The distribution of elements between co-existing phases in some marine ferromanganese-oxide deposits

Analyses of ferromanganese oxides from the Indian and Atlantic Oceans for the elements Mn, Fe, Co, Ni, Cu, Zn, Pb, Ca, AI, Ti, Cr and Cd have helped to elucidate some of the controls on their geochemistry. In most samples virtually all of the Mn and much of the Fe are present as acid-reducible phyllomanganates and Fe oxyhydroxides respectively. By contrast, in samples in which goethite was identified, much of the Fe and significant amounts of the Mn. are not acid-reducible. The partition patterns of the minor elements reflect to varying extents the mineralogy of the hydrous Mn and Fe oxide phases. In δ-Mn-O₂-rich samples the ratio of adsorbed to crystallographically-bound Ni. Cu and Zn, is higher than in todorokite-rich samples, but in each case these metals are virtually entirely phyllomanganate-associated. In goethite-rich samples, however, significant amounts of Ni. Cu and Zn may be associated with the goethite itself rather than with phyllomanganate minerals. Cobalt shows very close association with the phyllomanganates irrespective of the specific mineralogy, but Pb behaves in a way which cannot yet be fully characterised. The non-reducible fractions of the samples contain most of the Ca, Al. Ti and Cr. Some Ca however is also present in the phyllomanganates.     

Deposition of deep-sea manganese nodules

Manganese at equilibrium in seawater occurs dominantly as Mn²⁺ and inorganic complexes at a concentration ratio of about 1:0.72; solubility decreases exponentially with increasing pH or Eh. However, the nodule oxides birnessite and todorokite are at least four orders of magnitude undersaturated relative to the Mn concentrations of seawater, and are metastable relative to hausmannite and manganite. This apparent lack of equilibrium is explicable by the mechanism of precipitation.Surfaces assist Mn precipitation by catalyzing equilibration between dissolved and reactive O₂ and simultaneously also by adsorbing ionic Mn species. The effective Eh at the surface becomes 200–400 mV above that of seawater; the oxidation rate of Mn increases about 10⁸ ×, and the activation energies for Mn oxidation decrease ~ 11.5 kcal/mole. Consequently, marine Mn nodules and crusts form by adsorption and catalytic oxidation of Mn²⁺ and ferrous ions at nucleating surfaces such as sea-floor silicates, oxyhydroxides, carbonates, phosphates and biogenic debris. The resulting ferromanganese surfaces autocatalyze further growth. In addition, Mn-fixing bacteria may also significantly accelerate accretion rates on these surfaces.Mn which accumulates in submarine sediments may be diagenetically recycled in response to steep solubility gradients causing upward migration from more acidic and reducing horizons toward the sea floor. In contrast, the concentrations of the predominant ferric complexes, Fe(OH)₃⁰ and Fe(OH)₄^?, are relatively less sensitive to the Eh's and pH's found in this environment; Fe is therefore not as readily recycled within buried sediments. Consequently, Fe is not so effectively enriched on the sea floor, although it precipitates more readily than Mn because seawater is saturated in amorphous Fe(OH)₃.The metastable, perhaps kinetically-related, Mn oxides of nodules have a characteristic distribution: birnessite predominates in oxidizing environments of low sedimentation rate and todorokite where sedimentation rates and diagenetic Mn mobility are higher. Surface adsorption and cation substitution within the disordered birnessite-todorokite structure account for the high trace element content of Mn nodules.     

