Physics and Chemistry of Minerals最新文献

Three samples of fluorapophyllite-(K) with different cation composition were studied by the X-ray diffraction and IR-spectroscopy methods. The samples (K_0.72(NH₄)_0.20)_Σ0.92Na_0.08 Ca₄[Al_0.04Si_7.96O₂₀]F·8H₂O from Nidym and K_0.87Na_0.06 Ca₄[Si₈O₂₀]F·8H₂O from Akhaltsykhe localities are characterized by the deficiency of large cations K⁺ and (NH₄)⁺ in the A site; the chemical composition of Talnakh sample (K_0.94(NH₄)_0.06)_Σ1.00 Ca₄[Si₈O₂₀]F·8H₂O is close to the stoichiometric one. The large cations deficiency in the structure of Nidym and Akhaltsykhe samples is compensated by the presence of medium-sized Na⁺ cations, located 1.12–1.31 Å away from the A position. The planar coordination of Na is formed by four H₂O molecules. The occurrence of ammonium ions is found for the Nidym and Talnakh samples and their concentrations are estimated from the IR spectra and structure refinement. Polarized infrared measurements performed for fluorapophyllite-(K) samples oriented along the c-axis reveal pleochroic behavior of the OH absorption bands. The observed pleochroism is explained in terms of absorption from two OH vectors of the H₂O molecules in the crystal structure.

Carbonate minerals are major contributors to carbon sequestration in geological deposits; however, their nature and behavior remain unclear. Amorphous magnesium carbonate (AMC) is formed as a precursor to crystalline magnesium carbonates and as a product of thermal decomposition of nesquehonite (NSQ). In this study, the AMCs formed during the crystallization and decomposition of NSQ were investigated using X-ray diffraction (XRD) and atomic pair distribution function (PDF) methods. An AMC with a hydromagnesite-like structure (AMC-I) was formed immediately after mixing MgCl₂ and Na₂CO₃ solutions. After 5 min of stirring, no change was observed in the XRD pattern; however, the PDF pattern changed. This suggests that the medium-range ordered structure of AMC-I transformed into an intermediate structure (AMC-II) between AMC-I and NSQ. After 10 min of stirring, the AMC-II crystallized into NSQ. In the case of Rb₂CO₃, the AMC-II structure was formed immediately after the mixing of solutions and was stable for three days. AMC-II in the Rb₂CO₃ solution appeared to be in equilibrium with energetic local minima, indicating the existence of polyamorphism in AMC. When Cs₂CO₃ solution was used, the first precipitate had an AMC-I structure. By stirring for 5 min, the AMC-I was transformed to AMC-II, and after 10 min of stirring, a few quantities crystallized into NSQ. After three days, NSQ dissolved and transformed back into AMC-I. Thus, it is inferred that the crystallization of NSQ is significantly influenced by alkali cations in aqueous solutions. The AMC formed during the thermal decomposition also possesses the AMC-I structure.

Scapolite Na_2.25Ca_1.43K_0.30Fe_0.02[Al_4.23Si_7.77O₂₄]Cl_0.72(SO₄)_0.1(CO₃)_0.18 has been investigated by single-crystal X-ray diffraction, Infrared and Raman spectroscopy combined with diamond anvil cells (DAC) up to 19.6 GPa at room temperature, to understand its phase stability and the behaviors of CO₃²⁻. The experimental results show that a phase transition from the tetragonal phase (P4₂/n) to the triclinic phase occurs at 10.7 GPa, which is attributed to the pressure-induced rotation of tetrahedra around the bridging O atoms. The pressure–volume data were fitted to the second-order Birch–Murnaghan equation of state, yielding V₀ = 1102.6(8) ų and K₀ = 70.3(9) GPa for the tetragonal phase and V₀ = 1188.2(16) ų and K₀ = 33.0(3) GPa for the triclinic phase. In the triclinic phase, the larger compressibility is due to the increased degree of freedom in the crystal structure, and the anisotropy of the axial compressibility may be related to the ionic radius of the anionic group. From the Infrared absorption and Raman spectroscopy data, we speculate that CO₃²⁻ is extruded by the four-membered rings at the (001) plane during the whole pressure range. The high-pressure behavior of CO₃²⁻ in scapolite provides a possibility that carbon exists in the Earth’s interior as CO₃²⁻ coupled to silicates.

