Physics and Chemistry of Minerals最新文献

Menilites, intriguing botryoidal rocks found in Galera, Granada, Spain, have been examined through a multidisciplinary approach integrating mineralogical analysis and advanced imaging techniques. Characterized as opal and dolomite-bearing rocks, their complex morphologies and diverse internal structures prompted an investigation into their origin. Employing microfocus X-ray diffraction, scanning electron microscopy with energy-dispersive X-ray spectroscopy and X-ray computed tomography, we present a detailed study of the menilites, revealing opal-A, opal-CT, dolomite and quartz as primary constituents. Notably, the internal homogeneity contrasts with the diverse external shapes. The proposed hypothesis suggests a seismic influence in menilite formation. Seismic events in porous environments above the water table may induce fluidization, resulting in the distinctive menilite structures. Osmotic pressure differences between nodules and the surrounding rock, coupled with fluidization during seismic events, could explain the observed morphologies. Validation of the proposed hypothesis requires further fieldwork and analogue experimentation. This study contributes valuable insights into the mineralogical composition, internal structures and potential formation mechanisms of menilites, laying the groundwork for future research in the field of sedimentary geology and mineralogy.

Electrical resistivity measurements on oriented FeTiO₃ ilmenite using single crystals at high pressures proves that FeTiO₃ ilmenite shows anisotropic electrical resistivity. The resistivity in the direction perpendicular to the c-axis decreased monotonously with increasing pressure. In contrast, the resistivity in the parallel direction to the c-axis initially decreased and slightly increased with increasing pressure above 6 GPa. It then resumed decreasing above 8 GPa. The hallow-shape of the curvature was observed. Neutron and synchrotron X-ray diffraction experiments provided an accurate picture of the pressure-induced changes of the FeTiO₃ ilmenite structure. FeTiO₃ transforms neither into perovskite nor LiNbO₃ phase under pressures up to 28 GPa. However, different compression curves were observed for both FeO₆ and TiO₆ octahedra below 8 GPa. FeO₆ is more compressible and flexible than TiO₆. Among Fe–Fe, Ti–Ti and Fe–Ti interatomic distances, the shortest Fe–Ti distance presents the highest electrical restivity and electron mobility according to Fe²⁺Ti⁴⁺ and Fe³⁺Ti³⁺ by electron super-exchange mechanism, which is enhanced during compression. At high pressure, the electron configuration of Fe²⁺ (3d⁶) is more strongly changed than Ti⁴⁺ (3d⁰) and the former cation is the emphasized by Jahn–Teller effect in the ligand field of C_3v molecular symmetry. The anisotropic electrical resistivity and non-uniform structure change of Fe–Ti interatomic distance can be explained by possible spin transition. The spin transition of FeKβ from high-spin to intermediate-spin state is possible in the electronic state change of FeTiO₃.

The characteristic behavior of magnetic remanence correlated with mineralogical textures and composition was observed using low-temperature magnetometry, microscopy, and chemical analysis of three isocubanite samples collected from hydrothermal deposits in the Okinawa Trough and a sample transformed from natural cubanite via heating. Both zero-field remanence acquired at 5 K and field cooling remanence acquired at 300–5 K of all samples sharply decreased with increasing temperature at approximately 100 K. In addition, low-temperature cycling of isothermal remanence at 300 K exhibited a transition at approximately 100 K; remanence increased with decreasing temperature and vice versa. The intensity of remanence at low temperature and sharpness of the transition varied across samples with different compositions and microscopic textures, that is, the presence or absence of chalcopyrite lamellae and their widths. The sample obtained from a hydrothermal chimney, in which the magnetic transition was most clearly observed, was also subjected to X-ray diffraction, Mössbauer spectroscopy, electrical resistivity, and magnetic hysteresis measurements. The obtained results were generally consistent with those reported previously for unnamed mineral CuFe₃S₄ with an ordered cation arrangement. The low-temperature magnetic behavior of isocubanite possibly depends on the degree of cation ordering and can be regarded as an indicator of chemical composition and cooling history. Therefore, low-temperature magnetometry is useful for the detection of isocubanite and a potentially powerful technique for the prompt estimation of its composition and texture, contributing to our understanding of the formation process of hydrothermal deposits.