Composition and characteristics of the ferromanganese crusts from the western Arctic Ocean

Layered ferromanganese crusts collected by dredge from a water depth range of 2770 to 2200 m on Mendeleev Ridge, Arctic Ocean, were analyzed for mineralogical and chemical compositions and dated using the excess ²³⁰Th technique. Comparison with crusts from other oceans reveals that Fe-Mn deposits of Mendeleev Ridge have the highest Fe/Mn ratios, are depleted in Mn, Co, and Ni, and enriched in Si and Al as well as some minor elements, Li, Th, Sc, As and V. However, the upper layer of the crusts shows Mn, Co, and Ni contents comparable to crusts from the Atlantic and Indian Oceans. Growth rates vary from 3.03 to 3.97 mm/Myr measured on the uppermost 2 mm. Mn and Fe oxyhydroxides (vernadite, ferroxyhyte, birnessite, todorokite and goethite) and nonmetalliferous detrital minerals characterize the Arctic crusts. Temporal changes in crust composition reflect changes in the depositional environment. Crust formation was dominated by three main processes: precipitation of Fe-Mn oxyhydroxides from ambient ocean water, sorption of metals by those Fe and Mn phases, and fluctuating but large inputs of terrigenous debris.     

Simultaneous incorporation of Mn and Al in the goethite structure

Two series of (Al,Mn)-substituted goethites were synthesized from ferrihydrite made in alkaline media, with different Al/Mn mole ratios ([Al + Mn]/Fe molar ratio up to 0.12). Powder X-ray diffraction and extended X-ray absorption fine structure (EXAFS) techniques were used to assess the structural characteristics of the simultaneous substitution in goethite. XRD patterns revealed that all the obtained solids remain in a goethite-like structure. Rietveld refinement of X-ray diffraction data indicates that the increasing Mn substitution and consequent decrease of Al substitution causes an increase in the unit cell volume. This change is accompanied by the increment of the various Me-Me distances. XANES spectra at the Al and Mn K-edge confirm the octahedral coordination of Al and the trivalent oxidation state of the Mn ion in all the synthesized samples. EXAFS spectra at the Fe K-edge indicate that the local order around the Fe atom remains practically constant upon (Mn,Al) substitution. Measurements in the Mn K-edge show that distances Mn-Me suffer different changes with the increase in Mn substitution: a marked decrease in E and a slight decrease in E′, while DC remains constant. E and E′ values correspond to the distance between one Mn and one neighboring Me (Fe, Mn, Al) atom, both situated in two polyhedra linked by an edge. These polyhedra belong to the same double row of the goethite structure. DC value corresponds to the distance between one Mn and one Me (Fe, Mn, Al) atom, situated in two octahedral linked by one corner and belonging to two adjacent double chains. All the intermetallic distances are minor than the corresponding singly substituted goethites, this fact is attributed to the structure contraction due to the presence of Al(III) which restrains the axial distortion of Mn. Dissolution-time curves, resulting from exposure to 6 M HCl at 318 K, show that the dissolution rate slows with increasing Al substitution and consequent decrease of Mn substitution, and the shape of the curve becomes increasingly sigmoidal for mixed goethite with large Al content and Al-goethite. Dissolution kinetics of most samples are well described by the Kabai equation. Al dissolves almost congruently with respect to Fe, implying that it is homogeneously distributed in the structure. However, the convex χ_Mn:χ_Fe curve indicates that Mn tends to be concentrated in the outer layers of the goethite particles.     

Ferromanganese oxide deposits from the Central Pacific Ocean,II. Nodules and associated sediments

Bulk chemical, mineralogical and selective leach analyses have been made on a suite of abyssal ferromanganese nodules and associated sediments from the S.W. equatorial Pacific Ocean. Compositional relations between nodules, sediment oxyhydroxides and nearby ferromanganese encrustations are drawn assuming that the crusts represent purely hydrogenetic ferromanganese material. Crusts, δMnO₂-rich nodules and sediment oxyhydroxides are compositionally similar and distinct from diagenetic todorokitebearing nodules. Compared to Fe-Mn crusts, sediment oxyhydroxides are however slightly enriched, relative to Mn and Ni, in Fe, Cu, Zn, Ti and Al, and depleted in Co and Pb, reflecting processes of non-hydrogenous element supply and diagenesis. δMnO₂ nodules exhibit compositions intermediate between Fe-Mn crusts and sediment oxyhydroxides and thus are considered to accrete oxides from both the water column and associated sediments.Deep ocean vertical element fluxes associated with large organic aggregates, biogenic calcite, silica and soft parts have been calculated for the study area. Fluxes associated with organic aggregates are one to three orders of magnitude greater than those associated with the other phases considered, are in good agreement with element accumulation rates in sediments, and are up to four orders of magnitude greater than element accumulation rates in nodules. Metal release from labile biogenic material in surface sediments can qualitatively explain the differences between the composition of Fe-Mn crusts and sediment oxyhydroxides.Todorokite-rich diagenetic nodules are confined to an eastwards widening equatorial wedge. It is proposed that todorokite precipitates directly from interstitial waters. Since the transition metal chemistry of interstitial waters is controlled dominantly by reactions involving the breakdown of organic carbon, the supply and degradation rate of organic material is a critical factor in the formation of diagenetic nodules. The wide range of (trace metal/Mn) ratios observed in marine todorokite reflects a balance between the release of trace metals from labile biogenic phases and the reductive remobilisation of Mn oxide, both of which are related to the breakdown of organic carbon.     

