The previous Reviews in Mineralogy volume on spectroscopic methods (Vol. 18 Spectroscopic Methods in Mineralogy and Geology , Frank C. Hawthorne, ed. 1988), contained a single chapter on X-ray absorption spectroscopy which reviewed aspects of both EXAFS (Extended X-ray Absorption Fine Structure) and XANES (X-ray Absorption Near-Edge Structure) (Brown et al. 1988, Chapter 11) However, since publication of that review there have been considerable advances in our understanding of XANES theory and applications. Hence EXAFS and XANES have been separated into their own individual chapters in the current volume. In this chapter we endeavor to bring the reader up to date with regard to current XANES theories, as well as, introducing them to the common applications of the technique in mineralogy, geochemistry and materials science. There have been several reviews of XANES (cf., Brown et al. 1988, Brown and Parks 1989, Manceau et al. 2002, Brown and Sturchio 2002, Mottana 2004, Rehr and Ankudinov 2005, de Groot 2001, 2005, and papers therein). In this chapter on XANES it is not our intention to provide a comprehensive review of all the XANES studies since 1988 but to summarize what X-ray edges are commonly investigated and what one can expect to be able to extract from the data. The reader is also advised to read the chapters in this volume on analytical transmission electron microscopy by Brydson et al. (2014, this volume) where (core level) electron energy loss (EELS) spectroscopy is discussed, and by Lee et al. (2014, this volume) on X-ray Raman spectroscopy (XRS), as these techniques provide element specific information similar to XANES.

X-ray absorption near-edge structure (XANES) spectroscopy using synchrotron radiation is a well-established technique providing information on the electronic, structural and magnetic properties of matter. In XANES, …

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X-ray absorption near-edge structure (XANES) spectroscopy

Vivianite as the main phosphate mineral in digested sewage sludge and its role for phosphate recovery

Vivianite as an important iron phosphate precipitate in sewage treatment plants.

https://digitalcommons.uri.edu/cgi/viewcontent.cgi?article=1740&context=gsofacpubs

Determination of Fe3+/ΣFe of XANES basaltic glass standards by Mössbauer spectroscopy and its application to the oxidation state of iron in MORB

Sulfur-based denitrification process has attracted increasing attentions because it does not rely on the external addition of organics and avoids the risk of COD exceeding the limit. Traditionally, limestone is commonly employed to maintain a neutral condition (SLAD process), but it may reduce the efficiency as the occupied zone by limestone cannot directly contribute to the denitrification. In this study, we propose a novel sulfur-based denitrification process by coupling with iron(II) carbonate ore (SICAD system). The ore was demonstrated to play roles as the buffer agent and additional electron donor. Moreover, the acid produced through sulfur driven denitrification was found to promote the Fe(II) leaching from the ore and likely extend the reaction zone from the surface to the liquid. As a result, more biomass was accumulated in the SICAD system compared with the controls (sulfur, iron(II) carbonate ore and SLAD systems). Owing to these synergistic effects of sulfur and iron(II) carbonate on denitrification, SICAD system showed much higher denitrification rate (up to 720.35 g·N/m3·d) and less accumulation of intermediates (NO2- and N2O) than the controls. Additionally, sulfate production in SICAD system was reduced. These findings offer great potential of SICAD system for practical use as a highly efficient postdenitrification process.

Coupled Sulfur and Iron(II) Carbonate-Driven Autotrophic Denitrification for Significantly Enhanced Nitrate Removal.

Phosphate minerals, while relatively rare, show a broad range of crystal structure types with linkages among PO 4 tetrahedra mimicking the hierarchy of polymerization of SiO 4 tetrahedra seen in silicate minerals. To augment previous Mossbauer studies of individual phosphate species and groups of species, this paper presents new Mossbauer data on 63 different phosphate samples, and integrates them with data on more than 37 phosphate species in 62 other studies from the literature. Variations in Mossbauer parameters of different sites in each mineral are then related to both the local polyhedral environment around the Fe cations and the overall structural characteristics of each species. The entire aggregated Mossbauer data set on phosphate minerals is juxtaposed against parameters obtained for spectra from the MIMOS spectrometers on Mars. This comparison demonstrates that signatures from many different phosphate or sulfate mineral species could also be contributing to Mars Mossbauer spectra. Results underscore the conclusion that unique mineral identifications are generally not possible from Mossbauer data alone, particularly for paramagnetic phases, although combining Mossbauer results with other data sets enables a greater level of confidence in constraining mineralogy. This study provides a wealth of new data on Fe-bearing phosphate minerals to bolster future analyses of Mossbauer spectra acquired on Mars.

Mössbauer parameters of iron in phosphate minerals: Implications for interpretation of martian data

Measurements of Fe 3+ /∑Fe in geological materials have been intractable because of lack of access to appropriate facilities, the time-consuming nature of most analyses, and the lack of precision and reproducibility in most techniques. Accurate use of bulk Mossbauer spectroscopy is limited by largely unconstrained recoilless fraction ( f ), which is used to convert spectral peak area ratios into valid estimates of species concentrations and is unique to different mineral groups and compositions. Use of petrographic-scale synchrotron micro-XANES has been handicapped by the lack of a consistent model to relate spectral features to Fe 3+ /∑Fe. This paper addresses these two deficiencies, focusing specifically on a set of garnet group minerals. Variable-temperature Mossbauer spectra of the Fe 2+ -bearing almandine and Fe 3+ -bearing andradite end-members are used to characterize f in garnets, allowing Fe 3+ /∑Fe to be measured accurately. Mossbauer spectra of 19 garnets with varying composition were acquired and fit, producing a set of garnet-specific standards for Fe 3+ analyses. High-resolution XANES data were then acquired from these and 15 additional previously studied samples to create a calibration suite representing a broad range of Fe 3+ and garnet composition. Several previously proposed techniques for using simple linear regression methods to predict Fe 3+ /∑Fe were evaluated, along with the multivariate analysis technique of partial least-squares regression (PLS). Results show that PLS analysis of the entire XANES spectral region yields the most accurate predictions of Fe 3+ in garnets with both robustness and generalizability. Together, these two techniques present reliable choices for bulk and microanalysis of garnet group minerals.

