Trace Element Distribution in Zoned Kyanite of Thassos Island (Greece) Using Combined Spectroscopic Analyses.

Comprehensive mineralogical and petrographic studies require analytical methods capable to report the distribution of major to trace elements within crystals in order to unravel their formation conditions and subsequent evolution. Additionally, the investigation of transition elements (e.g., Ti, V, Cr, Mn, Fe, and Zn) is essential for the comprehension of substitution processes within colored minerals. This study is conducted on a zoned kyanite crystal from a deformed quartz vein found within garnet-kyanite-biotite-hematite-plagioclase±staurolite±sillimanite paragneiss of Thassos Island, Greece. Herein, we show the efficiency of combining conventional, for example, cathodoluminescence, electron probe microanalysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and new methods, for example, micro-laser-induced breakdown spectroscopy (µLIBS), micro-X-ray fluorescence (µXRF), and Raman spectroscopy, to determine the chemical and crystallographic features of minerals. The simple chemistry of this crystal offers an ideal case to compare and valuate the potential of combined spectroscopy techniques to analyze minerals. We demonstrate that µLIBS and µXRF are perfectly adapted to perform multi-element imaging of major to trace elements down to the ppm level within a pluricentimetric crystal (2.3 x 0.5 cm) prior to quantitative analyses. We also highlight the benefit of cathodoluminescence and Raman mapping in the investigation of crystallographic features within minerals. The multispectroscopic approach enabled us to correlate growth stages of kyanite with the polymetamorphic history of the sample. Our results also highlight the spatial dependence of Ti for the generation of blue zonation by Fe2+-Ti4+ substitutions with Al3+.

Identification and composition of carbonate minerals of the calcite structure by Raman and infrared spectroscopies using portable devices.

A portable Raman device with a 532 nm excitation laser and a portable infrared spectrometer with ATR (Attenuated Total Reflection) mode were used to analyse the spectral features associated with the identification and compositional variation of Ca-Mg-Fe-Mn natural carbonate minerals with a calcite structure (calcite, ankerite, dolomite, siderite, rhodochrosite, and magnesite). A systematic study of the variations of the peak positions with various compositional ratios was carried out. Most of the band positions were shifted to lower wavenumbers with increasing ionic radius or atomic mass of the divalent cations but the band of the translational lattice (T) mode in Raman and the symmetric bending (ν4) band in the mid-infrared were the most sensitive. Therefore, the elemental variation of the Ca-Mg-Fe-Mn ratio in this carbonate series can be estimated from Raman and infrared band positions from spectra acquired with portable spectrometers.

Interpretation of the pressure-induced Raman frequency shift of the ν1 stretching bands of CH4 and N2 within CH4-CO2, N2-CO2 and CH4-N2 binary mixtures.

The relationships between the frequency shift of the ν1 stretching bands of CH4 and N2 with pressure (or density) and composition have been previously provided in the literature as reliable parameters for accurate empirical barometers and densimeters for the direct determination of the pressure or density of gas mixtures. However, the latter results still remain a pure description of the experimental data without any interpretation of the physical mechanisms hidden behind the variation trend of the observed peak position. The present paper is devoted to interpreting the origin of the pressure-induced vibrational frequency shifts of the ν1 stretching bands of CH4 and N2 within CH4-CO2, N2-CO2 and CH4-N2 binary mixtures at the molecular level. Two different theoretical models (i.e., the Lennard-Jones 6-12 potential approximation - LJ, and the generalized perturbed hard-sphere fluid - PHF) are used to intuitively and qualitatively assess the variation trend as well as the magnitude of the frequency shift of the CH4 and N2ν1 bands for an in-depth understanding. Thereby, the contribution of the attractive and repulsive solvation-mean forces to the variation of the Raman frequency shift as a function of pressure and composition is assessed. A predictive model of the variation trend of the frequency shift of the CH4ν1 band as a function of pressure (up to 3000 bars), density and composition within CH4-N2 and CH4-CO2 binary mixtures is then provided.

Quantitative Measurements of Composition, Pressure, and Density of Microvolumes of CO2-N2 Gas Mixtures by Raman Spectroscopy.