Impacts of hydrous manganese oxide on the retention and lability of dissolved organic matter

Minerals constitute a primary ecosystem control on organic C decomposition in soils, and therefore on greenhouse gas fluxes to the atmosphere. Secondary minerals, in particular, Fe and Al (oxyhydr)oxides—collectively referred to as “oxides” hereafter—are prominent protectors of organic C against microbial decomposition through sorption and complexation reactions. However, the impacts of Mn oxides on organic C retention and lability in soils are poorly understood. Here we show that hydrous Mn oxide (HMO), a poorly crystalline δ-MnO₂, has a greater maximum sorption capacity for dissolved organic matter (DOM) derived from a deciduous forest composite O_i, O_e, and O_a horizon leachate (“O horizon leachate” hereafter) than does goethite under acidic (pH 5) conditions. Nonetheless, goethite has a stronger sorption capacity for DOM at low initial C:(Mn or Fe) molar ratios compared to HMO, probably due to ligand exchange with carboxylate groups as revealed by attenuated total reflectance-Fourier transform infrared spectroscopy. X-ray photoelectron spectroscopy and scanning transmission X-ray microscopy–near-edge X-ray absorption fine structure spectroscopy coupled with Mn mass balance calculations reveal that DOM sorption onto HMO induces partial Mn reductive dissolution and Mn reduction of the residual HMO. X-ray photoelectron spectroscopy further shows increasing Mn(II) concentrations are correlated with increasing oxidized C (C=O) content (r = 0.78, P < 0.0006) on the DOM–HMO complexes. We posit that DOM is the more probable reductant of HMO, as Mn(II)-induced HMO dissolution does not alter the Mn speciation of the residual HMO at pH 5. At a lower C loading (2 × 10² μg C m^?2), DOM desorption—assessed by 0.1 M NaH₂PO₄ extraction—is lower for HMO than for goethite, whereas the extent of desorption is the same at a higher C loading (4 × 10² μg C m^?2). No significant differences are observed in the impacts of HMO and goethite on the biodegradability of the DOM remaining in solution after DOM sorption reaches steady state. Overall, HMO shows a relatively strong capacity to sorb DOM and resist phosphate-induced desorption, but DOM–HMO complexes may be more vulnerable to reductive dissolution than DOM–goethite complexes.     

Pyrolusite and Chalcophanite Forming in Qixiashan,Nanjing

Weathering of manganese-bearing carbonate could form chalcophanite. In this paper, the occurrence of Fe (hydro) oxides and Mn-bearing minerals in Qixiashan were identified by XRD and SEM, mainly consisted of goethite, hematite, pyrolusite and chalcophanite. From the microscope investigation, stromatolite-like structure phenomenon is widespread existed, which may be caused by microbial activities. To identify the mineral structure in the Fe-Mn crust, Raman and XPS were used to identify the mineral structure and valence of Fe, Mn and Zn. This work could help us to know the relationship of Fe and Mn during the weathering of manganese-bearing carbonate. And the enrichment of Mn and Zn from the supergene environment could provide a path for the contamination of heavy metals.     

