Schol Of Science And Technology, Geology Division

We studied the oxidation state of Fe in silicate glasses produced during the fi rst atomic bomb blast at the Trinity test site (New Mexico) by X-ray absorption-near edge spectroscopy (XANES). The sample consists of green glass resulting from melting of the quartz-bearing sand present at the test site; some relict unmelted sand is still fused to the bottom of the sample. Comparison of the pre-edge peak data with model compounds of known Fe oxidation state and coordination number shows that in the Trinity glass sample, Fe is in the divalent state and, on average, in a mixture of 4-and 5-fold coordination. XANES spectra collected at various heights of the sample, from the bottom of the sample up to the exposed surface, show no variation of the pre-edge peak and, thus, of the Fe oxidation state with the distance from the sandglass interface. However, XANES analysis of a portion of the sand at the bottom of the sample shows Fe to be a mixture of Fe 2+ and Fe 3+ , with a Fe 3+ /(Fe 2+ + Fe 3+ ) ratio close to 0.5. This demonstrates that during the nuclear explosion, the ground rock was instantaneously reduced, transforming all the iron from mostly trivalent state to almost exclusively divalent.

Abstract— We determined the iron oxidation state and coordination number in five samples of yellow impact glass from the Cretaceous-Tertiary (K/T) boundary section at Beloc, Haiti, which formed as the result of impact melting during the Chicxulub impact event. The samples were analyzed by Fe K-edge XANES spectroscopy and the results were compared with published data on eight black impact glasses and one high Si-K impact spherule from the same impact layer. The pre-edge peak of our high-resolution XANES spectra displays evident variations indicative of significant changes in the Fe oxidation state, spanning a wide range from about 75 to 100 mole% Fe3+. Yellow K/T glasses have significantly higher Fe3+/(Fe2+ + Fe3+) ratios compared to black K/T impact glasses (from 20 to 75 mole% Fe3+) and high Si-K glass (20 mole% Fe3+). In particular, all the pre-edge peak data on these three types of impact glasses plot between two mixing lines joining a point calculated as the mean of a group of tektites studied so far (consisting of [4]Fe2+ and [5]Fe2+) to [4]Fe3+ and [5]Fe3+, respectively. Thus, the XANES spectra of the yellow K/T glasses can be interpreted as a mixture of [4]Fe2+, [5]Fe2+, [4]Fe3+, and [5]Fe3+.Our observations can be explained by a very large range of oxygen fugacity conditions during melt formation. Furthermore, there is a clear positive relationship between the Fe3+/(Fe2+ + Fe3+) ratio and the Ca content of these glasses, suggesting that the Fe oxidation state was influenced by the relative contribution of Ca-sulfate- and Ca-carbonate-bearing sedimentary rocks at the impact site.

Mg and Fe K-edge x-ray absorption near-edge spectroscopy spectra of the Pbnm olivine series end members forsterite, fayalite, monticellite, and kirschsteinite, as well as of a synthetic Fe-Mg olivine of intermediate composition ͑Fo50͒, have been recorded and compared with multiple-scattering simulations. A comparison of experimental and theoretical spectra shows that all transition features, energy positions, and relative intensities can be reproduced in the energy range up to 60 eV above threshold when using clusters containing at least 80 atoms and extending over a sphere at least 0.6 nm in radius away from the absorber. Mg and Fe are entirely in the expected structural sites in the four investigated end members. None of the models of Mg and Fe ordering in the octahedral site for the Fe50 olivine compound proposed so far is verified; thus Fe-Mg ordering still remains an open problem for olivines of intermediate compositions.

