Franklinite

Abstract: More than a century of non-ferrous metallurgical activities have had a severe impact on the natural environment leading, in most heavily contaminated sites, to a complete loss of the vegetation cover (that is, desert-like areas) or to the selection of metal-hyperaccumulator plant species. Identifying the chemical forms of toxic metals is of vital importance for a realistic assessment of the chemical risk posed by their presence in soils and selecting effective remedia- tion technologies. In this study, X-ray diffraction (XRD), X-ray texture goniom- etry, and powder and polarized extended X-ray absorption fine structure (EXAFS, P-EXAFS) have been used to investigate quantitatively the speciation of Zn in soils contaminated by three smelters from northern France and Belgium, and coupled synchrotron-based micro-X-ray radiation fluorescence (SXRF) and mi- cro-EXAFS (EXAFS) were also used for one of these soils. Of these techniques, the application of P-EXAFS and EXAFS to molecular environmental science was unprecedented, and we show that their complementarity greatly improves the sensitivity of powder EXAFS to identify the nature of metal-containing minerals in soils. Franklinite (ZnFe2O4), willemite (Zn2SiO4), hemimorphite (Zn4Si2O7(OH)2 ·H 2O), and Zn-containing magnetite ((Fe,Zn)Fe2O4) were identi- fied in dense soil fractions by XRD and powder EXAFS. These primary minerals originate from atmospheric fallout of Zn dusts emitted during the pyrometallurgi- cal smelting process, and they act as the main source of Zn in contaminated soils. In all soil samples, Zn released in solution during the weathering of these high-temperature minerals is taken up partly by phyllosilicates and, to a lesser extent, by Mn and Fe (oxyhydr) oxides. Zn-containing phyllosilicates were identi- fied by comparing powder EXAFS spectra to a library of model compounds and from the noteworthy angular dependence of EXAFS spectra collected on self- supporting films of clay soil fractions. Analysis of higher correlations in EXAFS spectra suggests that the local structure around Zn in phyllosilicates is trioctahe- dral. The phyllomanganate Zn-sorbed birnessite and Zn-containing Fe grains having a FeOOH-like local structure were unambiguously identified bySXRF— EXAFS. In birnessite Zn is sorbed in the interlayer space above/below vacant sites and can be either 4-fold or 6-fold coordinated depending, presumably, on the anionic stacking of birnessite layers. Based on this micro-mineralogical investiga- tion, a satisfactory fit of the three identified Zn species (that is, phyllosilicate, Mn, and Fe (oxyhydr)oxides) to experimental powder EXAFS spectra of all clay soil fractions was obtained. The significance, origin, and stability of Zn-phyllosilicates are discussed. Specifically, we show that the formation of Zn-containing phyllosili- cates is consistent with calculated thermodynamic solubilities. For the range of measured Zn 2 (D10 ppm), Si(OH)4 (10-20 ppm), and H (5.6 F pH F 7.5) concen- trations, soil solutions are supersaturated (pH G 6) or near saturation (pH F 6) with respect to the trioctahedral Zn phyllosilicate, Zn-kerolite. Finally, the plausi- bility of the formation of (Zn,Al) hydrotalcite-like species contemplated by Julliot (1999) is critically assessed.