Composition and formation of fossil manganese nodules in Jurassic to Cretaceous radiolarites from the Nicoya Ophiolite Complex (NW Costa Rica)

Horizons of several types of Upper Jurassic to Lower Cretaceous manganese nodules occur locally in sequences of radiolarian cherts within the Nicoya Ophiolite Complex (NW Costa Rica). Field studies, X-ray diffraction analysis, petrographic, chemical and experimental studies give evidence of a sedimentary, early diagenetic origin of the nodules, in contrast to earlier suggestions. Smooth, discoidal, compact and very dense nodules with diameters of some mm to 9 cm dominate. They are characterized by braunite, hollandite, pyrolusite and quartz as well as 39–61% Mn, 0.9–1.6% Fe, 5–26% SiO₂, 1.3–1.9% A1₂O₃, 1.5–3.0% Ba, 460–5400 ppm Cu, 85–340 ppm Ni and 40–130 ppm Co, among others. It is suggested that the original mineralogy (todorokite?) was altered during thermometamorphic (braunite) and hydrothermal (hollandite, pyrolusite) events. Petrographic similarities between the fossil nodules and modern deep-sea nodules are striking. Using standard hydrothermal techniques in an experimental study it is shown that under special conditions, braunite can be produced from modern nodule material.     

Structural relaxation in the MnCO3–CaCO3 solid solution: a Mn K-edge EXAFS study

Mn K-edge EXAFS spectroscopy of solid-solution samples encompassing the complete MnCO₃–CaCO₃ series shows that first-shell Mn–O distances deviate little from the 2.19-Å distance observed in pure MnCO₃. Very slight lengthening is observed only in the limiting case of dilute Mn(II) calcite solid solutions, where the Mn–O distance is 2.21 Å. The observed nearly complete structural relaxation and the composition independence of the Mn–O distance are consistent with the Pauling model behavior of solid solutions, and agree with previous studies showing a high degree of relaxation around hetero-sized substituents in the calcite structure. Strain occurs through bond bending, which is facilitated by the exclusively corner-sharing topology of calcite. Observed distances from Mn to more distant neighbors show significant variation across the solid-solution series that resembles Vegard's law-type behavior but reflects averaging. The high degree of relaxation suggests modest enthalpies of mixing in the solution, consistent with calorimetric studies.     

Structures and stabilities of Cd(II) and Cd(II)-phthalate complexes at the goethite/water interface

The complexation of Cd(II) and Cd(II)-phthalate at the goethite/water interface were investigated by EXAFS and IR spectroscopy, by batch adsorption experiments and by potentiometric titrations at 298.15 K. The EXAFS spectra showed Cd(II) to form only inner-sphere corner-sharing complexes with the goethite surface sites in the presence and absence of phthalate. EXAFS spectra also showed the presence of Cd(II)-chloride complexes in 0.1 mol/L NaCl. IR spectra also showed phthalate to form (1) an inner-sphere complex with adsorbed corner-sharing Cd(II) surface complexes in the pH 3.5 to 9.5 and (2) an outer-sphere complex with the same type of corner-sharing Cd(II) complex however at pH > 6, in addition to the inner- and outer-sphere complexes of phthalate reported in a previous study. The potentiometric titration and the batch adsorption data were used to constrain the formation constants of the different Cd(II)-phthalate surface complexes on the dominant {110} and the {001} planes of the goethite. The models were carried out with the Charge Distribution Multisite Complexation model coupled to the Three Plane Model and can predict the molecular-scale speciation of cadmium and phthalate in the presence of goethite. Cd(II) adsorption models calibrated on a 90 m²/g goethite also could accurately predict experimental data for a 37 m²/g goethite of slightly different basic charging properties.     