X-ray absorption spectra at the Mg and Al K edges have been recorded using synchrotron radiation on synthetic end member diopside (Di) and jadeite (Jd) and on a series of natural Fe-poor Ca-Na clinopyroxenes compositionally straddling the Jd-Di join. The spectra of C2/c end members and intermediate members of the solid solution series (C-omphacites) are different from those of the intermediate members having P2/n symmetry (P-omphacites). Differences can be interpreted and explained by comparing the experimental spectra with theoretical spectra calculated via the full multiple-scattering formalism, starting from the atomic positional parameters determined by single-crystal X-ray diffraction structure refinement on the same samples. Atomic clusters with at least 89 atoms, extending to more than 0.60 nm away from the Mg or Al absorbers, are needed to reproduce the experimental spectra. This shows that in the clinopyroxene systems XANES detects medium- rather than short-range order-disorder relationships. Theoretical spectra match the experimental ones well for all features in the regions from 16 to 60 eV above threshold. Experimental near-edge features in the first 16 eV are also reproduced, albeit less accurately. Certain near-edge features of C-omphacites reflect the octahedral arrangement of the back-scattering six O atoms nearest neighbours of the probed atom (Mg or Al) located at site M1 of the crystal structure, thus being indicators of short-range order. Others arise again from medium-range order. P-omphacites show more complicated spectra than C-omphacites. Their additional features reflect the increased complexity of the structure and the greater local disorder around the probed atom induced by the two alternative M1, M11 configurations of the six O atoms forming the first coordination shells. Mg and Al are confirmed to be preferentially partitioned in the M1 and M11 site of the P-omphacite crystal structure, however with a certain degree of local disorder. The relative heights of certain prominent features are directly related to sample composition in terms of Di:Jd ratio in the Al K-edge spectra, whereas they show abrupt variations in the Mg K-edge spectra. They demonstrate that XANES is directly related to composition and may be used to distinguish C- from P-omphacites.

A high-temperature furnace with an induction heater coil has been designed and constructed for in situ X-ray spectroscopic experiments under controlled atmospheric conditions and temperatures up to 3275 K. The multi-purpose chamber design allows working in backscattering and normal fluorescence mode for synchrotron X-ray absorption and emission spectroscopy. The use of the furnace is demonstrated in a study of the in situ formation of Cr oxide between 1823 K and 2023 K at log PO 2 values between À10.0 and À11.3 using X-ray absorption near-edge spectroscopy. The set-up is of particular interest for studying liquid metals, alloys and other electrically conductive materials under extreme conditions.

The structural environment of europium (Eu) in a synthetic silicate glass of Na 2 Si 2 O 5 composition has been determined with the final purpose to carry out a detailed investigation of the role of europium in glasses by building a structural model as a function of glass composition. The sample analyzed in this study has been characterized through Eu L 3 -edge X-ray Absorption Spectroscopy (XAS), which allowed to obtain Eu oxidation state, coordination number and the b Eu-ON bond distances in the glass. From EXAFS data analysis Eu results to be bonded to six oxygens (b Eu-ON = 2.26 ± 0.01 Å) at the corners of an almost regular octahedron.

The geochemical behavior of sulfur in magmas depends strongly on the oxidation state of sulfur, but this is not easily determined by standard analytical methods. We have measured XANES absorption spectra at the sulfur K-edge and have found that such measurements are useful to characterize the oxidation state and speciation of sulfur in silicate glasses of geological relevance. Measured spectra of a set of reference minerals show the effects of different oxidation states and coordination numbers of sulfur; there is a large shift in energy (~10-12 eV) of the sulfur K-edge between S 2and S 6+ . This large and easily detectable difference makes possible the measurement of the valence of sulfur in unknown samples by measuring the shift in energy of the absorption edge. This approach is applicable to both crystalline and glassy materials, and useful results have been obtained on samples with as little as 450 ppm S. We have used XANES measurements to characterize oxidation state and speciation of sulfur in a set of natural and synthetic sulfur-bearing glasses. The samples cover a range of composition from basaltic to almost rhyolitic, and some were synthesized over a range of pressure, temperature and oxygen fugacity; glass S content varies between 450 and 3000 ppm. XANES analyses, carried out in fluorescence mode at LURE, allowed determination of the sulfur oxidation state in all of the samples and clearly show that some samples contain a mixture of S 2and S 6+ ; no other sulfur species were observed. Quantitative determination of the abundance of sulfide and sulfate shows good agreement with independent measurements based on electron-microprobe determination of the wavelength shift of sulfur K␣ X-rays.