Abstract: A soil profile contaminated as a result of Zn smelting operations from the historic Palmerton, PA smelting facility was characterized using X-ray absorption fine structure spectroscopy (XAFS) and X-ray diffraction (XRD) as bulk techniques, coupled with electron microprobe (EM), and microfocused XAFS as microscopic techniques to determine the chemical forms of Zn and elucidate its geochemical fate. The black, organic matter-rich topsoil contained 6200 mg/ kg Zn and was strongly acidic (pH 3.2). Bulk XAFS revealed that about 2/3 of Zn was bound in franklinite and 1/3 bound in sphalerite. Both minerals may have been aerially deposited from the smelter operation. Microspec- troscopy detected also minor amounts of Zn adsorbed to Fe and Mn (hydr)oxides as inner-sphere sorption complexes, which may have formed after weathering of the Zn minerals. About 10% of the total Zn in this sample could be easily leached. In contrast, the yellowish, loamy subsoil contained less Zn (890 mg/kg) and had a higher pH of 3.9. XAFS revealed that Zn was mostly bound to Al-groups and to a lesser extent to Fe and Mn (hydr)oxides. Minor amounts of outer-sphere complexes or organic matter-bound Zn species could also be detected. About 70% of the total Zn content could be easily leached, indicating that outer- sphere sorption complexes have been underestimated and/ or inner-sphere sorption complexes are weak due to the low pH. The Zn forms in the subsoil most likely derive from weathering of the Zn minerals in the topsoil. Due to the lack of minerals incorporating Zn and due to the low pH, the availability of Zn in the subsoil is as high as in the topsoil, while the total concentration is almost 1 order of magnitude smaller.

Abstract: The distribution of cations between tetrahedral (A) sites and octahedral (B) sites in ferrite spinels has been studied using K-edge x-ray absorption spectroscopy. The samples include natural and synthetic end-member magnetites (Fe₃O₄), a natural Mn- and Zn-rich magnetite (franklinite) and synthetic binary, ternary and quaternary ferrites of stoichiometry M(²+)M₂(³+)O₄, where M(²+) = Mg, Co, Ni, Zn and M(³+) = Fe, Al. XAS data were obtained for all metals. Complete, unfiltered, EXAFS spectra were refined to determine the percentage distribution of each element over the A and B sites and these data were combined with microprobe analyses to quantify the tetrahedral occupancy for each element in each sample. Measured site occupancies and an internally consistent set of (M-O)(A) and (M-O)(B) bond lengths were used to calculate unit-cell parameters, which show excellent agreement with measured values, pointing to the reliability of the measured occupancy factors. The average occupancies determined for the tetrahedral sites in ferrites are (atoms per formula unit) Mg 0.44, Co 0.24, Ni 0.11, Zn 0.76, Al 0.11 and Fe(³+) 0.92-0.19. The wide range found for Fe(³+) is consistent with it playing a relatively passive role by making good any A-site deficit left by the other competing cations.

Abstract: Compounds with a spinel-type structure include mineral species with the general formula AB 2ϕ4, where ϕ can be O2−, S2−, or Se2−. Space group symmetry is Fd 3 m , even if lower symmetries are reported owing to the off-center displacement of metal ions. In oxide spinels (ϕ = O2−), A and B cations can be divalent and trivalent (“2–3 spinels”) or, more rarely, tetravalent and divalent (“4-2 spinels”). From a chemical point of view, oxide spinels belong to the chemical classes of oxides, germanates, and silicates. Up to now, 24 mineral species have been approved: ahrensite, brunogeierite, chromite, cochromite, coulsonite, cuprospinel, filipstadite, franklinite, gahnite, galaxite, hercynite, jacobsite, magnesiochromite, magnesiocoulsonite, magnesioferrite, magnetite, manganochromite, qandilite, ringwoodite, spinel, trevorite, ulvospinel, vuorelainenite, and zincochromite. Sulfospinels (ϕ = S2−) and selenospinels (ϕ = Se2−) are isostructural with oxide spinels. Twenty-one different mineral species have been approved so far; of them, three are selenospinels (bornhardtite, trustedtite, and tyrrellite), whereas 18 are sulfospinels: cadmoindite, carrollite, cuproiridsite, cuprokalininite, cuprorhodsite, daubreelite, ferrorhodsite, fletcherite, florensovite, greigite, indite, kalininite, linnaeite, malanite, polydymite, siegenite, violarite, and xingzhongite. The known mineral species with spinel-type structure are briefly reviewed, indicating for each of them the type locality, the origin of the name, and a few more miscellaneous data. This review aims at giving the state-of-the-art about the currently valid mineral species, considering the outstanding importance that these compounds cover in a wide range of scientific disciplines.