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The identification of iron (Fe) forms throughout a sediment sequence was investigated by X-ray Absorption Near Edge Spectroscopy (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) at the Fe K-edge, paired with Raman micro-spectroscopy. The contribution of different organic and inorganic Fe-bearing compounds was quantified by Linear Combination Fitting (LCF) carried out on both XANES and EXAFS spectra. Fe-XANES showed that the Fe(II)/Fe(III) ratio of different Fe-bearing minerals in sediments can be quantified with reasonable accuracy. The main Fe species detected were ferrihydrite, goethite, hematite, clay minerals (smectite, illite, nontronite), and Fe(III)-organic matter (Fe(III)-OM). A more accurate quantification of ferrihydrite was possible with LCF conducted on Fe-EXAFS spectra. With the exception of hematite, the concentration of these mineral species does not have a clear trend with depth, probably because water infiltration caused continuous Fe reduction and oxidation cycles in these sediments. From an analytical perspective, Fe oxide compounds can be difficult to identify or distinguish unless multiple techniques are used. X-ray diffraction (XRD; previous work) and Raman spectroscopy turn out to be not particularly useful in identifying ferrihydrite, while they are best suited for a broad mineralogical analysis that requires integrative spectral studies for an accurate Fe speciation. In detail, XANES and EXAFS allowed for the detection of Fe-bearing clay minerals and a more refined identification of Fe species, including Fe(III)-OM. Thermal analysis was useful to confirm some mineralogical components observed using both XRD (data previously published) and Raman spectroscopy (e.g., goethite, todorokite). In conclusion, this study underlines how a multi-technique approach is required to investigate peculiar environments such as karst pedosequences. Full article

Sulphur is the third most abundant volatile element in deep Earth systems. Analytical methods for accurately and efficiently determining the sulphur content and oxidation state in natural minerals are still lacking. Natural apatite is widely distributed in the Earth and incorporates a large amount of sulphur. Therefore, apatite is an ideal mineral for performing sulphur measurements. Here, we used spectroscopic, Raman, X-ray diffraction, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), electron microprobe (EMPA) and micro-X-ray fluorescence spectrometry (micro-XRF) analysis techniques and developed a new analytical approach (i.e., micro-X-ray absorption near-edge structure (micro-XANES) analysis of the sulphur K-edge) to investigate the chemical characteristics of natural apatite. These multiple methods were developed to measure in situ sulphur concentration and S oxidation states and to assess a potential natural apatite reference material. Apatite contains chemically homogeneous sulphur, with micro-XANES located at the peak energies corresponding to S⁶⁺ (sulphate; ~2482 eV), S⁴⁺ (sulfite; ~2478 eV), and S²⁻ (sulphide; ~2467, 2470 and 2474 eV). The Durango apatite contains total S presented as SO₃ at amount of 0.332 ± 0.012 wt.% (1σ), with a large amount of S⁶⁺ and a small contribution of S⁴⁺. The Kovdor apatite contains 44–100 ppm of S and is dominated by S⁶⁺. These results indicate that the Durango apatite crystallised under relative oxidising conditions, and the Kovdor apatite has a higher oxygen fugacity than Durango. In addition, this study indicates the potential use of the natural apatite reference material with its S composition and S oxidation state. Full article