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Accurate determination of ferric iron in garnets by bulk Mossbauer spectroscopy and synchrotron micro-XANES

Although Fe-sulfate minerals occur only rarely on Earth as alteration products of sulfidic basalts or in hydrothermal systems, multiple lines of evidence point to the importance of Fe- (and other) sulfate minerals on the surface of Mars. One such martian data set comes from the MIMOS II Mossbauer spectrometers on the Mars Exploration Rovers, which acquired hundreds of spectra from the martian surface at two locations. Interpretation of those spectra has been limited by the lack of a comprehensive set of laboratory analog spectra of the broad range of naturally occurring sulfate minerals. Accordingly, this study reports Mossbauer data of 98 samples representing 47 different sulfate mineral species, all containing six- or higher-coordinated Fe. The resultant Mossbauer parameters are related to the local polyhedral environment around the Fe cation in each mineral to explain variations in spectral characteristics. Results show that the size of the coordination polyhedron is the best predictor of quadrupole splitting, which increases with both octahedral volume and mean bond length. Species within groups of structurally similar minerals are shown to have comparable spectral peaks that generally fall within small ranges. Although coordination polyhedron geometry is not necessarily unique to any particular mineral species or group, Mossbauer data can be used to help constrain mineral identifications from martian spectra. The number of mineral species is large, but the range of crystal structures and hyperfine parameters may be small, so that in many cases, individual minerals cannot be uniquely fingerprinted. Examples would include quenstedtite, coquimbite, kornelite, and lausenite, which have indistinguishable spectra, as do apjohnite, bilinite, dietrichite, and romerite. Overlap of Mossbauer parameters is a particular complication for identification of Fe 3+ -rich phases because the range of Mossbauer parameters for Fe 3+ in any coordination number is so small. Previous analyses of martian Mossbauer spectra reported the presence of jarosite (Klingelhofer et al. 2004; Morris et al. 2004) and an unspecific ferric sulfate (Morris et al. 2008). New data presented here indicate that botryogen, metasideronatrite, and slavikite exhibit Mossbauer spectra similar to those attributed to jarosite at Meridiani Planum. Fibroferrite and rhomboclase have parameters similar to those observed at Arad Samra, and copiapite and parabutlerite could be present at Tyrone Mount Darwin and Berkner Island. Unique mineral identifications are generally not possible from Mossbauer data alone, particularly for paramagnetic phases, although combining Mossbauer results with other data sets enables a greater level of confidence in constraining mineralogy. This study provides a new expansive data set for future interpretation of iron sulfates on Mars.

Mössbauer parameters of iron in sulfate minerals

Remote sensing observations have identified aluminate spinel, in the absence of measureable olivine and pyroxene, as a globally distributed component of the lunar crust. Earlier remote sensing observations and returned samples did not indicate the presence of this component, leaving its geologic significance unclear. Here, we report visible to mid-infrared (V-IR) reflectance (300–25 000 nm) and Mossbauer spectra of aluminate spinels, synthesized at lunar-like oxygen fugacity (ƒO2), that vary systematically in Fe abundance. Reflectance spectra of particulate ( 6 Fe#. Although the 2000 and 2800 nm bands are assigned to Fe 2+ IV electronic transitions, spectra of aluminate spinels with excess Al2O3 demonstrate that the strengths of the 1000 nm bands are related to the abundance of Fe 2+ VI. The abundance of Fe 2+ VI depends on bulk Fe content as well as factors that control the degree of structural order-disorder, such as cooling rate. Consequently the strength of the 1000 nm bands are useful for constraining the Fe content and cooling rate of remotely sensed spinel. Controlling for cooling rate, particle size, and ƒO2, we conclude that spinels with >12 Fe# (<88 Mg#) have observable 1000 nm bands under ambient lunar conditions and that only very Mg-rich spinels lack 1000 nm bands in their spectra. This links remote observations of spinel anorthosite to Mg-Suite magmatism. The combined effects of Fe oxidation state, abundance of coexisting plagioclase, and space weathering have not been explored here, and may add additional constraints. The relative strengths of the distinctive 1000 and 2000 nm bands of the spinels associated with pyroclastic deposits at Sinus Aestuum suggest fast cooling rates, possibly in the absence of an extensive vapor cloud.

Melissa Nelms

Papers

Mössbauer parameters of iron in phosphate minerals: Implications for interpretation of martian data

Accurate determination of ferric iron in garnets by bulk Mossbauer spectroscopy and synchrotron micro-XANES

Mössbauer parameters of iron in sulfate minerals

Visible-infrared spectral properties of iron-bearing aluminate spinel under lunar-like redox conditions†