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Solfataric alteration at the South Sulfur Bank of the former Kilauea caldera produced opal, Mg- and Fe-rich smectites, gypsum, and jarosite through silica replacement of pyroclastic Keanakako'i ash and leaching of basaltic lavas. This site on the island of Hawaii serves as an analog for formation of several minerals found in altered deposits on Mars. Two distinct alteration environments were characterized in this study, including a light-toned, high-silica, friable outcrop adjacent to the vents and a bedded outcrop containing alternating orange/tan layers composed of smectite, gypsum, jarosite, hydrated silica, and poorly crystalline ferric oxide phases. This banded unit likely represents the deposition of pyroclastic material with variations in chemistry over time that was subsequently altered via moderate hydrothermal and pedogenic processes and leaching of basaltic caprock to enhance the Si, Al, Mg, Fe, and Ca in the altered layers. In the light-toned, friable materials closest to the vents along the base of the outcrop, glassy fragments were extensively altered to opal-A plus anatase. Lab measurements of samples returned from the field were conducted to replicate recent instruments at Mars and provide further characterization of the samples. These include elemental analyses, sample texture, XRD, SEM, VNIR/mid-IR reflectance spectroscopy, TIR emittance spectroscopy, and Mössbauer spectroscopy. Variations in the chemistry and mineralogy of these samples are consistent with alteration through hydrothermal processes as well as brines that may have formed through rain interacting with sulfuric fumes. Silica is present in all altered samples, and the friable pyroclastic ash material with the strongest alteration contains up to 80 wt% SiO2. Sulfate mineralization occurred at the South Sulfur Bank through fumarolic action from vents and likely included solfataric alteration from sulfuric gases and steam, as well as oxidation of sulfides in the basaltic caprock. Gypsum and jarosite are typically present in different layers of the altered wall, likely because they require different cations and pH regimes. The presence of both jarosite and gypsum in some samples implies high-sulfate concentrations and the availability of both Ca2+ and Fe3+ cations in a brine percolating through the altered ash. Pedogenic conditions are more consistent with the observed Mg-smectites and gypsum in the tan layers, while jarosite and nontronite likely formed under more acidic conditions in the darker orange layers. Assemblages of smectite, Ca-sulfates, and jarosite similar to the banded orange/tan unit in our study are observed on Mars at Gale crater, Noctis Labyrinthus, and Mawrth Vallis, while high-silica outcrops have been identified in parts of Gusev crater, Gale crater, and Nili Patera on Mars. [ABSTRACT FROM AUTHOR]

Some thermophilic bacteria from deep-sea hydrothermal vents grow by dissimilatory iron reduction, but our understanding of their biogenic mineral transformations is nascent. Mineral transformations catalyzed by the thermophilic iron-reducing bacterium Desulfovulcanus ferrireducens during growth at 55°C were examined using synthetic nanophase ferrihydrite, akaganeite, and lepidocrocite separately as terminal electron acceptors. Spectral analyses using visible-near infrared (VNIR), Fourier-transform infrared attenuated total reflectance (FTIR-ATR), and Mössbauer spectroscopies were complemented with x-ray diffraction (XRD) and transmission electron microscopy (TEM) using selected area electron diffraction (SAED) and energy dispersive X-ray (EDX) analyses. The most extensive biogenic mineral transformation occurred with ferrihydrite, which produced a magnetic, visibly dark mineral with spectral features matching cation-deficient magnetite. Desulfovulcanus ferrireducens also grew on akaganeite and lepidocrocite and produced non-magnetic, visibly dark minerals that were poorly soluble in the oxalate solution. Bioreduced mineral products from akaganeite and lepidocrocite reduction were almost entirely absorbed in the VNIR spectroscopy in contrast to both parent minerals and the abiotic controls. However, FTIR-ATR and Mössbauer spectra and XRD analyses of both biogenic minerals were almost identical to the parent and control minerals. The TEM of these biogenic minerals showed the presence of poorly crystalline iron nanospheres (50-200 nm in diameter) of unknown mineralogy that were likely coating the larger parent minerals and were absent from the controls. The study demonstrated that thermophilic bacteria transform different types of Fe(III) (oxyhydr)oxide minerals for growth with varying mineral products. These mineral products are likely formed through dissolution-reprecipitation reactions but are not easily predictable through chemical equilibrium reactions alone. [ABSTRACT FROM AUTHOR]