Ion beams have been employed to get micro-PIXE measurements and ionoluminescence analyses on a natural silica glass, known as Libyan desert glass (LDG), representing a probable fusion product of desert sandstone due to the impact of an extraterrestrial body. In particular some brown streaks, which are present in a few specimens and considered as traces of the impactor, have been characterized. The presence of discrete bands with dierent luminescence intensity has been revealed by both ionoluminescence and cathodoluminescence. Several hypotheses have been proposed in order to explain the luminescence properties of this material. Finally, the usefulness of the synergic combination of ionoluminescence and micro-PIXE, to characterize LDG, has been showed. Ó

We studied a group of North-American (NA) microtektites from five DSDP sites by Fe K-edge XANES in order to obtain information on the Fe oxidation state. Fe3+/Fetot ratios vary from 0 to 45% for NA microtektites, whereas is almost 0 in NA tektites.

The aluminium Is x-ray-absorptionspectra of a series of garnets, pyrope (Mg3Al 2 SÌ3O2), almandine (Fe 3 Al2SÌ3Oi2). spessartine (Mn3Al 2 Si 3 Oj2) and grossular (Ca3Al 2 SÌ3O 1 2). are compared to full multiple-scattering calculations using cluster models. An overall good agreement between experiment and calculation, extended also to the edge region, is obtained in the energy range up to 60 eV above the threshold, provided clusters containing at least 40 atoms are used. The analysis of these garnet XAS spectra provides clear evidence on the effect of atoms located far away from the first-coordination shell around the photoabsorber. As a local probe, XANES spectroscopy at the edge of low Z elements appears to be a perfect tool to investigate the role played by atoms located in higher-coordination shells.

Libyan Desert Glass (LDG) is an enigmatic type of glass that occurs in western Egypt in the Libyan Desert. Fairly convincing evidence exists to show that it formed by impact, although the source crater is currently unknown. Some rare samples present dark-colored streaks with variable amounts of Fe, and they are supposed to contain a meteoritic component. We have studied the iron local environment in an LDG sample by means of Fe K-edge high- resolution X-ray absorption near edge structure (XANES) spectroscopy to obtain quantitative data on the Fe oxidation state and coordination number in both the Fe-poor matrix and Fe-rich layers. The pre-edge peak of the high-resolution XANES spectra of the sample studied displays small but reproducible variations between Fe-poor matrix and Fe-rich layers, which is indicative of significant changes in the Fe oxidation state and coordination number. Comparison with previously obtained data for a very low-Fe sample shows that, while iron is virtually all trivalent and in tetrahedral coordination ([4]Fe3+) in the low-Fe sample, the sample containing the Fe-rich layers display a mixture of tetra-coordinated trivalent iron ([4]Fe3+) and penta-coordinated divalent iron ([5]Fe2+), with the Fe in the Fe-rich layer being more reduced than the matrix. From these data, we conclude the following: a) the significant differences in the Fe oxidation state between LDG and tektites, together with the wide intra-sample variations in the Fe-oxidation state, confirm that LDG is an impact glass and not a tektite-like glass; b) the higher Fe content, coupled with the more reduced state of the Fe, in the Fe-rich layers suggests that some or most of the Fe in these layers may be directly derived from the meteoritic projectile and that it is not of terrestrial origin.

Samples of impactite glass, known as Libyan Desert Glass (LDG), were analyzed by NMR MAS in order to study the coordination of silicon and aluminum. In all samples, silicon was observed in a fourfold coordination. A very small amount of aluminum in fourfold, ®vefold and sixfold coordinations was also observed in the three studied samples. The occurrence of ®ve and six coordinated aluminum may be due to high pressure eects or due to the presence of Al-enriched micro-domains. Ó

