Jacinta Enzweiler

Incorporation of Ni into natural goethite: An investigation by X-ray absorption spectroscopy

Abstract Goethite (α-FeOOH) is abundant at the Earth’s surface and has the unusual capacity to adsorb and fix ions from migrating solutions. Understanding the mechanisms by which foreign elements are incorporated into natural goethite has implications for environmental and mining problems. X-ray absorption spectroscopy (XAS) was used to obtain structural information on the local environment around Ni in natural Ni-containing goethite (1.8-4.1 mol% Ni) from Vermelho lateritic deposit of Serra dos Carajás (Brazil) and in synthetic analogues. The data were collected at the LNLS XAS beam line at the Ni and Fe K-edges, at room temperature, and at the Ni K-edge at 8 K. Nickel was found in essentially the same environment in all natural and synthetic samples, with negligible thermal disorder. The coordination polyhedron is a tetragonal dipyramid of oxygen atoms showing that Ni preserves its usual local symmetry. This finding is compatible with a model in which substitution of Ni for Fe is accompanied by a proton capture resulting in NiO2(OH)4 octahedra. The polyhedral linkages are similar to that of pure a-FeOOH, consisting of four shared edges at about the same metal-metal distances, as in the pure mineral. The third and longest metal-metal distance is about 6% larger than the expected corner-sharing distance in the α-FeOOH structure, showing that incorporation of Ni locally distorts and opens the structure.

Determination of platinum group elements and gold in geological materials using an ultraviolet laser ablation high-resolution inductively coupled plasma mass spectrometric technique

In this work, a UV laser system coupled to high-resolution ICP-MS was used in the determination of platinum group elements (PGE) and gold in nickel sulfide buttons prepared by the fire assay technique. A Nd:YAG laser operating at 266 nm quadrupled frequency was used. The laser operating parameters were established by studying signal intensity and the precision for different energy beams and frequencies. TheF-test was used to evaluate the homogeneity of the buttons, demonstrating that for most elements the distribution is homogeneous. The analyses were performed on three different sites in three replicates of the materials. The detection limits obtained (3σ) were in a range from 0.2 for Os (and Ir) to 7 ng g–1 for Pt. The accuracy of the results was better than 16.7% for most elements. The effects of isobaric interference on the 103Rh signal (206Pb2+ and 40Ar63Cu+) were evaluated by analysing NiS buttons doped with lead and copper. For Au, Pt and Pd, the standard deviations obtained were higher than expected and are related to a non-homogeneous distribution of these elements in the buttons. The concentrations of the elements in the geological reference materials UMT-1 and WPR-1 were obtained by external calibration curves.

Biogeochemical dynamics following land use change from forest to pasture in a humid tropical area (Rondônia, Brazil): a multi-element approach by means of XRF-spectroscopy

Forest burning for pastures in tropical areas represents an important component of biogeochemical cycles. In order to provide information concerning chemical modifications after forest burning, in this local study the total contents of 29 elements in topsoils were analyzed when forest is changed to pasture land. The work was carried out in 1999 in Rondĵnia state (Brazilian Amazon Basin) focussing on a native forest site and four neighboring pastures established in 1987, 1983, 1972 and 1911 after forest conversion. Chemical fingerprint graphs of the pasture soils related to the forest soil illustrated mainly higher contents for the vast majority of macro- and micro nutrients, but for other elements as well (e.g. Ba, Sr, Cr, Ni, V or Pb). Also increases of pH levels were measured in all pastures, which remained higher than the forest values for decades. After initial increases of most of the elements in pasture of 1987 the decreases of some macro elements (e.g. C, N, K, Mg, S) in pasture 1983 as well as again the enhanced levels in pasture 1972 and 1911 suggest both a persistent leaching of these elements and a function of pasture age where external element inputs exceed outputs. Ash deposition, accumulation of organic matter, animal excreta as well as natural soil conditions are discussed as influencing factors on the element contents of the original forest and the pasture soils. Nevertheless, in this particular area continuous pasturing after forest clearing primarily enriched the soils in elements.

Adsorption of colloidal gold on colloidal iron oxides

Abstract Lateritic gold enrichment can be associated with the formation of colloidal gold and its interaction with the iron-oxide-rich medium. Adsorption of colloidal gold on synthetic goethite and hematite was measured at 25°C. The observed high affinity between the oppositely charged particles depends on the medium composition. The presence of citrate anions can inhibit mutual coagulation, while inert electrolytes enhance this effect.

Ochreous laterite: a nickel ore from Punta Gorda, Cuba

Abstract The ‘ochreous laterite’ is the oxidised nickel ore from the Punta Gorda deposit, Cuba. Mossbauer spectroscopy, X-ray diffraction (XRD), thermal methods and selective extraction techniques have been used to characterise the ore. The ore consists mainly of poorly crystalline Al-substituted goethite. The poor crystallinity of goethite is demonstrated by broad lines in the diffraction patterns and hence, low mean crystallite dimension (MCD) values, low dehydroxylation temperatures, high weight losses between 100 and 1000°C, and the magnetic hyperfine field distribution. Also present in the ore are minor quantities of hematite, gibbsite, chromite, and manganese minerals. Most of the nickel is associated with goethite. Cobalt is mainly associated with the manganese minerals. The Ni-rich ore, with contents of Fe 2 O 3 between 72.1 and 77.2% and contents of Ni between 1.6 and 2.1%, is probably derived from a peridotite bedrock containing, in addition to olivine, Ni-bearing orthopyroxenes. Peridotite containing Al-clinopyroxene might be, on the other hand, the parent rock of the Ni-poor ore, with Fe 2 O 3 contents between 42.6 and 62.6% and Ni contents between 0.24 and 0.96%.