Fe(II) only occupies octahedral sites in phyllosilicates, whereas Fe(III) can occupy both octahedral and tetrahedral sites. The controls on Fe(III) distribution between tetrahedral and octahedral sites have been a matter of great interest to understand the interplay between formation environment (Fe abundance, redox conditions) and crystal-chemical factors (stability of the crystal lattice) during crystallization of Fe-phyllosilicates. Here, for the first time, we present a model of Fe(III) distribution in 2:1 phyllosilicates. We investigated 21 samples of 2:1 phyllosilicates of submarine hydrothermal origin using XRD, chemical analysis, and Mössbauer spectroscopy (and other supporting techniques not presented here). An additional data set of 49 analyses of 2:1 phyllosilicates from the literature was also used. Overall, the data cover a wide range of dioctahedral and trioctahedral phyllosilicates, including end-member minerals and interstratified phases. Dioctahedral phyllosilicates have a steric control whereby tetrahedral Fe(III) is only allowed if at least five out of six octahedral atoms are larger than Al (typically Fe[III], Fe[II], Mg) that produces an expanded structure where tetrahedral sites can accommodate Fe(III). After this threshold, further Fe(III) atoms occupy tetrahedral sites preferentially (~73% of further Fe[III] atoms) over octahedral sites. In trioctahedral 2:1 phyllosilicates there is no steric hindrance to tetrahedral Fe(III) because the crystal dimensions are such that tetrahedral sites can accommodate Fe(III). On average, Fe(III) enters tetrahedral and octahedral sites in similar proportion, and the only apparent control on tetrahedral Fe(III) abundance is Fe(III) availability during crystallization. This model allows to predict Fe(III) distribution between structural sites, provides an avenue for further exploration of the thermodynamic stability of phyllosilicates using cationic size, and provides a tool to better describe stability/reactivity of Fe-rich phyllosilicates, the most reactive of phyllosilicates and very relevant in geochemical and biological processes. [ABSTRACT FROM AUTHOR]

A synthetic hydrothermal beryl Fe-4-51, investigated previously by Taran and Rossman (Am Miner 86:973-980, 2001), was additionally studied by microprobe, Mössbauer, optical absorption, Raman and IR spectroscopy. For comparison, polarized spectra of natural blue aquamarine and Cr3+, Fe3+-bearing alexandrite, both from Brazil, are also presented. Fe-4-51 is a nearly pure Fe3+-bearing beryl, with a homogeneous composition as shown by electron microprobe. Averaging over 22 points gives a formula of Be3.07(Al1.94,Fe0.073+)Σ=2.01Si5.95O18, with Fe3+ replacing Al3+ in the octahedral site of the structure. The Mössbauer spectrum is dominated by a broad disordered pattern with beryl-suitable parameters; for Fe2+, IS = 1.21 mm/s, QS = 2.71 mm/s, area ≈ 5% and for Fe3+, IS = 0.34 mm/s, QS = 0.71 mm/s, and area ≈ 67%—are distinguished overlying a broad disordered continuum. The optical absorption spectrum is typical of octahedral Fe3+. From it, the crystal field strength Dq is derived as ~ 1520 cm−1 and the values of Racah parameters of interelectronic repulsion B and C are found to be 665 and 3415 cm−1, respectively. This rather low B value, compared with that of a free Fe3+ ion, 814 cm−1, suggests a comparatively high degree of covalency in the octahedral Fe3+-O bond. Infrared spectra show the presence of channel H2O of both I and II structural type in comparable quantities, about 0.5 and 1 mass%, respectively. Raman data show the expected five bands in the energy range from 300 to 1200 cm−1. [ABSTRACT FROM AUTHOR]

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 Mössbauer 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 Mössbauer data of 98 samples representing 47 different sulfate mineral species, all containing six- or higher-coordinated Fe. The resultant Mössbauer 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, Mössbauer 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 römerite. Overlap of Mössbauer parameters is a particular complication for identification of Fe3+-rich phases because the range of Mössbauer parameters for Fe3+ in any coordination number is so small. Previous analyses of martian Mössbauer spectra reported the presence of jarosite (Klingelhöfer 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 Mössbauer 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 Mössbauer data alone, particularly for paramagnetic phases, although combining Mössbauer 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. [ABSTRACT FROM AUTHOR]

