃(OH)₅·5H₂O. All the thirteen independent oxygen sites of the structure are involved in H-bonding, as donors or as acceptors. This confirms the pervasive nature and the important role played by the H-bonding network on the structural stability of inderite. The differences between the crystal structure of the two dimorphs inderite and kurnakovite are discussed.

Goethite (α-FeOOH) is an iron-oxyhydroxide mineral that is commonly found in soils and is of importance within the context of industrial mineralogy and aqueous geochemistry. The structure of goethite is such that vacant rows of octahedral sites form “channels” or nanopores. This study aims to investigate the response of goethite to dynamic shock compression in order to advance our understanding of minerals as potential shock-absorbing media. Shock compression of synthetic goethite powdered samples was achieved by using an inverted shock microscope and laser driven “flyer plates”. With this setup, a high-energy laser launches small  aluminum discs as projectiles or flyer plates at velocities of the order of a few km/s towards the sample. The resulting impact sends a shock wave through the sample, thereby compressing it. The compression is precisely controlled by the plate-impact speed, which in turn is controlled by laser-power. In this work, 25 µm aluminum flyer plates with 3.5 km/s impact velocities were used. The impact resulted in a planar shock wave with shock velocity (U_s) ~ 6.78 km/s and an estimated pressure of ~ 41.6 GPa. The shock wave compressed the target goethite for 5 ns. Subsequent, post-shock investigations via transmission electron microscopy (TEM) documented that crystal morphology persisted, and that goethite’s “bird’s nest” texture was maintained. Lattice fringe images revealed localized zones of distortion and amorphous regions within single goethite particles. Raman spectra appear to indicate structural changes after shock compression with the shocked goethite spectra matching that of synthetic hematite. X-ray diffraction (XRD) interestingly identified two major phases: goethite and magnetite. Irrespective of the mineral phases present, the goethite particles persist post shock. A thixotropic-like model for accompanying shock compression is proposed to account for goethite’s shock resistant behavior.