available online at http://meteoritics.org 1575 Abstract-We examined the local iron environment in nine impact glasses from the Cretaceous-Tertiary (K/T) boundary section at Beloc, Haiti, which formed as the result of impact melting during the Chicxulub impact event. The samples have been analyzed by Fe K-edge high-resolution X-ray absorption near edge structure (XANES) spectroscopy to obtain data on both the Fe oxidation state and the coordination number. The pre-edge peak of our high-resolution XANES spectra display noticeable variations indicative of significant changes in the Fe oxidation state spanning a wide range from about 20 to 75 mol% trivalent Fe. All data plot along the same trend, falling between two mixing lines joining a point calculated as the mean of a group of tektites studied so far (consisting of four-and five-coordinated Fe 2+ ) to [4] Fe 3+ and [5] Fe 3+ , respectively. Thus, the XANES spectra can be interpreted as a mixture of [4] Fe 2+ , [5] Fe 2+ , [4] Fe 3+ , and [5] Fe 3+ . There is no evidence for six-fold coordinated Fe; however, its presence in small amounts cannot be excluded from XANES data alone. Our observations can be explained by two possible scenarios: either these impact glasses formed under very reducing conditions and, because of their small size, were easily oxidized in air while still molten, or they formed under a variety of different oxygen fugacities resulting in different Fe oxidation states. In the first case, the oxidation state and coordination number would imply similar formation conditions as splash-form tektites, followed by progressive oxidation.

The local structure of iron in three tektites has been studied by means of Fe K-edge extended X-ray absorption fine structure (EXAFS) and high-resolution X-ray absorption near-edge structure (XANES) spectroscopy in order to provide quantitative data on ϽFe-OϾ distance and Fe coordination number. The samples studied are a moldavite and two australasian tektites. Fe model compounds with known Fe oxidation state and coordination number were used as standards in order to extract structural information from the XANES pre-edge peak. EXAFS-derived grand mean ϽFe-OϾ distances and Fe coordination numbers for the three tektite samples are constant within the estimated error (ϽFe-O Ͼ ϭ2.00 Å Ϯ 0.02 Å, CN ϭ 4.0 Ϯ 0.4). In contrast to other data from the literature on Fe-bearing silicate glasses, the tektites spectra could not be fitted with a single Fe-O distance, but rather were fit with two independent distances (2 ϫ 1.92 Å and 2 ϫ 2.08 Å). High-resolution XANES spectra of the three tektites display a pre-edge peak whose intensity is intermediate between those of staurolite and grandidierite, thus suggesting a mean coordination number intermediate between 4 and 5. Combining the EXAFS and XANES data for Fe, we infer the mean coordination number to be close to 4.5.

Samples of impactite glass, known as Libyan Desert Glass (LDG), were analyzed by NMR MAS in order to study the coordination of silicon and aluminum. In all samples, silicon was observed in a fourfold coordination. A very small amount of aluminum in fourfold, ®vefold and sixfold coordinations was also observed in the three studied samples. The occurrence of ®ve and six coordinated aluminum may be due to high pressure eects or due to the presence of Al-enriched micro-domains. Ó

