Na–H3O jarosite was synthesized hydrothermally at 413 K for 8 days and investigated using single-crystal X-ray diffraction (XRD) and electron microprobe analysis (EMPA). The chemical composition of the studied crystal is [Na0.57(3) (H3O)0.36 (H2O)0.07]A Fe2.93(3) (SO4)2 (OH)5.70 (H2O)0.30, and Fe deficiency was confirmed by both EMPA and XRD analysis. The single-crystal XRD data were collected at 298 and 102 K, and crystal structures were refined in space group $$ R\overline{3}m$$ . The room-temperature data match structural trends of the jarosite group, which vary linearly with the c axis. The low-temperature structure at 102 K shows an anisotropic decrease in the unit cell parameters, with c and a decreasing by 0.45 and 0.03 %, respectively. Structural changes are mainly confined to the A site environment. Only minor changes occur in FeO6 and SO4 polyhedra. The structure responds upon cooling by increasing bond length distortion and by decreasing quadratic elongation of the large AO12 polyhedra. The structural parameters at low temperature follow very similar patterns to structural changes that correspond to compositional variation in the jarosite group, which is characterised by the flexibility of AO12 polyhedra and rigidity of Fe(OH)4O2–SO4 layers. The most flexible areas in the jarosite structure are localized at AO12 edges that are not shared with neighbouring FeO6 octahedra. Importantly, for the application of XRD in planetary settings, the temperature-related changes in jarosite can mimic compositional change.
A number of marine sequences across the K/Pg boundary have been identified that offer reasonably continuous records and relatively high sedimentation rates, most notably those near Tethyan continental margins. However, few Eastern Tethys K/Pg localities have been studied compared to the well-known North African and Southern European sites. Here we present a high-resolution stable carbon isotope and palynological record of a 2m thick section across the K/Pg boundary from the eastern Tethys at Sumbar in Turkmenistan (38°28’N, 56°14’E). The stratigraphy and inorganic geochemistry of the section used in this study, SM-4, has been described in detail by [1].
Introduction: There is an established relationship between organic matter content and aqueous alteration processes [1,2]. However, the relationship between meteoritic organic matter and individual aqueously generated mineral matrix phases is poorly understood. Meteoritic organic matter is primarily composed of C, H and N and therefore their bulk abundances in chondrites are strongly controlled by the organic matter content. Mossbauer Spectroscopy can characterise the ferric iron bearing matrix minerals associated with aqueous alteration, such as Febearing clays and magnetite. A combination of these two parameters may indicate the presence of any organic-mineral interactions. Experimental: 22 whole-rock samples representing types CI, CM, CR, CO, CV and CK carbonaceous chondrites, in powder or chip form were analysed. C, N and H concentrations were determined using an Elemental Analyser-Isotope Ratio Mass Spectrometer (Europa ANCA-SL). Sample size varied between 2.0919.92 mg. Fe Mossbauer spectra were recorded at 298K with a microprocessor-controlled spectrometer using a 57Co/Rh source. Sample size varied between 0.2-0.3mg. Results and discussion: Combining Mossbauer and elemental abundance data indicates that there is a general correlation between C, H and N abundances and established extent of pre-terrestrial aqueous alteration as quantified by total oxidation of Fe-bearing mineral phases [3]. This has been supported by Spearmans Correlation Coefficients (rs). The data also indicates there are strong relationships between C, H and N and the paramagnetic iron within hydrothermally produced clay minerals (Fig. 1A). Surprisingly, no correlation is seen with magnetite alone (Fig. 1B). Within terrestrial systems, it has been suggested that clay surfaces may adsorb labile organic compounds allowing them to accumulate, condense and polymerise [4,5] However, the amount of organic matter associated with clays is greater than that which can be accommodated through adsorption processes. It has been proposed that the trapping of organic matter within the inter-layer spacings of clays [6] combined with adsorption may offer an improved mechanism. This sorptive protection mechanism has been proposed as a means for the sequestration and protection of organic material. Within chondrites, this is supported by the observation that the majority of carbonaceous material is located within clay rich, aqueously altered rims around anhydrous precursors [2]. Furthermore, it has been noted that significant amounts of organic molecules are liberated from solvent extracted carbonaceous chondrites following demineralisation procedures [7]. Conclusion: Elemental abundances indicative of organic matter correlate with the abundance of hydrothermally produced clay minerals but not magnetite. This association may be due to the adsorption and trapping of the organic molecules between clay interlayer spacings. These interactions may explain the preservation of macromolecular organic material within carbonaceous chondrites. References: [1] Cronin J.R. and Chang S. (1993) in The Chemistry of Lifes Origins (Ed. Greenburg J.M.) 209-258. [2] Bunch T.E. and Chang S. (1980) GCA, 44, 1543-1577. [3] Bland P. et al. (2001) EPSL, submitted. [4] Collins M.J. et al. (1995) GCA, 59, 2387-2391. [5] Mayer L.M. (1994) GCA, 58, 12711284. [6] Salmon V. et al. (2000) Org. Geochem. 31, 463-474. [7] Becker R.H. and Epstein S. (1982) GCA, 46, 97-103.