Geochemistry of ferromanganese nodules from DOMES site a,Northern Equatorial Pacific: Multiple diagenetic metal sources in the deep sea

The major and minor element composition of ferromanganese nodules from DOMES Site A has been determined by X-ray fluorescence methods. Three phases appear to control the bulk compositions: Mn and Fe oxyhydroxides and aluminosilicates. Relatively wide compositional variations are evident throughout the area. Nodules with high Mn/Fe ratios, high Cu, Mg, Mo, Ni and Zn concentrations and high todorokite/δ-MnO₂ ratios have granular surface textures and are confined to an east-west trending depression with thin Quaternary sediment cover. Nodules with low Mn/Fe ratios, high concentrations of As, Ca, Ce, Co, La, P, Sr, Ti, V, Y and Zr and low todorokite/δ-MnO₂ ratios have smooth surfaces and are confined to shallower areas with relatively thick Quaternary sediment to the north and south of the depression.All nodules in the area have compositions which are influenced by diagenesis, but those with the most marked diagenetic signature (high Mn/Fe and Cu/Ni ratios, low Ce/La ratios and more todorokite) are found in areas of very slow or non-existent sedimentation; many of these nodules are actually in contact with outcropping Tertiary sediment. This paradox may be resolved by postulating, by analogy with some shallow-water occurrences, that the nodules accrete from bottom waters which have enhanced particulate and dissolved metal contents derived from diagenetic reaction in areas remote from the site of nodule formation. The metals are supplied in a bottom flow (probably Antarctic Bottom Water) which also erodes, or prevents modern sedimentation in, the depression. Nodules on the flanks of the depression are not evidently affected by this flow and derive at least pan of their constituent metals from diagenetic reaction in the underlying Quaternary sediment.Apparently, abyssal diagenetic nodules can have an immediate and a remote diagenetic metal source. Metal fluxes derived from pore water dissolved metal gradients may not be relevant to particular accreting nodules if a significant fraction of their metals is derived from outside the area in which they form.     

Germanium crystal chemistry in hematite and goethite from the Apex Mine,Utah, and some new data on germanium in aqueous solution and in stottite

At the Apex Mine in southwest Utah, fine-grained hematite contains as much as 1.0 wt. percent Ge, and fine-grained goethite contains as much as 0.7 wt. percent Ge. The mode of Ge incorporation in these minerals was investigated by high-resolution K-edge fluorescence spectroscopy using synchrotron radiation. Analysis of extended fine structure (EXAFS) K-edge data for Ge and Fe shows that Ge substitutes for Fe in the octahedral metal sites of the studied hematite and goethite, with average Ge-ligand bond lengths of 1.88 Å. The solid solution in hematite probably occurs through the coupled substitution 2Fe(III) = Ge(IV) + Fe(II), similar to the coupled substitution 2Fe(III) = Ti(IV) + Fe(II) that occurs in the solid solution series hematite-ilmenite. The solid solution in goethite probably occurs by the loss of an H atom from an OH group, through the coupled substitution Fe(III) + H(I) = Ge(IV). In related experiments, EXAFS data indicate that in a neutral aqueous solution containing 790 ppm Ge, the Ge occurs predominately as Ge(OH)₄, with tetrahedral Ge-OH bond lengths of 1.74 Å. In stottite, FeGe(OH)₆, Fe(II) and Ge(IV) occur in octahedral sites with average Fe-OH and Ge-OH bond lengths of about 2.20 Å and 1.88 Å.     

Distribution and partitioning of major and trace elements in pyrite-bearing sediments of a Mediterranean coastal lagoon

The formation of iron sulphide minerals exerts significant control on the behaviour of trace elements in sediments. In this study, three short sediment cores, retrieved from the remote Antinioti lagoon (N. Kerkyra Island, NW Greece), are investigated concerning the solid phase composition, distribution, and partitioning of major (Al, Fe) and trace elements (Cd, Cu, Mn, Pb, and Zn). According to ²¹⁰Pb, the sediments sampled correspond to depositions of the last 120 years. The high amounts of organic carbon (4.1–27.5%) result in the formation of Fe sulphides, predominantly pyrite, already at the surface sediment layers. Pyrite morphologies include monocrystals, polyframboids, and complex FeS–FeS₂ aggregates. According to synchrotron-generated micro X-ray fluorescence and X-ray absorption near-edge structure spectra, authigenically formed, Mn-containing, Fe(III) oxyhydroxides (goethite type) co-exist with pyrite in the sediments studied. Microscopic techniques evidence the formation of galena, sphalerite and CuS, whereas sequential extractions show that carbonates are important hosts for Mn, Cd, and Zn. However, significant percentages of non-lattice held elements are bound to Fe/Mn oxyhydroxides that resist reductive dissolution (on average 60% of Pb, 46% of Cd, 43% of Zn and 9% of Cu). The partitioning pattern changes drastically in the deeper part of the core that is influenced by freshwater inputs. In these sediments, the post-depositional pyritization mechanism, illustrated by overgrowths of Fe monosulphides on pre-existing pyrite grains, results in relatively high degree of pyritization that reaches 49% for Cd, 66% for Cu, 32% for Zn and 7% for Pb.     