Quantitative analysis of gases by Raman spectroscopy is based on relative Raman scattering cross sections (RRSCS) and the evolution of different spectral parameters (peak position, peak area, peak intensity, etc.). However, most of the calibration data were established at low pressure (low density) and without evaluating the effect of the composition. Using these data may lead to considerable errors, especially when applied to gas mixtures at high pressure as found in natural fluid inclusions. The aim of this study is to reevaluate the RRSCS of CO2 and to establish new calibration data based on the variation of CO2 Fermi diad splitting as a function of pressure (density) and composition over a pressure range of 5-600 bar at 22 and 32 °C. A high-pressure optical cell system (HPOC) and a heating-cooling stage were used for Raman in situ analyses at controlled PTX conditions. Our experimental results show that the RRSCS of CO2 varies slightly with pressure but can be considered constant over the studied pressure range. It can be used to measure the proportion of CO2 in gas mixtures with an uncertainty of about ±0.5 mol%. Different polynomial equations were provided to calculate pressure and density of CO2-N2 gas mixtures with an uncertainty of ±20 bar or 0.01 g·cm-3. A comparison of PVTX properties of natural CO2-N2 fluid inclusions hosted in quartz from the Central Alps (Switzerland) obtained by Raman measurement and as derived from phase transition temperatures by microthermometry experiments shows comparable values.

Redox control on nitrogen isotope fractionation during planetary core formation.

The present-day nitrogen isotopic compositions of Earth's surficial (15N-enriched) and deep reservoirs (15N-depleted) differ significantly. This distribution can neither be explained by modern mantle degassing nor recycling via subduction zones. As the effect of planetary differentiation on the behavior of N isotopes is poorly understood, we experimentally determined N-isotopic fractionations during metal-silicate partitioning (analogous to planetary core formation) over a large range of oxygen fugacities (ΔIW -3.1 < logfO2 < ΔIW -0.5, where ΔIW is the logarithmic difference between experimental oxygen fugacity [fO2] conditions and that imposed by the coexistence of iron and wüstite) at 1 GPa and 1,400 °C. We developed an in situ analytical method to measure the N-elemental and -isotopic compositions of experimental run products composed of Fe-C-N metal alloys and basaltic melts. Our results show substantial N-isotopic fractionations between metal alloys and silicate glasses, i.e., from -257 ± 22‰ to -49 ± 1‰ over 3 log units of fO2 These large fractionations under reduced conditions can be explained by the large difference between N bonding in metal alloys (Fe-N) and in silicate glasses (as molecular N2 and NH complexes). We show that the δ15N value of the silicate mantle could have increased by ∼20‰ during core formation due to N segregation into the core.

A vibrational study of the nature of hydroxyl groups chemical bonding in two aluminium hydroxides.

Lengths, strengths and valences of OH bonds in the two aluminium hydroxides gibbsite and bayerite were determined on the basis of vibrational spectral data. The uncoupled OD stretching modes in the range 2400-2800cm(-1) were recorded by means of infrared diffuse reflectance, thereby avoiding effects of surface, vibrational coupling or particle shape. The assignment of the corresponding Raman spectra resulted in the determination of harmonic wavenumbers, force constants and anharmonicity coefficients of bulk OH groups in the two minerals. OH bond lengths deduced from these data varied from 0.964A to 0.975A in gibbsite and 0.962A to 0.973A in bayerite. These lengths appear to correspond to weak H-bonds contrary to previously recognized data from X-ray diffraction and neutron diffraction studies. Finally, bond valences were calculated on the basis of these new bond lengths and discussed as a function of crystallographic structures and the nature of hydrogen bonding in these two structures.

Uranyl interaction with the hydrated (001) basal face of gibbsite: a combined theoretical and spectroscopic study.

The sorption of uranyl cations and water molecules on the basal (001) face of gibbsite was studied by combining vibrational and fluorescence spectroscopies together with density functional theory (DFT) computations. Both the calculated and experimental values of O-H bond lengths for the gibbsite bulk are in good agreement. In the second part, water sorption with this surface was studied to take into account the influence of hydration with respect to the uranyl adsorption. The computed water configurations agreed with previously published molecular dynamics studies. The uranyl adsorption in acidic media was followed by time-resolved laser-induced fluorescence spectroscopy and Raman spectrometry measurements. The existence of only one kind of adsorption site for the uranyl cation was then indicated in good agreement with the DFT calculations. The computation of the uranyl adsorption has been performed by means of a bidentate interaction with two surface oxygen atoms. The optimized structures displayed strong hydrogen bonds between the surface and the -yl oxygen of uranyl. The uranium-surface bond strength depends on the protonation state of the surface oxygen atoms. The calculated U-O(surface) bond lengths range between 2.1-2.2 and 2.6-2.7 A for the nonprotonated and protonated surface O atoms, respectively.