Fe-Ni alloys in a unique Antarctic meteorite: Yamato-791694

Phase composition and structure of iron-nickel alloys in Y-791694 are discussed and compared to other non-Antarctic Ni-rich ataxites using the results obtained by Mossbauer spectroscopy and X-ray diffraction

Fe2+-Mg order-disorder study in orthopyroxenes from São João Nepomuceno (IVA) iron meteorite

São João Nepomuceno (SJN) is an iron meteorite belonging to IVA chemical group. It consists of Fe-Ni metal matrix and silicate inclusions made of a coarse-grained mixture of tridymite, orthopyroxene and clinopyroxene. In spite of the extensive work performed on the IVA group there is still no consensus about their origin and its thermal history is subject of ongoing debates. In this work, we report preliminary results on Fe2 + distribution in the non-equivalent octahedral sites in orthopyroxene crystals extracted from São João Nepomuceno in order to infer on the thermal history of this meteorite and therefore acquire more information related to the origin of the IVA iron meteorite group.Fil: Dos Santos, Eder Paulo. Centro Brasileiro de Pesquisas Fisicas; Brasil;Fil: Scorzelli, R. B.. Centro Brasileiro de Pesquisas Fisicas; Brasil;Fil: Varela, Maria Eugenia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Cientifico Tecnológico - Conicet - San Juan. Instituto de Ciencias Astronómicas de la Tierra y del Espacio; ArgentinaFil: Munayco, P.. Centro Brasileiro de Pesquisas Fisicas; Brasil

57Fe Mössbauer spectroscopy studies of chondritic meteorites from the Atacama Desert, Chile: Implications for weathering processes

Some terrestrial areas have climatic and geomorphologic features that favor the preservation, and therefore, accumulation of meteorites. The Atacama Desert in Chile is among the most important of such areas, known as dense collection areas. This desert is the driest on Earth, one of the most arid, uninhabitable locals with semi-arid, arid and hyper-arid conditions. The meteorites studied here were collected from within the dense collection area of San Juan at the Central Depression and Coastal Range of Atacama Desert. [superscript 57]Fe Mössbauer spectroscopy was used for quantitative analysis of the degree of weathering of the meteorites, through the determination of the proportions of the various Fe-bearing phases and in particular the amount of oxidized iron in the terrestrial alteration products. The abundance of ferric ions in weathered chondrites can be related to specific precursor compositions and to the level of terrestrial weathering. The aim of the study was the identification, quantification and differentiation of the weathering products in the ordinary chondrites found in the San Juan area of Atacama Desert

Fe-bearing phases in Antarctic carbonaceous chondrites Yamato-82162 and Yamato-86720: A Mossbauer study

We report the results of variable temperature Mossbauer spectroscopy measurements on samples of Yamato-82162 and Yamato-86720 carbonaceous chondrites. Transmission Mossbauer spectra were taken through the temperature range from 4.2K to 300K. The Mossbauer spectra at room temperature clearly indicate the presence of magnetic splits and quadrupole doublets in both meteorites, which can be attributed to troilite and olivine, respectively. The measurements also indicate the presence of a superparamagnetic component, probably ferryhidrite, in Y-86720 but not in Y-82162. These results are mostly consistent with those obtained by mineralogical studies. However, our Mossbauer measurements indicate the presence of a previously unknown magnetic component having a field of 250kOe in both meteorites; the mineral is unidentified so far. Our study supports the view that these meteorites were affected by thermal metamorphism, and that Y-82162 was less thermally metamorphosed than Y-86720

Mossbauer spectroscopy study of the metallic particles extracted from the Antarctic chondrite Allan Hills-769

Pieces of the Antarctic L6 chondrite Allan Hills-769 (ALH-769) have been submitted to magnetic and chemical separation in order to obtain small metallic grains of the special ordered crystal structure of AuCu-type tetrataenite. The samples have been analyzed in each step of preparation by Mossbauer spectroscopy and other techniques

Hyperfine interactions associated with iron substituted superconducting oxides

Theoretical and experimental Moessbauer spectroscopy studies have been made concerning charge and spin densities and magnetic hyperfine fields (H{sub hf}) in iron-substituted superconducting oxides. Calculations were carried out in the self-consistent-field embedded cluster model using local density theory (SCF-X{alpha}) with a variational atomic orbital basis. Spectral densities and changes in charge and spin density were monitored around neighboring Cu sites, as well as Fe impurity site, and La{sub 2}Cu{sub 1-x}Fe{sub x}O{sub 4} and YBa{sub 2}Cu{sub 3-x}Fe{sub x}O{sub 7-y} compounds. Moessbauer isomer shifts (IS), quadrupole splittings (QS) and H{sub hf} are obtained by fitting multiline models to the observed spectra and are compared with SCF-X{alpha} results for specific lattice sites. The influence of oxygen vacancies and partial oxygen disorder is modelled and compared with the experimental data on variable oxygen content and disorder. 10 refs., 3 tabs

