C. S. M. Partiti

Mineralogical characterization of a highly-weathered soil by the Rietveld Method

The mineralogical characterization through mineral quantification of Brazilian soils by X-ray diffraction data using the Rietveld Method is not common. A mineralogical quantification of an Acric Ferralsol from the Ponta Grossa region, state of Parana, Brazil, was carried out using this Method with X-Ray Diffraction data to verify if this method was suitable for mineral quantification of a highly-weathered soil. The A, AB and B 3 horizons were fractioned to separate the different particle sizes: clay, silt, fine sand (by Stokes Law) and coarse sand fractions (by sieving), with the procedure free of chemical treatments. X-ray Fluorescence, Inductively Coupled Plasma Atomic Emission Spectrometry, Infrared Spectroscopy and Mossbauer Spectroscopy were used in order to assist the mineral identification and quantification. The Rietveld Method enabled the quantification of the present minerals. In a general way, the quantitative mineralogical characterization by the Rietveld Method revealed that quartz, gibbsite, rutile, hematite, goethite, kaolinite and halloysite were present in the clay and silt fractions of all horizons. The silt fractions of the deeper horizons were different from the more superficial ones due to the presence of large amounts of quartz. The fine and the coarse sand fractions are constituted mainly by quartz. Therefore, a mineralogical quantification of the finer fraction (clay and silt) by the Rietveld Method was successful.

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%.

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.

Mössbauer spectroscopic study of iron oxides and oxyhydroxides in gossans

Abstract Two sets of gossan samples derived from the oxidation of sulphides of a NiCu and a PbZn deposit, respectively, have been investigated by Mossbauer spectroscopy. For the first set of samples all spectra consisted of one magnetically split sextet with parameters similar to those of goethite, except for one sample that presented an additional sextet typical of hematite. The magnitude of the magnetic hyperfine field in goethite was found to decrease linearly with increasing (Ni + Cu)/(Ni + Cu + Fe) mole fraction, suggesting that Ni and Cu could be structurally incorporated in the goethite lattice, in an analogous way to previously reported Al substitution. The PbZn gossan samples yielded spectra with two sextets that could be assigned to goethite and hematite. One sample, in addition, presented a doublet that was attributed to jarosite. No correlation was observed between the goethite hyperfine fields and the (Zn + Pb)/(Zn + Pb + Fe) mole fractions. Reasons for such behavior are discussed.

Magnetic study of Brazilian peats from São Paulo state

The magnetic properties of peats from the Parao´ba do Sul Valley (Sao Paulo State) Brazil have been studied in cores up to 6.4 m long with a basal radiocarbon age of 11,080 years BP. In general, the magnetic susceptibility is low, but is consistently higher near the bottom of the profiles. Anhysteretic remanence is surprisingly strong in comparison to other magnetic properties, and is attributed to the preservation of fossil bacterial magnetosomes in the peat. Results from Mossbauer spectroscopy indicate the presence of Fe 3+ and Fe 2+ in the bottom of the cores, but only Fe 3+ toward the top. The additional Fe 2+ may be related to the higher magnetic susceptibility. Susceptometer measurements of magnetisations, in fields up to 6 Tat both room temperature and 10 K, reveal predominantly paramagnetic behavior. Laboratory-induced remanences and magnetisations point to the presence of three additional magnetic phases, namely, ferrite (both low and medium stability) and a haematite or goethite component. D 2003 Elsevier B.V. All rights reserved.

Anomalous temperature dependence of Fe3+ quadrupole interaction in (ND4)2FeCl5·H2O

The quadrupole splitting of the title compound was measured in the 12–295 K temperature range. A strong increase of Δ is observed on cooling. This effect can be explained in terms of thermally activated reorientation of the ND4+ ion, which makes a sizeable contribution to the EFG at the Fe site. Analysis of our data yields an activation energy for the ions hindered rotation,E/kB=470±30 K. A comparison with previous results for the non-deuterated salt is made.

Phonon softening in (C2H5)4NFeX4 (X=Cl, Br)

Mössbauer spectra of the Fe(III) compounds (C2H5)4NFeX4 (X=Cl, Br) exhibit the following anomalies above ∼ 260 K: (i) thef-factor drops with respect to normal thermal behavior, (ii) the second-order Doppler shift drops similarly, (iii) the single-line spectrum broadens, becoming asymmetric at room temperature. These phenomena are discussed in relation to possible phase transitions in these systems.

Magnetic Characterization of Sediments of Urban Supply Lakes in SÃo Paulo, Brazil

We report on Mossbauer spectroscopy, X-ray diffraction and chemical analysis studies of samples from urban supply lakes. Our results are in accordance with literature data that have recently shown that the presence of foreign metals and P inhibits the transformation from ferrihydrite (paramagnetic phase) to goethite and hematite (more stable magnetic phase).