Nickel geochemistry of a Philippine laterite examined by bulk and microprobe synchrotron analyses

The Ni geochemistry of limonite and saprolite laterite ores from Pujada in the Philippines has been investigated using a mixture of laboratory and synchrotron techniques. Nickel laterite profiles are typically composed of complicated mineral assemblages, with Ni being distributed heterogeneously at the micron scale, and thus a high degree of spatial resolution is required for analysis. This study represents the first such analysis of Philippine laterite ores. Synchrotron bulk and microprobe X-ray absorption spectroscopy (XAS), comprising both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies, together with synchrotron microprobe X-ray fluorescence microscopy (XFM) and diffraction (XRD) have been applied to provide quantitative analysis of the mineral components and Ni speciation.Synchrotron microprobe EXAFS spectroscopy suggests that the limonite Ni is associated with phyllomanganate via adsorption onto the Mn oxide layers and substitution for Mn within these layers. Laboratory scanning electron microscopy, coupled to electron dispersive spectroscopy analyses, indicates that Ni is also associated with concentrated Fe containing particles and this is further confirmed by synchrotron bulk and microprobe investigation. Linear combination fitting of the bulk EXAFS limonite data suggests 60 ± 15% of the Ni is associated with phyllomanganate, with the predominant fraction adsorbed above vacancies in the MnO₆ layers with the remainder being substituted for Mn within these layers. The remaining 40 ± 10% of the Ni in the limonite ore is incorporated into goethite through replacement of the Fe. In the saprolite ore, 90 ± 23% of the Ni is associated with a serpentine mineral, most likely lizardite, as a replacement for Mg. The remaining Ni is found within phyllomanganate adsorbed above vacancies in the MnO₆ layers.     

Fe(II) reduction of pyrolusite (β-MnO<Subscript>2</Subscript>) and secondary mineral evolution

Iron (Fe) and manganese (Mn) are the two most common redox-active elements in the Earth’s crust and are well known to influence mineral formation and dissolution, trace metal sequestration, and contaminant transformations in soils and sediments. Here, we characterized the reaction of aqueous Fe(II) with pyrolusite (β-MnO₂) using electron microscopy, X-ray diffraction, aqueous Fe and Mn analyses, and ⁵⁷Fe Mössbauer spectroscopy. We reacted pyrolusite solids repeatedly with 3 mM Fe(II) at pH 7.5 to evaluate whether electron transfer occurs and to track the evolving reactivity of the Mn/Fe solids. We used Fe isotopes (56 and 57) in conjunction with ⁵⁷Fe Mössbauer spectroscopy to isolate oxidation of Fe(II) by Fe(III) precipitates or pyrolusite. Using these complementary techniques, we determined that Fe(II) is initially oxidized by pyrolusite and that lepidocrocite is the dominant Fe oxidation product. Additional Fe(II) exposures result in an increasing proportion of magnetite on the pyrolusite surface. Over a series of nine 3 mM Fe(II) additions, Fe(II) continued to be oxidized by the Mn/Fe particles suggesting that Mn/Fe phases are not fully passivated and remain redox active even after extensive surface coverage by Fe(III) oxides. Interestingly, the initial Fe(III) oxide precipitates became further reduced as Fe(II) was added and additional Mn was released into solution suggesting that both the Fe oxide coating and underlying Mn phase continue to participate in redox reactions when freshly exposed to Fe(II). Our findings indicate that Fe and Mn chemistry is influenced by sustained reactions of Fe(II) with Mn/Fe oxides.

