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Hematite ( a-Fe2O3) and goethite ( a-FeOOH) can incorporate considerable amounts of tungsten (W). Although W concentrations up to several wt% in hematite and goethite have been reported in the literature, none of the proposed models for a structural incorporation has been generally accepted yet. Here, the first combination of X-ray absorption fine structure (XAFS) measurements with X-ray diffraction (XRD), Raman spectroscopy (RS), electron microprobe (EMPA) and total reflection X-ray fluorescence (TXRF) provides a general relation between W content and its structural incorporation into hematite and goethite. Botryoidal specimens of goethite and hematite, obtained from the Schwarzwald ore district, Black Forest, SW Germany, and from the Grantcharitza W deposit, Bulgaria, display W concentrations of up to 5.5 and 2.15 mol% W for goethite and hematite, respectively. In addition to these natural specimens, goethite and hematite were synthesized in the presence of W and incorporate up to 7 and 1.3 mol% W, respectively. X-ray diffraction analysis does not indicate the presence of separate W-phases, supporting the structural incorporation of W into the hematite and goethite. Refined unit-cell parameters indicate no changes with increasing W concentration in hematites but a rising structural disorder within the structure of the synthetic goethites. Raman spectroscopy, however, shows an increasing structural disorder for both synthetics, indicating an increase of Fe vacancies in both hematite and goethite. A deprotonation mechanism for the goethite structure is unlikely according to the Raman results. XAS near-edge spectra indicate a strong distortion of the WO6 octahedra in both hematite and goethite. Extended XAFS spectra of the natural and synthetic goethites and hematites show striking similarities and suggest that W6+ resides in all samples on the Fe3+ position, again without developing separate W phases. Calculations of the Fe-loss related to W incorporation reach mean values of ~2.9 and ~2.8 for goethite and hematite, respectively. The formation of two Fe3+ vacancies in close proximity to the newly incorporated W6+ in addition to a protonation of the structures achieves charge balance within the hematite and goethite structure. Hematite and goethite record the presence of W in fluids even in the absence of visible W minerals. After W adsorption to ferrihydrite (the hematite and goethite precursor phase) and after its transformation to either hematite or goethite, only hematite with up to 0.4 wt% W is clearly able to continuously monitor a changing W signature as a record of the fluid history within its oscillatory growth zones. In contrast, goethite is probably not a good monitor of a primary W fluid history. Their combination, however, could be particularly useful, as hematite records the W concentrations in a fluid during ferihydrite precipitation, while goethite records W concentrations during later ferrihydrite maturation. Botryoidal Fe-ores have never been considered for W recovery but could play an important role to fight a potential supply risk of W as a high-technology metal. [ABSTRACT FROM AUTHOR]

Cerium is the most abundant rare earth element (REE) within the waste product of alumina production (red mud), but its speciation in this complex material is still barely understood. Previous studies showed evidence for a correlation between Ce and the main constituent of red mud, iron oxides, which led us to investigate the most abundant iron oxide in red mud, hematite, as possible carrier phase for Ce. Synthetic hematite can incorporate up to 1.70 ± 0.01 wt% Ce, which leads to a systematical increase of all unit cell parameters. Investigations by extended X-ray absorption fine structure spectroscopy suggest an incorporation of Ce 4 + O 6 into the hematite structure by a novel atomic arrangement, fundamentally different from the close-range order around Fe 3 + in hematite. Samples of red mud were taken in Lauta (Saxony), Germany and analyzed by powder X-ray diffraction, inductively coupled plasma mass and optical emission spectrometry, electron microprobe analysis and X-ray absorption near-edge structure spectroscopy. Red mud samples consist of hematite (Fe 2 O 3 ) (34–58 wt%), sodalite (Na 8 Al 6 Si 6 O 24 Cl 2 ) (4–30 wt%), gibbsite (Al(OH) 3 ) (0–25 wt%), goethite (FeOOH) (10–23 wt%), böhmite (AlOOH) (0–11 wt%), rutile (TiO 2 ) (4–8 wt%), cancrinite (Na 6 Ca 2 Al 6 Si 6 O 24 (CO 3 ) 2 ) (0–5 wt%), nordstrandite (Al(OH) 3 ) (0–5 wt%) and quartz (SiO 2 ) (0–4 wt%). While the main elemental composition is Fe > Al > Na > Ti > Ca (Si not included), the average concentration of REE is 1109 ± 6 mg/kg with an average Ce concentration of 464 ± 3 mg/kg. The main carrier of Ce was located in the Fe-rich fine-grained fraction of red mud (0.10 wt% Ce 2 O 3 ), while other potential Ce carriers like monazite, lead oxides, secondary Ce-minerals and particles of potentially anthropogenic origin are of subordinated relevance. Cerium in red mud occurs predominantly as Ce 4 + , which further excludes Ce 3 + minerals as relevant sources. [ABSTRACT FROM AUTHOR]

Hematite (α-Fe2O3) is one of the most common iron oxides and a sink for the toxic metalloid arsenic. Arsenic can be immobilized by adsorption to the hematite surface; however, the incorporation of As in hematite was never seriously considered. In our study we present evidence that, besides adsorption, the incorporation of As into the hematite crystals can be of great relevance for As immobilization. With the coupling of nanoresolution techniques and X-ray absorption spectroscopy the presence of As (up to 1.9 wt %) within the hematite crystals could be demonstrated. The incorporated As5+ displays a short-range order similar to angelellite-like clusters, epitaxially intergrown with hematite. Angelellite (Fe4As2O11), a triclinic iron arsenate with structural relations to hematite, can epitaxially intergrow along the (210) plane with the (0001) plane of hematite. This structural composite of hematite and angelellite-like clusters represents a new immobilization mechanism and potentially long-lasting storage facility for As5+ by iron oxides. [ABSTRACT FROM AUTHOR]