Lithological control on soil chemistry and microbial diversity

The geological diversity of Northern Ireland provides a unique opportunity to investigate the effects of differing lithologies on the geochemistry of overlying soils. This will increase our understanding of how lithology influences the microbial populations within the soil and their role in biochemical cycling. Whilst several factors contribute to the properties of developing soils, source rock is one of the major controls in determining chemistry and formation rate. The array of geological ages and formations within Northern Ireland occurs over a relatively small area. When this is combined with the glacial history of the British-Irish ice sheet during the last glacial maximum, any soils present now will all be of the same age and have undergone the same climatic conditions. The Tellus project, an intense high resolution geochemical mapping study which analysed soil samples across Northern Ireland, has been utilised to select sites of potential interest where specific elements occur in high concentrations and evidence of anthropogenic activity is limited. There are several components to this interdisciplinary study; one of which is the chemical analysis, of inorganic and organic constituents throughout the depth profile. This has been undertaken upon soils from varying lithologies (Paleogene basalts, Devonian sandstones and Carboniferous limestones). The study also includes biological assessment in the form of various DNA profiling techniques and lab based microcosm experiments using soils obtained from the field. This will enable a thorough investigation of the microbial communities present which will identify microbial diversity arising from differences in lithology through the soil profile

Variations in the compositional, textural and electrical properties of natural pyrite: a review

Variability in the chemical composition, textures and electrical properties of the major sulphide mineral pyrite may be one cause of variation in the flotation and leaching properties of different sulphide ores. This report summarises the results of a review that has been conducted to establish the range in variability of natural pyrite with regard to chemical composition, texture and electrical properties. The S/Fe ratio in pyrite is generally very close to 2, suggesting that stoichiometric variability is small in this mineral. However, minor deviations from the ideal are reported. Pyrite typically contains a host of minor and trace elements, including: Ag, As, Au, Bi, Cd, Co, Cu, Hg, Mo, Ni, Pb, Pd, Ru, Sb, Se, Sb, Sn, Te, Tl and Zn. Minor elements are often present within the mineral lattice at levels up to several percent, these include; As, Co, Ni, Sb and possibly Cu, Ag, Au and Sn. Arsenian pyrites may contain up to 10% As, and such specimens are typically rich in other minor and trace elements, particularly Au. As-rich pyrites typically appear to have formed at relatively low temperatures and often exhibit habits that suggest rapid precipitation; these As-rich pyrites may be metastable and, therefore, relatively reactive. Considerable variation has been reported in the semiconducting properties of pyrite. Natural pyrites are either n- or p-type semiconductors and reported conductivities vary by four orders of magnitude. The rates of galvanic processes in mineral pulps are likely to vary with the mineral conductivities. Typically p-type pyrites exhibit low conductivities and are As-rich and are suggested to have formed at relatively low temperature, whereas relatively highly conducting n-type pyrites are typically As-poor and suggested to have formed at high temperature. Based on comparative measurements of rest potential, iron pyrite is the most electrochemically inert of the common sulphide minerals, with a rest potential of the order of 0.6 (cf. sphalerite and galena with rest potentials of 0.46 and 0.40 V. vs. SHE, respectively). Variability in the rest potential of different samples of this mineral are generally small. However, based on the extent of pyrite reaction during a peroxide dissolution procedure, the chemical reactivity of pyrite samples may differ significantly

Gold-Silver vein mineralization at Tyndrum, Scotland

Electrum, hessite, petzite and sylvanite have been recorded from veins at Tyndrum, Scotland. Electron probe micro-analyses have also revealed two un-named Ag-Te-S phases. Fluid inclusion studies suggest that the mineralising fluids responsible for the precious metal mineralization contained ∼ 7.0 mol % CO2 and 7 wt % NaCl. TH (temperature of homogenisation) determinations were in the range 295°C to 325°C and a depth of vein formation ∼ 4 km is indicated. Mineral precipitation was probably caused by cooling and adsorption of gold onto pyrite. Δ34S values of + 1.8%o for galena from the Au + Ag + Te veins suggest a different (possibly igneous) sulphur source to that producing the Pb +Zn vein mineralization in the Tyndrum area. Although an age of ∼ 380 Ma was obtained using K-feldspar in the veins the data are not conclusive. It is argued that the Au + Ag mineralization at Tyndrum is due to hydrothermal activity related to Cu +Mo mineralization associated with the Late Caledonian granites