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X-ray K-edge absorption spectra of Fe minerals and model compounds: II. EXAFS

K-edge extended X-ray absorption fine structure (EXAFS) spectra of Fe in varying environments in a suite of well-characterized silicate and oxide minerals were collected using synchrotron radiation and analyzed using single scattering approximation theory to yield nearest neighbor Fe-O distances and coordination numbers. The partial inverse character of synthetic hercynite spinal was verified in this way. Comparison of the results from all samples with structural data from X-ray diffraction crystal structure refinements indicates that EXAFS-derived first neighbor distances are generally accurate to ±0.02 Å using only theoretically generated phase information, and may be improved over this if similar model compounds are used to determine EXAFS phase functions. Coordination numbers are accurate to ±20 percent and can be similarly improved using model compound EXAFS amplitude information. However, in particular cases the EXAFS-derived distances may be shortened, and the coordination number reduced, by the effects of static and thermal disorder or by partial overlap of the longer Fe-O first neighbor distances with second neighbor distances in the EXAFS structure function. In the former case the total information available in the EXAFS is limited by the disorder, while in the latter case more accurate results can in principle be obtained by multiple neighbor EXAFS analysis. The EXAFS and XANES spectra of Fe in Nain, Labrador osumulite and Lakeview, Oregon plagioclase are also analyzed as an example of the application of X-ray absorption spectroscopy to metal ion site occupation determination in minerals.     

Surface complexation of arsenic(V) to iron(III) (hydr)oxides: structural mechanism from ab initio molecular geometries and EXAFS spectroscopy

Arsenic(V), as the arsenate (AsO₄)³⁻ ion and its conjugate acids, is strongly sorbed to iron(III) oxides (α-Fe₂O₃), oxide hydroxides (α-,γ-FeOOH) and poorly crystalline ferrihydrite (hydrous ferric oxide). The mechanism by which arsenate complexes with iron oxide hydroxide surfaces is not fully understood. There is clear evidence for inner sphere complexation but the nature of the surface complexes is controversial. Possible surface complexes between AsO₄ tetrahedra and surface FeO₆ polyhedra include bidentate corner-sharing (²C), bidentate edge-sharing (²E) and monodentate corner-sharing (¹V). We predicted the relative energies and geometries of AsO₄-FeOOH surface complexes using density functional theory calculations on analogue Fe₂(OH)₂(H₂O)_nAsO₂(OH)₂³⁺ and Fe₂(OH)₂(H₂O)_nAsO₄⁺ clusters. The bidentate corner-sharing complex is predicted to be substantially (55 kJ/mole) more favored energetically over the hypothetical edge-sharing bidentate complex. The monodentate corner-sharing (¹V) complex is very unstable. We measured EXAFS spectra of 0.3 wt. % (AsO₄)³⁻ sorbed to hematite (α-Fe₂O₃), goethite(α-FeOOH), lepidocrocite(γ-FeOOH) and ferrihydrite and fit the EXAFS directly with multiple scattering. The phase-shift-corrected Fourier transforms of the EXAFS spectra show peaks near 2.85 and 3.26 Å that have been attributed by previous investigators to result from ²E and ²C complexes. However, we show that the peak near 2.85 Å appears to result from As-O-O-As multiple scattering and not from As-Fe backscatter. The observed 3.26 Å As-Fe distance agrees with that predicted for the bidentate corner-sharing surface (²C) complex. We find no evidence for monodentate (¹V) complexes; this agrees with the predicted high energies of such complexes.     

