Show Abstract ▼

Abstract: Sodium chlorate (NaClO3) crystals change from a cuboid to a tetrahedron of {1 ̅1 ̅1 ̅} morphology when crystallized in the presence of sodium dithionate (Na2S2O6) impurity. Polarized Extended X-Ray Absorption Fine Structure at the S K-edge, used to probe the local structure around this impurity with respect to its orientation within the bulk crystal lattice, reveals that the S-S bond of the S2O62- ions is closely aligned along the <111>/<1 ̅1 ̅1 ̅> lattice direction. High resolution diffraction studies using X-ray Multiple Diffraction reveal growth-induced anisotropy in the doped crystals associated with subtle lattice distortions in the symmetry-independent {1 ̅1 ̅1 ̅} and {111} growth sectors. The data is consistent with a mechanistic model involving creation of lattice vacancies and the substitution of one of the SO3 anionic groups of the dopant ion for a host ClO3 ion when incorporated at the {1 ̅1 ̅1 ̅} growth interface with the other SO3 group substituting for one or more anionic sites in the succeeding growth layer depending on the degree of impurity concentration within the crystallization solution. This mechanism is also fully consistent with the formation of twinning at higher impurity concentrations previously reported by Lan et.al.

Show Abstract ▼

Abstract: Marine and lacustrine archaeological waterlogged wood encounters serious problems after excavation due to the accumulation of sulfur and iron compounds during burial. Exposure of these compounds to oxygen results in precipitation of salts and acidification, which can lead to serious structural damage, and ultimately the loss of important cultural heritage. In this study, we evaluated the capacity of the bacterium Thiobacillus denitrificans to transform sulfur compounds commonly found in waterlogged wooden objects, to more readily extractable compounds thereby eliminating the threat of degradation. Oak samples, impregnated with a solution containing iron(II) and sulfides, were used to assess the efficiency of the bacterial treatment. The model wood samples were characterized before and after treatment using different techniques such as ESEM-EDS, micro-Raman spectroscopy, XRD and Sy-XRF mapping. Before treatment, mackinawite (FeS) and mineral sulfur (α-S8) were detected in the impregnated wood. After treatment with T. denitrificans, even though some mineral sulfur remained in the samples, the predominant phase corresponded to oxidized sulfur. This demonstrates that T. denitrificans was able to use the reduced sulfur compounds present in the wood samples as an energy source, thereby producing more soluble oxidized sulfur compounds. In addition, non-invasive techniques such as Fourier transform infrared (FTIR) spectroscopy, were carried out to assess the consequences of the biological treatment on the wood structure. No negative effect on the wood was detected after the treatment in comparison with the reference-impregnated wood. This study demonstrates the feasibility of a biotechnological procedure for the preventive extraction of sulfur species from archaeological waterlogged wood.

Show Abstract ▼

Abstract: The spinel group minerals, found in a range of igneous rocks, are resistant to weathering and can incorporate several multivalent elements, meaning they have the potential to provide insight into redox conditions of parental magmas. Naturally occurring spinel can contain varying quantities of Mn, an element which occurs terrestrially and extraterrestrially as Mn2+, Mn3+, Mn4+ and Mn5+. However, a lack of information on the effects of oxygen fugacity (fO2) on: (1) Mn valence state and cation distribution, and (2) on spinel-melt partitioning means that the potential for a Mn-in-spinel oxy-barometer remains largely untested. Here, we use electron probe microanalysis, micro-focus X-ray Absorption Near Edge Structure (XANES) spectroscopy and single crystal X-ray diffraction (SC-XRD) to investigate cation distribution and valence state in spinels in the Al-Mn-O and Fe-Mn-O systems synthesized at ambient pressure under varying fO2 conditions. In contrast to previous studies, we find that the spectral resolution of the Mn K edge XANES spectra is insufficient to provide quantitative data on Mn valence state and site occupancy, although it does verify that Mn is incorporated as both Mn2+ and Mn3+, distributed over tetrahedral and octahedral sites. Combination of data from XANES and SC-XRD refinements can, however, be used to model Mn, Al and Fe valence and site occupancy. It would be expected that Mn-Fe spinels have the potential to record fO2 conditions in parental melts due to changes to the octahedral site under more reducing conditions. However, decoupling the effects of temperature and oxygen fugacity on the TFe3+-TMn2+ exchange in the Mn-Fe spinels remains challenging. In contrast, little variation is noted in Mn-Al spinels as a function of fO2, implying that crystal chemistry and cation site geometry may significantly influence cation distribution, and by inference, crystal-melt partitioning, in spinel group minerals.