The high-pressure structural evolution of a natural hydroxyapophyllite-(K) K_0.96 Ca_4.01[Al_0.01Si_7.99O₂₀]((OH)_0.95F_0.05)·(H₂O)_8.14, Z = 2, a = 8.9699(1), c = 15.8934(3) Å, space group P4/mnc, from the Hatrurim Basin, Negev Desert, compressed in penetrating (ethanol:water 8:1 mixture) medium up to 5 GPa, was studied by single-crystal X-ray diffraction with a diamond anvil cell. The results clearly demonstrate the absence of pressure-induced hydration in the structure. Within 3 GPa, the compression mechanism is similar to that known in fluorapophyllite-(K). The compression in the plane of silicate layer proceeds via the relative rotation of the four-membered rings. The compression along the c-axis proceeds through the shortening of the inter-layer distance, whereas the thickness of silicate layer remains almost unchanged. As a result, the pressure-induced changes in the unit-cell metrics are similar to those for fluorapophyllite-(K). At about 3 GPa, hydroxyapophyllite-(K) undergoes a phase transition with the symmetry lowering to orthorhombic (space group Pnnm). The symmetry of the high-pressure phase allows deformation of the four-membered rings of the silicate layer, which is impossible within tetragonal symmetry. In this case, the structure is compressed much more along the a-axis than along the b-axis. As a result, the orthorhombic phase of hydroxyapophyllite-(K) is more compressible compared to fluorapophyllite-(K).

The crystal structure and bonding environment of K₂Ca(CO₃)₂ bütschliite were probed under isothermal compression via Raman spectroscopy to 95 GPa and single crystal and powder X-ray diffraction to 12 and 68 GPa, respectively. A second order Birch-Murnaghan equation of state fit to the X-ray data yields a bulk modulus, ({K}_{0}=46.9) GPa with an imposed value of ({K}_{0}^{prime}= 4) for the ambient pressure phase. Compression of bütschliite is highly anisotropic, with contraction along the c-axis accounting for most of the volume change. Bütschliite undergoes a phase transition to a monoclinic C2/m structure at around 6 GPa, mirroring polymorphism within isostructural borates. A fit to the compression data of the monoclinic phase yields ({V}_{0}=322.2) Å³(,) ({K}_{0}=24.8) GPa and ({K}_{0}^{prime}=4.0) using a third order fit; the ability to access different compression mechanisms gives rise to a more compressible material than the low-pressure phase. In particular, compression of the C2/m phase involves interlayer displacement and twisting of the [CO₃] units, and an increase in coordination number of the K⁺ ion. Three more phase transitions, at ~ 28, 34, and 37 GPa occur based on the Raman spectra and powder diffraction data: these give rise to new [CO₃] bonding environments within the structure.

We synthesized six samples in the compositional field NaMgAl(SO₄)₃–KMgAl(SO₄)₃ in 20 mol% increments from pure Na to pure K compounds. We investigated them by Powder X-Ray diffraction, ²³Na, and ²⁷Al Nuclear Magnetic Resonance spectroscopy. The results confirm NaMgAl(SO₄)₃ as a unique phase identical to a presumed new mineral found in the fumaroles of Eldfell and Hekla volcanoes in Iceland. It tolerates less than 10 mol% K substitution for Na. There exists a compositional gap to approximately Na_0.65K_0.35MgAl(SO₄)₃ from where a solid solution extends to KMgAl(SO₄)₃. The mineral koryakite [NaKMg₂Al₂(SO₄)₆] is a member of the latter solid solution series. The crystal structures of all (Na,K)MgAl(SO₄)₃ phases are akin to NASICON (NA Super Ionic CONductor). NaMgAl(SO₄)₃ has (Roverline{3}c) symmetry and a disordered distribution of Mg and Al among the octahedral sites with only one unique site for the alkali atom. The members of the solid solution have (Roverline{3}) symmetry with ordered Mg–Al distribution and two unique alkali sites with different preferences for Na and K. In the crystal structure, the coordination of Na and/or K is trigonal antiprismatic, and these share bases with two octahedral Mg (Na) or Al (K) coordinations. These polyhedra are arranged in columns parallel to [001] and interconnected by SO₄ tetrahedral groups. The alkali atoms from a column lie in the same (001) layers as the octahedrally coordinated atoms from the three neighboring rows. On the same level, parallel to (001), there are gaps in the other three neighboring columns forming channels containing Na⁺ or K⁺ ions.