Oxidation of As^Ⅲ by Several Manganese Oxide Minerals in Absence and Presence of Goethite

Oxidation of As^Ⅲ by three types of manganese oxide minerals affected by goethite was investigated by chemical analysis, equilibrium redox, X-ray diffraction （XRD） and transmission electron microscopy （TEM）. Three synthesized Mn oxide minerals of different types, birnessite, todorokite, and hausmannite, could actively oxidize As^Ⅲ to Asv, and greatly varied in their oxidation ability. Layer structured birnessite exhibited the highest capacity of As^Ⅲ oxidation, followed by the tunnel structured todorokite. Lower oxide hansmannite possessed much low capacity of As^Ⅲ oxidation, and released more Mn^2＋ than birnessite and todorokite during the oxidation. The maximum amount of Asv produced during the oxidation of As^Ⅲ by Mn oxide minerals was in the order： birnessite （480.4 mmol/kg） 〉 todorokite （279.6 mmol/kg） 〉 hansmannite （117.9 mmol/kg）. The oxidation capacity of the Mn oxide minerals was found to be relative to the composition, crystallinity, and surface properties. In the presence of goethite oxidation of As^Ⅲ by Mn oxide minerals increased, with maximum amounts of Asv being 651.0 mmol/kg for birnessite, 332.3 mmol/kg for todorokite and 159.4 mmol/kg for hansmannite. Goethite promoted As^Ⅲ oxidation on the surface of Mn oxide minerals through adsorption of the Asv produced, incurring the decrease of Asv concentration in solutions. Thus, the combined effects of the oxidation （by Mn oxide minerals）-adsorption （by goethite） lead to rapid oxidation and immobilization of As in soils and sediments and alleviation of the As^Ⅲ toxicity in the environments.     

Phase E: A high pressure hydrous silicate with unique crystal chemistry

The unique cation-disordered crystal structures of two samples of phase E, a non-stoichiometric, hydrous silicate synthesized in a uniaxial, split-sphere, multi-anvil apparatus at conditions above 13 GPa and 1000° C, have been solved and refined in space group $\bar 3$ . The compositions and unit cells for the two materials, assuming six oxygens per cell, are Mg_2.08Si_1.16H_3.20O₆, a=2.9701(1) Å, c=13.882(1) Å V = 106.05(4) ų for sample 1, and Mg_2.17Si_1.01H_3.62O₆, a=2.9853(6) Å, c=13.9482(7) Å, V= 107.65(4) ų for sample 2. The structure contains layers with many features of brucite-type units, with the layers stacked in a rhombohedral arrangement. The layers are cross linked by silicon in tetrahedral coordination and magnesium in octahedral coordination, as well as hydrogen bonds. Interlay er octahedra share edges with intralayer octahedra. Interlayer tetrahedra would share faces with intralayer octahedra. To avoid this situation, there are vacancies within the layers. There is, however, no long-range order in the occupation of these sites, as indicated by the lack of a superstructure. Selected-area electron diffraction patterns show walls of diffuse intensity similar in geometry and magnitude to those observed in short-range-ordered alloys and Hågg phases. Phase E thus appears to represent a new class of disordered silicates, which may be thermodynamically metastable.     

Mn, Fe, Zn and As speciation in a fast-growing ferromanganese marine nodule

The speciation of Mn, Fe, As, and Zn in a fast-growing (0.02mm/yr), shallow-marine, ferromanganese nodule has been examined by micro X-ray fluorescence, micro X-ray diffraction, and micro X-ray absorption spectroscopy. This nodule exhibits alternating Fe-rich and Mn-rich layers reflecting redox variations in water chemistry. Fe occurs as two-line ferrihydrite. The As is strictly associated with Fe and is mostly pentavalent, with an environment similar to that of As sorbed on or coprecipitated with synthetic ferrihydrite. The Mn is in the form of turbostratic birnessite with ∼10% trivalent manganese in the layers and probably ∼8% corner-sharing metal octahedra in the interlayers. The Zn is enriched on the rim of the nodule, associated with Mn. The Zn is completely (>90%) tetrahedrally coordinated and sorbed in the interlayers of birnessite on vacant layer Mn sites. The Zn and Mn species are similar to ones found in soils, suggesting common structural principles despite the differing formation conditions in these systems.