TEM-EDS microanalysis: Comparison among the standardless, Cliff & Lorimer and absorption correction quantification methods

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.ultramic.2023.113845Available quantification methods for energy dispersive X-ray microanalysis in transmission electron microscopy, such as the standardless method (SLM), the Cliff-Lorimer approximation (CLA) and the absorption correction method (ACM), are compared. As expected, the CLA and ACM give superior results with respect to the SLM. As far as absorption can be considered negligible, CLA and ACM perform similarly. However, starting from mass-thickness of the order of 22 × 10−6 g/cm2, absorption become significant and the ACM gives better results. More accurate analyses can be obtained with the ACM if distinct kO/Si factors are determined for light and heavy minerals, respectively, placing a divide at 2.90 g/cm3. Caution must be used when k-factors are derived indirectly from minerals with very different structure/chemistry, suggesting that separate k-factors data sets are required for accurate EDS quantification, at least for the major and diverse broad classes of minerals. Element diffusion of monovalent cations and channelling effects may represent a complication, especially in very anisotropic minerals such as phyllosilicates, where these two phenomena may occur together

High-temperature study of basic ferric sulfate, FeOHSO 4

AbstractWe report in this paper a new crystal-chemical study of synthetic basic ferric sulfate FeOHSO4. The structure solution performed by the Endeavour program, from new X-ray powder diffraction (XRPD) data, indicated that the correct space group of the monoclinic polytype of FeOHSO4 is C2/c. Selected Area Electron Diffraction (SAED) patterns are also consistent with this structure solution. The arrangement of Fe and S atoms, based on linear chains of Fe3+ octahedra cross-linked by SO4 tetrahedra, corresponds to that of the order/disorder (OD) family. The positions of the hydrogen atoms were located based on DFT calculations. IR and Raman spectra are presented and discussed according to this new structure model. The decomposition of FeOHSO4 during heating was further investigated by means of variable temperature XRPD, thermogravimetry, and differential thermal analysis as well as IR and Raman spectroscopies

Ni-serpentine nanoflakes in the garnierite ore from Campello Monti (Strona Valley, Italy): Népouite with some pecoraite outlines and the processing of Ni-containing ore bodies

The garnierite ore at Campello Monti occurs as dark green colloform concretions covering surfaces, fractures, and filling veins in harzburgite rocks. The representative composition (Ni2.45Mg0.14Cu0.12Co0.05)(Sigma 2.76)Si2.10O5(OH)(4) is consistent with a 7 angstrom phase, namely pecoraite or nepouite. Relevant chemical features are an exceptionally high Ni/Mg ratio, a significant level of Cu substituting for Ni, and a low content of S, possibly in tetrahedral sites.Olivine and orthopyroxene in the harzburgite host rock are only partially serpentinized, do not contain detectable Ni, and are almost iron free. The green coating probably originated from ground-water solutions that leached nearby weathered peridotites and sulfide ores, and deposited less-mobile elements along fractures and voids of the host peridotite, just outside their provenance area.Bulk techniques such as X-ray powder diffraction and infrared spectroscopy do not confidently distinguish between nepouite and pecoraite, although the comparison with synthetic, implicitly pure polymorphs indicates nepouite as the best matching phase. On the other hand, HRTEM clearly shows that garnierite is mostly constituted by plumose aggregates made of curved crystals with frayed tips, a few nanometers thick along the stacks and a few tens of nanometers long (nanoflakes). All known lizardite stacking sequences, namely 1T, 2H(1), and 2H(2), have been locally observed, even though most crystals show stacking disorder.The recorded nanostructure suggests possible explanations for the recurrent anomalies (low oxide totals, high T-IV/M-VI cation ratios, etc.) found in EMP analyses of garnierites. The small grain size, the high density of defects, and the structural arrangement actually intermediate between lizardite and chrysotile probably explain the ambiguities that occurred during the characterization with bulk techniques. The results obtained in this study may have important implications for ore processing methods

Solid state transformations of iron-bearing hydrated sulfate to a-Fe2O3

Iron oxides are transition metal oxides of paramount importance for their technological applications. Their synthesis can be performed by a variety of methods, most of which are chemical methods. Hematite, α-Fe2O3, can also be produced from iron sulfates by heating them sufficiently in air. In this work we have employed the thermal decomposition method to obtain hematite from the dehydration of fibroferrite, FeOH(SO4)·5H2O, a secondary iron-bearing hydrous sulfate. The study was performed via Rietveld refinement based on in-situ synchrotron X-ray powder diffraction combined with thermogravimetric analysis and mass spectrometry. The integration of the data from these techniques allowed to study the structural changes of the initial compound, determining the stability fields and reaction paths and its high temperature products. Six main dehydration/transformation steps from fibroferrite have been identified in the heating temperature range 30-798 °C. In the last step of the heating process, above 760 °C, hematite is the final phase. The temperature behavior of the different phases was analyzed and the heating-induced structural changes are discussed