Precious metals determination in ancient coins by portable ED-XRF spectroscopy with a 238Pu source

A portable X-ray fluorescence system (pXRF) was employed to analyse a set of gold, silver and billon coins from the collection of the Museu Histórico Nacional do Rio de Janeiro (MHN), struck during the Brasil Colônia period, under the Dutch occupation, and in Portuguese mints under the reigns of Fernando I (1367 to 1383) and his successor João I (1383 to 1433). The experimental setup consisted of a mini Si-PIN detector and a radioactive source of 238Pu for the excitation of the samples. The aim of the study was to test the possibilities and limitations of this portable system for the determination of the coins’ base alloys, and to verify whether the results obtained were consistent with historical facts. The Brasil Colônia period coins show about 80.5% gold and 96.8% silver. For the Portuguese coins, we could identify a possible monetary debasement during the reign of João I. No matrix corrections were made for these preliminary results.Un équipement portable à fluorescence X (pFX) a été utilisé pour l’analyse de monnaies en or, en argent et en billon appartenant à la collection du Museu Histórico Nacional de Rio de Janeiro (MHN). Certaines de ces monnaies ont été frappées au Brésil pendant la période Brasil Colônia et sous l’occupation hollandaise, alors que d’autres ont été frappées au Portugal sous les règnes de Ferdinand I (1367-1383) et de son successeur Jean I (1383-1433). Le montage expérimental est constitué d’un mini détecteur Si-PIN et d’une source radioactive de 238Pu, dont la radiation émise est utilisée comme source d’excitation. Le but de ce travail est double : vérifier les possibilités et les limites de ce système portable lors qu’il s’agit de déterminer les alliages constitutifs de monnaies en métal précieux et vérifier les rapports entre l’évolution des compositions des monnaies et les différents faits historiques. Les monnaies appartenant à la période Brasil Colônia sont constituées de 80,5 % d’or et de 96,8 % d’argent et les monnaies frappées au Portugal montrent une dévaluation sous Jean I

Druse clinopyroxene in D'Orbigny angritic meteorite studied by single-crystal X-ray diffraction, electron microprobe analysis and Mössbauer spectroscopy

The crystal structure of druse clinopyroxene from the D'Orbigny angrite, (Ca0.944 Fe 2+ 0.042 Mg0.010Mn0.004) (Mg0.469Fe 2+ 0.317Fe 3+ 0.035Al0.125Cr0.010Ti0.044) (Si1.742Al0.258) O6, a = 9.7684(2), b = 8.9124(2), c = 5.2859(1) A, β = 105.903(1)°, V = 442.58 A 3 , space group C2/c, Z = 2, has been refined to an R1 index of 1.92% using single-crystal X-ray diffraction data. The unit formula, calculated from electron microprobe analysis, and the refined site scattering values were used to assign site populations. The distribution of Fe 2+ and Mg over the M1 and M2 sites suggests a closure temperature of 1000 °C. Mossbauer spectroscopy measurements were done at room temperature on a single crystal and a powdered sample. The spectra are adequately fit by a Voigt-based quadrupole-splitting distribution model having two generalized sites, one for Fe 2+ with two Gaussian components and one for Fe 3+ with one Gaussian component. The two ferrous components are assigned to Fe 2+ at the M1 site, and arise from two different next-nearest-neighbor configurations of Ca and Fe cations at the M2 site: (3Ca,0Fe) and (2Ca,1Fe). The Fe 3+ /Fetot ratio determined by Mossbauer spectroscopy is in agreement with that calculated from the electron microprobe analysis. The results are discussed in connection with the redox and thermal history of D'Orbigny.Instituto de Física La Plat

MÖSSBAUER SPECTRA AND ELECTRON EXCHANGE IN TOURMALINE AND STAUROLITE

Les spectres Mössbauer de la tourmaline et de la staurotide présentent, à la température ambiante, deux doublets de Fe2+. Le doublet interne de plus faible intensité augmente avec la température et par l'irradiation avec des électrons de 2 MeV. Ce doublet disparaît reversiblement aux basses températures (inférieures à 77 K). Ceci a été interprété comme une conséquence de l'échange d'un électron entre le Fe2+ et la lacune dans le réseau.The Mössbauer spectra of tourmaline and staurolite presents at room temperature two doublets of Fe2+. The inner doublet is less intense and increases with temperature and by irradiation with 2 MeV electrons. This doublet vanishes reversibly at low temperatures (below 77 K). This behaviour has been interpreted as due to an electron exchange between Fe2+ and the vacancy