Santabarbaraite is a new amorphous ferric iron hydroxy phosphate hydrate from Valdarno, Tuscany, Italy, the type locality, and Wannon Falls, Victoria, Australia. The mineral is the result of in situ oxidation of vivianite, occurring as pseudomorphs after vivianite crystals that are up to 3 mm across in concretionary nodules at the type locality and somewhat larger at Wannon Falls. Santabarbaraite is brown to light-brown in hand specimen, but appears yellowish amber under the microscope and has a similar streak. It is translucent with a distinct vitreous to greasy lustre and is nonfluorescent. It is brittle with a distinct parting along the perfect cleavage of vivianite. The hardness was not measured but the measured density of the type material is 2.42 g/cm 3 . Optically isotropic, the mineral has a refractive index of 1.695(5). The empirical formula for the type material is (Fe 2.64 Mn 0.13 Mg 0.07 ) (PO 4 ) 2 (OH) 2.45 · 5.1H 2 O, giving a simplified formula of Fe 3+ 3 (PO 4 ) 2 (OH) 3 ·5H 2 O. Santabarbaraite from both localities was analysed by several techniques of X-ray absorption (XANES and EXAFS) and infrared spectroscopy, as well as DTA, DTG and TG. The results show that all the Fe in santabarbaraite is trivalent, associated with the presence of both H 2 O and hydroxyl. This is consistent with an oxidation series from vivianite through metavivianite to santabarbaraite, involving progressive oxidation of Fe 2+ accompanied by conversion of H 2 O ligands to OH ions. Such a process leads to a gradual collapse of the vivianite structure as hydrogen bonds are eliminated. Santabarbaraite is the end product of this process and can be thus considered the phosphorus analogue of ferrisymplesite. E. Schweizerbart'sche Verlagsbuchhandlung, D-70176 Stuttgart Kertschenit = oxidized vivianite, Min. Mag. (1907) Kertchenit = oxidized vivianite, Chem. Abstr. (1907) Oxykertschenite = oxidized vivianite, Min. Mag. (1910) Kertchenite = oxidized vivianite, English (1939) Oxykertchenite = oxidized vivianite, English (1939) Oxykerchenite = oxidized vivianite, Min. Mag. (1946) Kertschenite-= oxidized vivianite, Dana 7th (1951) Kertschenite-= oxidized vivianite, Dana 7th (1951) Kerchenite-, , = oxidized vivianite, Hey (1962) Kertschenite-= oxidized vivianite, Hey (1962) Oxykertschenite = oxidized vivianite, Hey (1962) Oxykerchenite = amorphous phosphate, Bull. Min. (1980) Oxykerchenite = amorphous phosphate, Bull. Min. (1987) Kerchenite = metavivianite, Min. Mag.(1986) Kertschenite = oxidized vivianite, metavivianite, Hey (1993) Oxykerchenite = metavivianite, Hey (1993) Kerchite = metavivianite(?), Dana 8th (1997) Kertschenite = metavivianite, Dana 8th (1997) Oxykertschenite = metavivianite, Dana 8th (1997)

The optical properties of chromium-doped sodium silicate glass synthesized under oxidizing conditions have been investigated. The method of preparation provides a large concentration of 0953-8984/8/46/011/img10 ions, but absorption and emission spectra showed that a small amount of 0953-8984/8/46/011/img11 (and 0953-8984/8/46/011/img12) ions are always present. The Dq-, B- and C-parameters of the crystal-field theory for the 0953-8984/8/46/011/img11 ion were obtained, confirming the glassy nature of the sample. 0953-8984/8/46/011/img11 luminescence was observed in the near-infrared region, even at room temperature, confirming the low value of the crystal-field Dq. Moreover, the high ultraviolet transparency of this matrix allows observation of a well resolved absorption spectrum of the 0953-8984/8/46/011/img10 ion.

Powders of R-(Al 1-x Fe x ) 2 O 3 solid solutions prepared by the calcination in air of the corresponding γ-(Al 1-xFe x ) 2 O 3 powders were studied by several techniques including X-ray diffraction, field-emission-gun scanning electron microscopy, transmission Mössbauer spectroscopy, integral low-energy electron Mössbauer spectroscopy (ILEEMS), and Fe K-edge X-ray absorption near-edge structure (XANES) measurements. The asymmetry of the characteristic Mössbauer doublet representing Fe 3+ ions substituting for Al 3+ ions in the corundum lattice of R-(Al 1-x Fe x ) 2 O 3 solid solutions was resolved and explained for the first time by using two additional subspectra, i.e., a broad second doublet characteristic of a very distorted octahedral site for Fe 3+ and a singlet attributable to R-Fe, suggesting the presence of metallic iron nanoclusters consisting of only a few number of atoms within the solid solution grains. ILEEMS studies showed that the Fe nanoclusters are evenly distributed among the surface layers and the cores of the grains. Fe K-edge XANES measurements further confirmed the occurrence of metallic iron. The proportion of Fe nanoclusters increases when the total iron content is decreased, as does the proportion of distorted octahedral site, suggesting that they are located around the iron nanoclusters. The formation of the metallic Fe nanoclusters in the R-(Al 1-x Fe x ) 2 O 3 grains is thought to be a consequence of the γ f R phase transition which implies structural rearrangement on both the cationic and anionic sublattices.