Organic materials isolated from carbonaceous meteorites provide us with a record of pre-biotic chemistry in the early Solar System. Molecular, isotopic and in situ studies of these materials suggest that a number of extraterrestrial environments have contributed to the inventory of organic matter in the early Solar System including interstellar space, the Solar nebula and meteorite parent bodies. There are several difficulties that have to be overcome in the study of the organic constituents of meteorites. Contamination by terrestrial biogenic organic matter is an ever-present concern and a wide variety of contaminant molecules have been isolated and identified including essential plant oils, derived from either biological sources or common cleaning products, and aliphatic hydrocarbons, most probably derived from petroleum-derived pollutants. Only 25% of the organic matter in carbonaceous chondrites is amenable to extraction with organic solvents; the remainder is present as a complex macromolecular aromatic network that has required the development of analytical approaches that can yield structural and isotopic information on this highly complex material. Stable isotopic studies have been of paramount importance in understanding the origins of meteoritic organic matter and have provided evidence for the incorporation of interstellar molecules within meteoritic material. Extending isotopic studies to the molecular level is yielding new insights into both the sources of meteoritic organic matter and the processes that have modified it. Organic matter in meteorites is intimately associated with silicate minerals and the in situ examination of the relationships between organic and inorganic components is crucial to our understanding of the role of asteroidal processes in the modification of organic matter and, in particular, the role of water as both a solvent and a reactant on meteorite parent bodies.
Excessive acid rainfall associated with emplacement of the Siberian Traps magmatic province is increasingly accepted as a major contributing factor to the end-Permian biosphere crisis. However, direct proxy evidence of terrestrial acidification is so far not available. In this paper, we seek to determine the probability that relative proportions of extractable monophenolic components from soil-derived organic matter in marine sediments provide a molecular proxy for estimating soil acidity. Intermittently low and high ratios of vanillic acid to vanillin detected in latest Permian and earliest Triassic deposits of the southern Alps, Italy, support concepts of pulses of severe acidification (pH <4) during the main phase of the biosphere crisis.
Extraterrestrial organic matter has been widely studied; however, its response to pressure has not. Primitive organic matter bearing meteorites, such as CI and CM carbonaceous chondrites, have experienced variable pressures, up to 10 GPa. To appreciate the effects of these pressures on the organic content of these bodies, the model compounds isophthalic acid, vanillin, and vanillic acid were subjected to pressures of up to 11.5 GPa and subsequently decompressed. High-resolution synchrotron source Fourier transform infrared spectroscopy was used to determine the effects of different benzene substituents at high pressure on both the vibrational assignments of the benzene core of the molecules and the ability of the aromatic compounds to form intermolecular hydrogen bonds. The presence of additional peaks at high pressure was found to coincide with molecules that contain carboxyl groups; these features are interpreted as C–H···O intermolecular hydrogen bonds. The formation of these hydrogen bonds has implications for the origination of macromolecular organic matter (MOM), owing to the importance of such attractive forces during episodes of cross-linking, such as esterification. Pressure-induced hydrogen-bond formation is a process by which aromatic MOM precursors could have cross-linked to generate the organic polymers found within extraterrestrial bodies today.
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