XPEEM valence state imaging of mineral micro-intergrowths with a spatial resolution of 100 nm

The crystal chemistry and textural relationships of minerals hold a vast amount of information relating to the formation, history and stability of natural materials. The application of soft X-ray spectroscopy to mineralogical material has revealed that 2p (L2,3 spectra provide a sensitive fingerprint of the electronic states of 3d metals. In bulk powdered samples much of the textural and microstructural information is lost, but the area-selectivity capability of X-ray Photo-Emission Electron Microscopy (XPEEM) provides the ability to obtain valence state information from mineral intergrowths with a submicron spatial resolution. Using the state-of-the-art PEEM2 facility on beamline 7.3.1.1 at the Advanced Light Source, Berkeley, USA, a range of minerals, mineral intergrowths and mineralogical textures have been studied for a broad suite of geological, planetary and environmental science materials. High-quality, multi-element valence images have been obtained showing the distribution/variation of the metal valence states across single grains or mineral intergrowths/textures at the l00 nm scale and quantitative valence state ratios can be obtained from areas of ~0.01

A sulphur isotopic investigation of the potential sulphur sources for Lower Palaeozoic-hosted vein mineralization in the English Lake District

Current models for Cu–Fe–As (Devonian), Pb–Zn (Carboniferous) and barite (Upper Carhoniferous/Permian) vein mineralization in the Lake District invoke a variety of sources for fluids and metals including the Skiddaw Group sediments, the Borrowdale Volcanic Group, the Lake District batholith, meteoric water and Carboniferous seawater. This study isotopically characterizes the sulphur reservoirs within the Lower Palaeozoic sedimentary and igneous sequences and Late Caledonian intrusives, and assesses their importance as sources of sulphur for the vein mineralization. The Ordovician Skiddaw Group is a reservoir of isotopically heavy sulphur with a range of δ34S from +11‰ to +29‰ [& x~ (1σ) = 18.53 ±6.7‰ (n = 10)]. Four analyses of Borrowdale Volcanic Group rocks yielded values of +1‰ to +15‰. Such positive values are thought to be the result of closed system bacteriogenic reduction of Ordovician seawater sulphate in the case of the Skiddaw Group, and mixing of mantle-derived magmatic sulphur with Ordovician seawater sulphate for the Borrowdale Volcanic rocks. The sulphides in the vein systems hosted by the Skiddaw Group in the Vale of Newlands formed from fluids with δ34SH2S signatures ranging from +16.7‰, to +22.5‰: partially homogenized Skiddaw Group sulphur is considered to be the source. Mineralization at Coniston, hosted by the Borrowdale Volcanic Group, shows similar sulphur isotopic characteristics, and Skiddaw Group at depth is considered to be the predominant source of sulphur with a possible minor input from the wallrock volcanic rocks at the site of deposition. Although the Lake District batholith provided a heat source for the Cu–Fe–As vein mineralization, the granite did not supply any sulphur to those systems. However, magmatic sulphur was an important constituent in mineralization associated with the Shap and Skiddaw granites. The δ34S values for barite mineralization at Force Crag, Greenside, Skiddaw and Shap ranges from +13‰ to +18‰, and Carboniferous seawater provides the most reasonable source for sulphate

An XAS Study of the Semi-Conducting Sulfides M2S3 (M = As, Sb, Bi)

XANES has been used to probe the low-lying vacant states in the Period 15 sulfide semi-conductors M2S3 (M = As, Sb, Bi). The As K- and L3-, Sb K- and L3-, and Bi L3- and L1-edges are related to the S K-edge XANES in terms of bands of mixed orbital character. In the K-edge spectra transitions from the 1s state to states of some p character can be seen in the region between 5 eV before the edge and 15 eV after it. The S spectra are alike showing the similar nature of the electronic structure of these compounds. The white line intensity decreases down the period showing the density of empty S 2p states at the Fermi level is also decreasing. The Period 15 spectra are related to theoretical band structure calculations for As2S3