The cations of an ordered omphacite from the Tauern window were gradually disordered in piston cylinder experiments at temperatures between 850 and 1150 °C. The samples were examined by X-ray powder diffraction and then investigated using low-temperature calorimetry and IR spectroscopy. The low-temperature heat capacity data were used to obtain the vibrational entropies, and the line broadening of the IR spectra served as a tool to investigate the disordering enthalpy. These data were then used to calculate the configurational entropy as a function of temperature. The vibrational entropy does not change during the cation ordering phase transition from space group C2/c to P2/n at 865 °C but increases with a further temperature increase due to the reduction of short-range order.

Iron-rich rocks of Orosirian Period in the Chilpi Group on the northern margin of the Bastar Craton, Central India, contain an association of hematite-magnetite-greenalite-chamosite-quartz in oxide-silicate facies. Additionally chert (quartz) and siderite occur in chert and carbonate facies. Presence of these mineral assemblages was investigated to infer the redox state of the depositional basin. The results have indicated formation temperature variation of 116–255 °C (average 198 °C) and log P_(O2) between  – 37 and  – 60 (average –44). A ferruginous state of the shallow water depositional environment, having oxygen content of 10^–2 to 10^–5 times the present atmospheric level, is inferred. The variations in composition of greenalite-chamosite association indicate development of the mineral phases from the reaction involving kaolinite-illite and magnetite-siderite as end-members. Thermodynamic requirements for the formation of the rare association of magnetite-greenalite-cronstedtite indicate the precipitation of the mineral phases from seawater with enhanced activities of Fe²⁺, Al, Si, Mg and C compared to the level in the present day seawater. The results indicate a steep fall in the atmospheric oxygen content immediately after the Great Oxidation Event of 2400–2000 Ma.

Spurrite from Negra Mine, Queretaro, Mexico is characterized by a complex chemical composition. Its empirical formula derived based on electron microprobe, wet chemical analyses and gas chromatography of annealing products is H_0.18Ca_5.01Na_0.05[(SiO₄)_1.91(SO₄)_0.08)][(CO₃)_0.71(BO₃)_0.28]O₁₁. The mineral was studied by single-crystal X-ray diffraction (SCXRD) as well as infrared (IR), Raman and nuclear magnetic resonance (NMR) spectroscopy. According to spectroscopic data, boron has three-fold coordination and sulfur occurs in the mineral in the sulfate form. A significant portion of carbonate groups is substituted by BO₃^3– anions. Charge compensation is achieved due to the substitution of a part of SiO₄^4– anions by SO₄^2– groups, as well as to the admixture of sodium. SCXRD shows that sodium occurs in its own site with a low occupancy. The studied sample is isotypic with the synthetic NaCa₅(SiO₄)₂(BO₃) compound. The IR spectrum shows possible partial protonation of the SiO₄ tetrahedra whereas bands of H₂O molecules and isolated OH^– anions are not observed. Thermal behavior of B,S,Na-bearing spurrite from Negra Mine has been studied using powder high-temperature X-ray diffraction (HTXRD) together with boron poor and S-free spurrite from Fuka Area (Japan). The studied samples are stable up to ~ 1200 °C and ~ 1100 °C, respectively, whereas synthetic B,S-free spurrite decomposes at about 900 °C. The thermal expansion is significantly anisotropic and is observed mainly in the direction perpendicular to the ac plane which is coplanar with the layers of calcium polyhedra and anionic pseudo-layers formed by (C,B)O₃ triangles and (Si,S)O₄ tetrahedra. Isomorphism and a similarity of the thermal, baric and compositional (C-B substitution) deformations of spurrite-like structures are discussed.