Determination of trace elements in silicate rocks by X-ray fluorescence spectrometry on 1:5 glass discs: comparison of accuracy and precision with pressed powder pellet analysis

Abstract A fusion procedure commonly used for major element analysis of silicate rocks (1:5 sample to flux ratio, using 0.7 g of sample) was used to determine a small group of trace elements (Cu, Zn, Rb, Sr, Y, Zr, Nb) by X-ray fluorescence. Compton scatter peak ratioing was successfully used for matrix corrections, despite the thickness of the glass discs being only 1.7 mm. Precision and detection limits were compared with pressed pellet values. For most elements, routine analytical precision is better with powder pellets and detection limits are worse in glass discs, by an average factor of 2, considering all elements analyzed. Accuracy was evaluated with international reference materials and for most of the determinations, good or excellent agreement with recommended values was obtained. The main advantages of using glass discs for the trace element determination are the possibility of analyzing small amounts of sample and the suppression of any mineralogical effects in the glass discs, which can also be used to determine major elements.

Reference Materials in Geochemical and Environmental Research

In all fields of geochemistry, cosmochemistry, and environmental research, reference materials play an important role. They are used as samples for calibration of analytical instruments and to establish metrological traceability to measurements, quality control and assurance, and interlaboratory comparisons. The International Organization for Standardization Committee on Reference Materials (ISO/REMCO) works to establish and update internationally agreed guidelines concerning terminology, production, characterization, certification, and use of reference materials. Many reference materials are homogeneous rock powders, synthetic and natural silicate glasses, minerals, and environmental materials. So-called delta zero, radioactive and radiogenic isotopic reference materials are necessary for normalization purposes of isotope ratio measurements. Geochemists can easily have access to information about reference materials using the GeoReM database. Although many rock reference materials have been produced, only few of them have been certified by ISO-compatible procedures. There is a need for microanalytical reference materials, especially for homogeneous and well-characterized mineral samples. The situation is also unsatisfactory for isotopic reference materials. Many isotopic reference materials are heterogeneous in the isotopic composition, because they were made from oxides or metal, do not have certified values, and have isotope ratios much far from any natural sample or not available any more.

Characterization of Ni-Containing Goethites by Mössbauer Spectroscopy and Other Techniques

A set of goethite samples from the oxidized level of Vermelho Ni-lateritic deposit was investigated by TEM, XRD, TGA-DTA, FTIR and Mössbauer spectroscopy and compared to a series of synthetic goethite samples, with similar Ni content. Natural Ni-containing goethite presents poor crystallinity and does not incorporate more than about 4 mol% Ni. Natural goethite particles also contain significant amounts of Al and Cr (1.6 and 0.7 mol%, in average, respectively). The wavenumbers of the OH deformation vibration bands δ-OH (in-plane vibration) and lengthening γ-OH (out of plane vibration) are coarsely negatively correlated with Ni content. The weight loss during the thermal analysis of synthetic samples shows a good linear relationship with Ni content, pointing to a proton uptake in order to compensate a possible charge imbalance due to a Ni2+ for Fe3+ substitution. The Mössbauer spectra of Ni-goethite present one sextet whose parameters are in agreement with those usually reported for goethite. The magnitude of the hyperfine field slightly decreases with increasing Ni, while the other parameters remain unchanged. These variations may be explained by Ni incorporation, but also by differences in the crystallinity. The effect of the low Ni concentration in our samples was a weak to medium correlation of the analytical signals with the Ni content in goethite, but when considered together, the results indicate that isomorphic substitution is occurring.

NAA and UV Laser Ablation ICP-MS for Platinum Group Elements and Gold Determination in NiS Fire Assay Buttons: A Comparison between Two Methods

NAA and UV laser ablation ICP-MS were used to determine platinum group elements (PGEs) and gold in the geological reference materials UMT-1, WPR-1, WMG-1, GPT-4 and GPt-6, after NiS fire assay. Both methods presented results were good agreement with the recommended values. NAA gave more accurate values for Ir (relative errors between 0 to 9%) and UV-LA-ICP-MS presented better results for Pt (relative errors less than 12%, except for WPR-1). UV-LA-ICP-MS showed better sensitivity than NAA for Pd and Os. On the other hand, NAA showed lower detection limits for Ir and Au. Advantages and disadvantages of each method are discussed.

Multi-element analysis of soils and sediments by wavelength-dispersive X-ray fluorescence spectrometry

Soil and sediment reference materials were used to calibrate and evaluate an analytical method for the determination of major (Si, Al, Fe, Mg, Ca, Na, K, Mn, P, Ti) and trace elements (As, Ba, Cd, Co, Cr, Cu, Ga, Mo, Mb, Ni, Pb, Rb, S, Sb, Sn, Sr, Th, U, V, Y, Zn, Zr) by sequential wavelength X-ray fluorescence spectrometry. Samples were prepared as pressed pellets and analysis was done with a total measuring time of thirty minutes per sample. Special attention was given to the selection of the thirty reference materials used for calibration of the spectrometer. Another set of eleven RM (reference materials) was analyzed for the evaluation of accuracy. Detection limits for trace elements (1-2 mg kg-1) are adequate both for geochemical and environmental purposes, except for Cd and Sb. Accuracy for trace elements falls within the expected interval of certified or recommended values in most cases, but for some major elements, like SiO2, some results showed discrepancies, evidencing difficulties associated with the determination of light elements in complex matrices. But when quality criteria proposed by mapping programs are applied to the results, their requirements are fulfilled. Both instrumental precision, obtained by twelve sample replicate analyses, and analytical precision, considering also sub-sampling and pellet preparation, lie between the limits of the Horwitz expression, except at concentrations close to the detection limits.