Lithological control on soil chemistry, Northern Ireland

The geological diversity of Northern Ireland provides a unique opportunity to investigate the effects of differing lithologies upon the (organic) geochemistry of overlying soils. This will aid the understanding of how this influences the microbial populations within the soil and their role in biochemical cycling. Whilst several factors contribute to the properties of developing soils, source rock is one of the major controls in determining chemistry and formation rate (Chengmin et al., 2004). The array of geological ages and formations within Northern Ireland occurs over a relatively small area. When this is combined with the glacial history of the British-Irish ice sheet during the last glacial maximum which stripped away the regolith (Evans et al., 2005), any soils present now will all be of the same age and will have undergone the same climatic conditions. The Tellus project, an intense high resolution geochemical mapping study which analysed soil samples across the whole of Northern Ireland between 1994 and 2006, has been utilised to select sites of potential interest where specific elements (e.g. Ni and Co) occur in high concentrations and evidence of anthropogenic activity is absent. Analysing the organic components of soils derived from different geological sequences within Northern Ireland is just one part of this interdisciplinary study. Soil organic matter varies in response to geology, vegetation and climate and has a vital role within element cycling. However, the relationship between organics and element cycling within soils is not well known. A recent study of Irish and UK soils identified linear correlations between TOC and TSe and a more specific correlation with acid functional groups (Fellowes, J. et al., unpublished data). Soils derived from Palaeogene Basalts, Carboniferous Limestone, Devonian Sandstone and Granite as well as Ordovician Shale were selected for analysis, including bulk organic chemistry and biomarker analysis of selected soil horizons. The results of other geochemical analyses (e.g. ICP-MS, XRF/XRD), investigations of the microbial communities present using TR-FLP profiling and results of lab based microcosm experiments using soils obtained from the field, are combined to identify the different processes operating. References Chengmin, H., Zitong, G., Yurong, H., 2004. Elemental geochemistry of a soil chronosequence on basalt on northern Hainan Island, China. Chinese Journal of Geochemistry 23, 245-254. Evans, D.J.A., Clark, C.D., Mitchell, W.A., 2005. The last British Ice Sheet: A review of the evidence utilised in the compilation of the Glacial Map of Britain. Earth-Science Reviews 70, 253-312

Enigmatic x-ray magnetic circular dichroism in greigite (Fe3S4)

Greigite (Fe3S4), a widely occurring iron thiospinel, was investigated using soft X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). XAS and XMCD spectra were recorded at the Fe L2,3 edges for pure synthetic and natural greigite samples. At the Fe L3 edge, the XAS spectra reveal two main absorption peaks at 707.2 and 708.6 eV, which are interpreted to originate from greigite and an oxidized surface layer on greigite crystals. The XMCD spectra, which are dominated by a greigite signal, contain three peaks at 705.3, 706.2, and 707.7 eV, all with the same sign. The expectation is that the spectrum would have two negative peaks representing Fe2+ and Fe3+ in octahedral coordination, and a positive peak representing Fe3+ in tetrahedral coordination, as found in stoichiometric magnetite (Fe3O4). A reasonable fit of the XMCD data can be achieved without the tetrahedral Fe component, which contradicts magnetic structural information provided by neutron diffraction analysis, and uses unreasonable parameters. The conundrum between theory and experimental data may be caused by the strong covalent effect in sulfides, which causes broadening of the hybridized XMCD peaks and also indicates that multiplet calculations cannot fully predict the properties of greigite. Our results indicate covalent 3d states in greigite, which can destroy the half-metallicity that is present in magnetite. Our measurements represent the best available XAS and XMCD spectra for greigite, but further experimental and modeling information are needed to explain the observed XMCD spectra and to understand what it represents in terms of electronic and magnetic structure. This is important because greigite widely contributes to the magnetic properties of sedimentary rocks