Physics and Chemistry of Minerals最新文献

To investigate the quantitative relationship between the crystal structure and composition of Mn-bearing calcite, the solid solutions of Ca_1–xMn_xCO₃ (x = 0.1, 0.3, 0.5, 0.7, 0.9) with continuous MnCO₃ mol% content were synthesized at 1 GPa and 700 °C using high-purity CaCO₃ and MnCO₃ powders as starting materials. The run products were analysized by electron probe, powder X-ray diffraction and Raman spectroscopy. The CaO wt% and MnO wt% of the resulting products are consistent with the expected compositions. The powder X-ray diffraction results show that the products are single phase without any impurities. All diffraction peaks of samples with varying MnCO₃ mol% contents can be indexed by the calcite-type structure carbonates ACO₃ (R-3c space group; A is a divalent cation), confirming the previous results that there is the completely continuous solid solution between CaCO₃ and MnCO₃ end members. The unit-cell parameters and volumes of the solid solutions decrease as the MnCO₃ mol% content increases, presenting a linear relationship of Ca–Mn ideal miscibility, which is perfectly consistent with the rigid body model of A-site substitution in ACO₃. Besides, as MnCO₃ mol% content increases, the bond distance of A–O decreases linearly, while the bond distance of C–O changes like a parabola. Therefore, the addition of Mn makes the bond distance of A–O shorten, resulting in the decrease of unit-cell parameters and volumes for Ca_1–xMn_xCO₃. Furthermore, two exterior vibrations (T and L) of the crystal lattice and two internal vibrations (ν₄ and ν₁) within the CO₃²⁻ unit are assigned in the Raman spectra of these solid solutions. The characteristic vibration modes T, L, and ν₄ as a whole increase with the increasing of MnCO₃ mol% content, whereas the characteristic vibration mode ν₁ as a whole decreases with the increase of MnCO₃ mol% content. These variations in Raman vibration modes are related to the radius of substituted ions.

The equilibrium partitioning of Na and K between alkali feldspar and NaCl–KCl salt melt was determined at 800 (^circ)C, 850 (^circ)C, 900 (^circ)C, 950 (^circ)C and 1000 (^circ)C and close to ambient pressure. Four different natural gem-quality alkali feldspars with low degree of Al–Si ordering covering the range from orthoclase to high sanidine and with slightly different minor element concentrations were used as starting materials. The partitioning curves obtained for the four feldspars are indistinguishable indicating that Na–K partitioning independent of the differences of Al–Si ordering state and minor element concentrations existing amongst these feldspars. A sub-regular two parameter Margules type solution model was fitted to the partitioning data, and the excess Gibbs energy describing the thermodynamic non-ideality of the alkali feldspar solid-solution and the respective Margules parameters (W_{text {g}text {K}}) and (W_{text {g}text {Na}}) including their temperature dependence expressed as (W_g=W_h-TW_s) were determined:

The corresponding solvus has a critical temperature slightly above 650 (^circ)C and is well comparable with earlier direct experimental determinations of the low-sanidine-albite solvus curve. Comparison of the vibrational excess entropy determined from low-temperature heat capacity measurements with the total excess entropy derived from the temperature dependence of the excess Gibbs energy yields a negative configurational contribution to the excess entropy pointing towards short-range Na–K ordering on the alkali site.

The metamict fergusonite-(Y) with the formula (Y_0.70Ln_0.20Ca_0.13U_0.02Th_0.02)_∑1.07(Nb_0.72Ta_0.17W_0.06Ti_0.04)_∑1(O_3.97(OH)_0.11F_0.08Cl_0.03) · 2.12H₂O from the Blyumovskaya Pit, Ilmeny Mountains (Russia) was studied by the means of high-temperature X-ray diffraction, thermal analysis, Raman spectroscopy and microprobe analysis. Thermal expansion was studied for both tetragonal (α-fergusonite) and monoclinic (β-fergusonite) polymorphs. The expansion of β-fergusonite is anisotropic and strongly negative along the α₃₃. In contrast, α-fergusonite exhibits a relatively isotropic thermal expansion upon heating. The volume CTE (α_V) for β-fergusonite varies in the range 22.87(94)–75.4(2.5) × 10^–6 ºC⁻¹, whereas α-fergusonite has α_V = 32.33(57)–31.66(49) × 10^-6 ºC⁻¹ in the temperature range 850–1200 °C. After heating to 1100 °C, the mineral develops a porous texture, and the radioactivity is reduced by 37%, which can be attributed to the partial volatilization of some radionuclides. In situ experiments revealed the complete sequence of the thermal evolution of the metamict fergusonite-(Y) for the first time.

High-temperature X-ray diffraction measurements of calcium ferrite (CF)-type MgAl₂O₄ were performed in a temperature range of 300–673 K at atmospheric pressure. From temperature dependence of the unit cell volume, thermal expansivity (α) was determined to be α(T) = (2.46 ± 0.13) × 10^–5 + (1.2 ± 0.3) × 10^–8 T in 1/K. Thermoelastic parameters of isothermal bulk modulus at zero pressure (K_T0), its pressure derivative (K_T′) and temperature derivative [(∂K_T0/∂T)_P] of MgAl₂O₄ CF were optimized by iteration calculation combining the least squares fitting of a third-order Birch–Murnaghan equation of state to previous P–V–T data with α calculation using the Grüneisen relation equation, α = γ_thC_V/(K_T0V) where γ_th and C_V are thermal Grüneisen parameter and isochoric heat capacity, respectively. γ_th was constrained by the α measured in this study. When pressure data were rescaled by Au equations of state which are different from that adopted in the previous study and temperature data were corrected using pressure dependence of electromotive force of a W–Re thermocouple, K_T0, K_T′ and (∂K_T0/∂T)_P were assessed to be 216(4) GPa, 3.9(3) and − 0.027(3) GPa/K, respectively. It was suggested that the optimized α was about 17% lower than that determined by the previous study at 2000 K.

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.