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 Fe2+ 0.042 Mg0.010Mn0.004) (Mg0.469Fe2+ 0.317Fe3+ 0.035Al0.125Cr0.010Ti0.044) (Si1.742Al0.258) O6, a = 9.7684(2), b = 8.9124(2), c = 5.2859(1) Å, β = 105.903(1)°, V = 442.58 Å3, 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 Fe2+ and Mg over the M1 and M2 sites suggests a closure temperature of 1000 °C. Mössbauer 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 Fe2+ with two Gaussian components and one for Fe3+ with one Gaussian component. The two ferrous components are assigned to Fe2+ 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 Fe3+/Fetot ratio determined by Mössbauer 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.Fil: Abdu, Yassir A.. University of Manitoba; CanadáFil: Scorzelli, Rosa B.. Centro Brasileiro de Pesquisas Físicas; BrasilFil: Varela, Maria Eugenia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Complejo Astronómico "El Leoncito". Universidad Nacional de Córdoba. Complejo Astronómico "El Leoncito". Universidad Nacional de la Plata. Complejo Astronómico "El Leoncito". Universidad Nacional de San Juan. Complejo Astronómico "El Leoncito"; ArgentinaFil: Kurat, Gero. Naturhistorisches Museum; AustriaFil: Souza Azevedo, Izabel de. Centro Brasileiro de Pesquisas Físicas; BrasilFil: Stewart, Silvana Jacqueline. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Física La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Física La Plata; ArgentinaFil: Hawthorne, Frank C.. University of Manitoba; Canad

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

Infrared Spectroscopy of Carbonaceous-chondrite Inclusions in the Kapoeta Meteorite: Discovery of Nanodiamonds with New Spectral Features and Astrophysical Implications

We report the finding of nanodiamonds, coexisting with amorphous carbon, in carbonaceous-chondrite (CC) material from the Kapoeta achondritic meteorite by Fourier-transform infrared (FTIR) spectroscopy and micro-Raman spectroscopy. In the C-H stretching region (3100-2600 cm-1), the FTIR spectrum of the Kapoeta CC material (KBr pellet) shows bands attributable to aliphatic CH2 and CH3 groups, and is very similar to IR spectra of organic matter in carbonaceous chondrites and the diffuse interstellar medium. Nanodiamonds, as evidenced by micro-Raman spectroscopy, were found in a dark region (¡­400 ¥ìm in size) in the KBr pellet. Micro-FTIR spectra collected from this region are dramatically different from the KBr-pellet spectrum, and their C-H stretching region is dominated by a strong and broad absorption band centered at ¡­2886 cm-1 (3.47 ¥ìm), very similar to that observed in IR absorption spectra of hydrocarbon dust in dense interstellar clouds. Micro-FTIR spectroscopy also indicates the presence of an aldehyde and a nitrile, and both of the molecules are ubiquitous in dense interstellar clouds. In addition, IR peaks in the 1500-800 cm-1 region are also observed, which may be attributed to different levels of nitrogen aggregation in diamonds. This is the first evidence for the presence of the 3.47 ¥ìm interstellar IR band in meteorites. Our results further support the assignment of this band to tertiary CH groups on the surfaces of nanodiamonds. The presence of the above interstellar bands and the absence of shock features in the Kapoeta nanodiamonds, as indicated by Raman spectroscopy, suggest formation by a nebular-condensation process similar to chemical-vapor deposition.Fil: Abdu, Yassir A.. American University of Sharjah. Department of Physics; Emiratos Árabes UnidosFil: Hawthorne, Frank C.. University of Manitoba; CanadáFil: Varela, Maria Eugenia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio. Universidad Nacional de San Juan. Instituto de Ciencias Astronómicas, de la Tierra y del Espacio; Argentin

Oxidation state of iron in alteration minerals associated with sandstone-hosted unconformity-related uranium deposits and apparently barren alteration systems in the Athabasca Basin, Canada: Implications for exploration

Sandstone-hosted unconformity-related U deposits in the Athabasca Basin at McArthur River Zone 4 and Maurice Bay as well as barren alteration systems at Wheeler River Zone “K” and Spring Point are surrounded by an outer, peripheral zone of illite and an inner, central zone of chlorite alteration. The oxidation state of Fe determined by 57Fe Mössbauer spectroscopy of illite, sudoite, and clinochlore from these systems indicates that pre-ore illite contains mostly Fe3+ (Fe3+/ΣFe = 0.77–1.00), reflecting formation from a predominantly oxidizing basinal fluid. Lower Fe3+/ΣFe ratios are recorded by late pre-ore sudoite and clinochlore occurring near faulted unconformities, consistent with the contribution to the chlorites from reducing basement- derived fluids. Sudoite and clinochlore in the alteration halo of barren systems are more reduced (Fe3+/ ΣFe median = 0.3–0.4) than sudoite from mineralized systems (Fe3+/ΣFe median = 0.45–0.55). Sudoite from both systems becomes broadly more oxidized with increasing distance from the fault zone. Variations in the Fe3+/ΣFe ratios of sudoite and clinochlore support a mechanism in which Fe2+ transported by reduc- ing basement-derived fluids is partially oxidized to Fe3 + by basinal fluids prior to incorporation into the chlo- rite. Thus, scarcity of oxidizing U-bearing basinal fluids available for interaction with reducing basement fluids is a critical factor precluding U mineralization in barren systems. Trends of decreasing Fe3+/ΣFe ratios in sudoite coupled with decreasing 207Pb/206Pb ratios of leachable Pb can be used as geochemical vectors to the sites of potential U mineralization

Oxidation state of iron in alteration minerals associated with sandstone-hosted unconformity-related uranium deposits and apparently barren alteration systems in the Athabasca Basin, Canada: Implications for exploration

Sandstone-hosted unconformity-related U deposits in the Athabasca Basin at McArthur River Zone 4 and Maurice Bay as well as barren alteration systems at Wheeler River Zone “K” and Spring Point are surrounded by an outer, peripheral zone of illite and an inner, central zone of chlorite alteration. The oxidation state of Fe determined by 57Fe Mössbauer spectroscopy of illite, sudoite, and clinochlore from these systems indicates that pre-ore illite contains mostly Fe3+ (Fe3+/ΣFe = 0.77–1.00), reflecting formation from a predominantly oxidizing basinal fluid. Lower Fe3+/ΣFe ratios are recorded by late pre-ore sudoite and clinochlore occurring near faulted unconformities, consistent with the contribution to the chlorites from reducing basement- derived fluids. Sudoite and clinochlore in the alteration halo of barren systems are more reduced (Fe3+/ ΣFe median = 0.3–0.4) than sudoite from mineralized systems (Fe3+/ΣFe median = 0.45–0.55). Sudoite from both systems becomes broadly more oxidized with increasing distance from the fault zone. Variations in the Fe3+/ΣFe ratios of sudoite and clinochlore support a mechanism in which Fe2+ transported by reduc- ing basement-derived fluids is partially oxidized to Fe3 + by basinal fluids prior to incorporation into the chlo- rite. Thus, scarcity of oxidizing U-bearing basinal fluids available for interaction with reducing basement fluids is a critical factor precluding U mineralization in barren systems. Trends of decreasing Fe3+/ΣFe ratios in sudoite coupled with decreasing 207Pb/206Pb ratios of leachable Pb can be used as geochemical vectors to the sites of potential U mineralization