Show Abstract ▼

Abstract: Previous work has shown that Mo isotopes measurably fractionate between metal and silicate liquids, even at temperatures appropriate for core formation. However, the effect of variations in the structural environment of Mo in the silicate liquid, especially as a function of valence state, on Mo isotope fractionation remained poorly explored. We have investigated the role of valence state in metal-silicate experiments in a gas-controlled furnace at 1400 °C and at oxygen fugacities between 10−12.7 and 10–9.9, i.e. between three and 0.2 log units below the iron-wüstite buffer. Two sets of experiments were performed, both with a silicate liquid in the CaO-Al2O3-SiO2 system. One set used molybdenum metal wire loops as the metal source, the other liquid gold alloyed with 2.5 wt% Mo contained in silica glass tubes. X-ray absorption near-edge spectroscopy analysis indicates that Mo6+/ΣMo in the silicate glasses varies between 0.24 and 0.77 at oxygen fugacities of 10–12.0 and 10–9.9 in the wire loop experiments and between 0.15 and 0.48 at 10–11.4 and 10–9.9 in the experiments with Au-Mo alloys. Double-spiked analysis of Mo isotope compositions furthermore shows that Mo isotope fractionation between metal and silicate is a linear function of Mo6+/ΣMo in the silicate glasses, with a difference of 0.51‰ in 98Mo/95Mo between purely Mo4+-bearing and purely Mo6+-bearing silicate liquid. The former is octahedrally and the latter tetrahedrally coordinated. Our study implies that previous experimental work contained a mixture of Mo4+ and Mo6+ species in the silicate liquid. Our refined parameterisation for Mo isotope fractionation between metal and silicate can be described as Δ98/95Mometal–silicate=−1.43±0.14×106Mo6+/ΣMo+8±6×104T2 Molybdenum isotope ratios therefore have potential as a proxy to constrain the oxygen fugacity during core formation on planetary bodies if the parameterisation of Mo6+/ΣMo variation with oxygen fugacity is expanded, for instance to include iron-bearing systems. On Earth literature data indicate that the upper mantle is depleted in heavy Mo isotopes relative to the bulk Earth, as represented by chondrites. As previously highlighted, this difference is most likely not caused by core formation, which either enriches the mantle in heavy Mo isotopes or causes no significant fractionation, depending on temperature and, as we determined here, Mo6+ content. We reaffirm that core formation does not account for the Mo isotope composition of the modern upper mantle, which may instead reflect the effect of fractionation during subduction as part of global plate recycling.

Show Abstract ▼

Abstract: Trace element inventories are known to correlate with specific histological structures in bone, reflecting organismal physiology and life histories. By studying trace elements in fossilised bone, particularly in individuals with cyclic bone growth (alternating fast/slow bone deposition), we can improve our understanding of the physiology of extinct organisms. In this study we present the first direct comparison between optical histology (bone tissue identification) and synchrotron-based chemical mapping, quantification, and characterisation of trace elements (biochemistry) within cyclic growth tissues, in this case within bones of a cave hyaena (Crocuta crocuta spelaea). Results show distributions of zinc, an element strongly associated with active ossification and bone growth, correlating with (1) fast-growing tissue of zonal bone (cyclic growth) in an extinct hyaena and (2) secondary osteons (remodelling) in both extant and extinct hyaena. Concentrations and coordination chemistry of zinc within the fossil sample are comparable to those seen in extant bone suggesting that zinc is endogenous to the sample and that the chemistry of bone growth has been preserved for 40 ka. These results demonstrate that the study of trace elements as part of the histochemistry has wide utility for reconstructing growth, diet and other lifestyle factors in archaeological and fossil bone.