Diamonds from placers in the northern provinces of the Siberian Platform attract attention not only due to the unsolved problem of their sources but also due to the variety of properties they exhibit. In this work, electron paramagnetic resonance (EPR), infrared (IR), and photoluminescence (PL) methods were used to study cuboid diamonds from Istok (25 diamonds of Ib-IaA type) and dodecahedron diamonds from Mayat (21 diamonds of IaAB type) placers. The main impurity defects in the crystals studied are nitrogen, nitrogen-titanium, and oxygen-containing centers. The total amount of nitrogen in the form of C (P1 in EPR nomenclature), A, and B centers ranges from 34 to 903 ppm. For 19 out of 25 crystals from the Istok placer, the N3 titanium-related center is observed in the EPR spectra, but only one optical system with zero-phonon line (ZPL) at 635.7 nm is detected in the PL spectra in most cases. It can be assumed that the absence of an accompanying system with ZPL at 440.3 nm is due to its quenching by other centers in these crystals. For 12 out of 21 crystals from the Mayat placer, photoluminescence of oxygen-containing centers in the region of 600–800 nm in the form of a series of equidistant bands with an interval of ~ 31 meV was revealed. A comparison of the EPR and PL data has shown that this red broad band is not correlated with titanium-nitrogen paramagnetic centers OK1 and N3. Thus, the results obtained for diamond crystals from the Istok and Mayat placers show no connection between the N3 and OK1 paramagnetic centers and band in the 600–800 nm region, which questions the presence of oxygen in the structure of the N3 and OK1 centers. The nickel-nitrogen centers were found in crystals from the Mayat placer testifying about the peridotitic paragenesis of these diamonds.

The luminescence of the uranyl cation UO₂²⁺ depends on the local crystalline environment and is sensitive to structural influences. Steady-state photoluminescence emission spectra of the related uranyl silicates uranophane-α, uranophane-β, sklodowskite and haiweeite from various locations are presented and discussed in the light of structure–property relation. The four mineral species were chosen for their close relationships: uranophane-α and uranophane-β are polymorphs and share the underlaying topology with sklodowskite. Haiweeite, with different topology, shares the composing elements Ca, U, Si, O with uranophane, while in sklodowskite Mg replaces Ca. All species show some variability in their spectra, parameterized as a variation of the centroid wavelength. Those variations are linked to defects and structural disorder, relevant in studies of uranyl speciation and migration. We present empiric spectra of the four mineral species with the least influence of structural disorder. As an unexpected feature, a prominent—partly dominating—double peak structure occurs in the case of uranophane-α only, while it is absent in the spectra of the other species. Considering a model of luminescent transitions in the uranyl ion in more detail, this observation is discussed in the light of the polymorphism of uranophane. We show evidence that variable amounts of uranophane-β phase embedded in uranophane-α are possibly at the origin of this spectral signature. Growth of those uranophane-β clusters might be induced by defects in the uranophane-α lattice and further promoted by the polymorphism of uranophane.

To calculate the thermodynamic properties of recently discovered high-pressure mixed valence iron oxides in the system Fe–Mg–O, information on the equation of state of precursor inverse spinel phases along the magnetite–magnesioferrite join is needed. The existing equation of state data, particularly for magnesioferrite, are in poor agreement and no data exist for intermediate compositions. In this study, the compressibility of nearly pure magnesioferrite as well as of an intermediate ({{mathrm{Mg}}_{0.5}}^{vphantom{2+}}mathrm{Fe}_{0.5}^{2+}{mathrm{Fe}}_{2}^{3+}{mathrm{O}}_{4}^{vphantom{2+}}) sample have been investigated for the first time up to approximately 19 and 13 GPa, respectively, using single-crystal X-ray diffraction in a diamond anvil cell. Samples were produced in high-pressure synthesis experiments to promote a high level of cation ordering, with the obtained inversion parameters larger than 0.83. The room pressure unit cell volumes, V₀, and bulk moduli, K_T0, could be adequately constrained using a second-order Birch–Murnaghan equation of state, which yields V₀ = 588.97 (8) ų and K_T0 = 178.4 (5) GPa for magnesioferrite and V₀ = 590.21 (5) ų and K_T0 = 188.0 (6) GPa for the intermediate composition. As magnetite has K_T0 = 180 (1) GPa (Gatta et al. in Phys Chem Min 34:627–635, 2007. https://doi.org/10.1007/s00269-007-0177-3), this means the variation in K_T0 across the magnetite–magnesioferrite solid solution is significantly non-linear, in contrast to several other Fe–Mg spinels. The larger incompressibility of the intermediate composition compared to the two end-members may be a peculiarity of the magnetite–magnesioferrite solid solution caused by an interruption of Fe²⁺–Fe³⁺ electron hopping by Mg cations substituting in the octahedral site.

Understanding the thermal behaviour of iron-containing amphiboles (AB₂C₅T₈O₂₂W₂, C₅ = M(1)₂ M(2)₂ M(3)) at atomic-level scale may have important implications in several fields, including metamorphic petrology, geophysics, and environmental sciences. Here, the thermally induced oxidation and decomposition of actinolite are studied by in situ high-temperature Raman spectroscopy and complementary thermogravimetric/mass-spectrometry analyses as well as X-ray diffraction of the products of amphibole decomposition. The effect of ^CFe²⁺ on dehydrogenation/dehydroxylation is followed by comparing the results on actinolite with those for tremolite. We show that mobile charge carriers, namely polarons (conduction electrons coupled to FeO₆ phonons) and H⁺ cations, exist in actinolite at elevated temperatures ~ 1150–1250 K. The temperature-induced actinolite breakdown is a multistep process, involving (i) delocalization of e⁻ from ^CFe²⁺ as well as of H⁺ from hydroxyl groups shared by Fe-containing M(1)M(1)M(3) species, which, however, remain in the crystal bulk; (ii) dehydrogenation and ejection of e⁻ between 1250 and 1350 K, where actinolite can be considered as “oxo-actinolite”, as H⁺ also from hydroxyl groups next to ^M(1,3)(MgMgMg) configurations become delocalized and mostly remain in the crystal bulk; (iii) complete dehydroxylation and consequent structure collapse above 1350 K, forming an Fe³⁺-bearing defect-rich augitic pyroxene. The dehydrogenation of tremolite occurs at 1400 K, triggering immediately a disintegration of the silicate double-chain into single SiO₄-chains and followed by a rearrangement of the amphibole octahedral strips and ^BCa²⁺ cations into pyroxene-type octahedral sheets at 1450 K. The result of tremolite decomposition is also a single-phase defect-rich clinopyroxene with an intermediate composition on the diopside–clinoenstatite join.

Characterization of the behavior of zeolites at high pressures is of interest both in fundamental science and for practical applications. For example, zeolites occur as a major mineral group in tuffaceous rocks (such as those at the Nevada Nuclear Security Site), and they play a key role in defining the high-pressure behavior of tuff in a nuclear explosion event. The crystal structure, Si/Al ratio, and type of pressure-transmitting media (PTM) used in high-pressure experiments influence the compressional behavior of a given zeolitic phase. The heulandite-type (HEU) zeolites, including heulandite and clinoptilolite, are isostructural but differ in their Si/Al ratios. Thus, HEU-type zeolites comprise an ideal system in unraveling the effects of Si/Al ratio and type of PTM on their pressure-induced structural behavior. In this study, we performed in situ high-pressure angle-dispersive powder synchrotron X-ray diffraction (XRD) experiments on a natural HEU zeolite, clinoptilolite, with a Si/Al ratio of 4.4, by compressing it in a diamond anvil cell (DAC) up to 14.65 GPa using a non-penetrating pressure-transmitting medium (KCl). Unit cell parameters as a function of pressure up to 9.04 GPa were obtained by Rietveld analysis. Unit cell volumes were fit to both a second and a third-order Birch–Murnaghan equation of state. The mean bulk modulus (K₀) determined from all the fittings is 32.7 ± 0.9 GPa. The zero-pressure compressibility of the a-, b-, and c-axes for clinoptilolite are 10.6 (± 0.8) × 10^–3 GPa^–1, 5.3 (± 0.7) × 10^–3 GPa^–1, and 17.1 (± 1.8) × 10^–3 GPa^–1, respectively. The pressure–volume equations of states of this type of zeolite are important for characterizing high-pressure behavior of the broader family of microporous materials and for developing reliable geophysical signatures for underground nuclear monitoring.

The high-pressure behavior of the natural arsenate gasparite-(Ce) [Ce_0.43La_0.24Nd_0.15Ca_0.11Pr_0.04Sm_0.02Gd_0.01(As_0.99Si_0.03O₄)] from the Mt. Cervandone mineral deposit (Piedmont Lepontine Alps, Italy), has been studied by in situ single-crystal synchrotron X-ray diffraction up to 22.01 GPa. Two distinct high-pressure ramps have been performed, using a 16:3:1 methanol:ethanol:water solution and helium as P-transmitting fluids, respectively. No phase transition occurs within the pressure range investigated, whereas a change in the compressional behavior has been observed at ~ 15 GPa. A second-order Birch-Murnaghan EoS was fitted to the P-V data, leading to a refined bulk modulus of 109.4(3) GPa. The structural analysis has been carried out on the basis of the refined structure models, allowing the description of the deformation mechanisms accommodating the bulk compression in gasparite-(Ce) at the atomic scale, which is mainly controlled by the compression of the Rare Earth Elements coordination polyhedra, while the AsO₄ tetrahedra behave as a quasi-rigid units. A micro-Raman spectroscopy analysis, performed at ambient conditions, suggests the presence of hydroxyl groups into the structure of the investigated gasparite-(Ce).