Understanding of Fe site occupancy across the Ca-Fe-Mg pyroxene quadrilateral requires knowledge of space groups and appreciation of the diversity of site geometries across Ca-Mg-Fe composition space. Most commonly, site occupancies are measured using some combination of single-crystal structure refinements (SREF) from X-ray diffraction data and Mössbauer spectroscopy for bulk measurements. The vast majority of previous Mössbauer studies have been hampered by the lack of differential recoil-free fraction data that describe how the Fe2+ and Fe3+ cations are bonded in the M1 and M2 sites in pyroxene. To remedy this situation, this paper examines 658 Mössbauer spectra acquired from 64 synthetic samples covering the pyroxene quadrilateral in roughly 10 mol% increments, and determines their fundamental Mössbauer parameters as a function of composition. Results show variations in all the Mössbauer parameters studied: center shift (δ), quadrupole splitting (Δ), area, recoil-free fraction (f), Mössbauer temperature (ΘM), and intrinsic isomer shift (δI). The most systematic variations with composition are seen for δ and f, while small variations are seen for Δ, ΘM, and δI. These data are then related to characteristics of the pyroxene crystal structure to examine the relationship between site geometry and recoil-free fraction. In general, smaller bond lengths (e.g., in the M1 site along the enstatite-ferrosilite join) result in higher f values. As Ca is added to the structure and Mg is removed, the f value for M1 increases as the site becomes larger and more regular. Larger sites with lower bond strengths result in lower values of f because the cation is less tightly bound in the crystal structure and thus encounters more recoil. This result is in keeping with theoretical expectations, but has not previously been clearly demonstrated for minerals with experimental data. Values of recoil-free fraction determined in this study will facilitate more accurate determinations of cation site occupancies in pyroxenes from Mössbauer data and lend insights into the geometries of the M1 and M2 sites [ABSTRACT FROM AUTHOR]

Measurements of Fe3+/ΣFe 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 Mössbauer 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 Fe3+/ΣFe. This paper addresses these two deficiencies, focusing specifically on a set of garnet group minerals. Variable-temperature Mössbauer spectra of the Fe2+-bearing almandine and Fe3+-bearing andradite end-members are used to characterize f in garnets, allowing Fe3+/ΣFe to be measured accurately. Mössbauer spectra of 19 garnets with varying composition were acquired and fit, producing a set of garnet-specific standards for Fe3+ 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 Fe3+ and garnet composition. Several previously proposed techniques for using simple linear regression methods to predict Fe3+/ΣFe 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 Fe3+ in garnets with both robustness and generalizability. Together, these two techniques present reliable choices for bulk and microanalysis of garnet group minerals. [ABSTRACT FROM AUTHOR]

Fe2+- and Mn2+-rich tourmalines were used to test whether Fe2+ and Mn2+ substitute on the Z site of tour- maline to a detectable degree. Fe-rich tourmaline from a pegmatite from Lower Austria was characterized by crystal-structure refinement, chemical analyses, and Mössbauer and optical spectroscopy. The sample has large amounts of Fe2+ (~2.3 apfu), and substantial amounts of Fe3+ (~1.0 apfu). On basis of the collected data, the structural refinement and the spectroscopic data, an initial formula was determined by assigning the entire amount of Fe3+ (no delocalized electrons) and Ti4+ to the Z site and the amount of Fe2+ and Fe3+ from delocalized electrons to the Y-Z ED doublet (delocalized electrons between Y-Z and Y-Y): Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed with a = 16.039(1) and c = 7.254(1) Å. This formula is consistent with lack of Fe2+ at the Z site, apart from that occupancy connected with delocalization of a hopping electron. The formula was further modified by considering two ED doublets to yield: Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. This formula requires some Fe2+ (~0.3 apfu) at the Z site, apart from that connected with delocalization of a hopping electron. Optical spectra were recorded from this sample as well as from two other Fe2+-rich tourmalines to determine if there is any evidence for Fe2+ at Y and Z sites. If Fe2+ were to occupy two different 6-coordinated sites in significant amounts and if these polyhedra have different geometries or metal-oxygen distances, bands from each site should be observed. However, even in high-quality spectra we see no evidence for such a doubling of the bands. We conclude that there is no ultimate proof for Fe2+ at the Z site, apart from that occupancy connected with delocalization of hopping electrons involving Fe cations at the Y and Z sites. A very Mn-rich tourmaline from a pegmatite on Elba Island, Italy, was characterized by crystal-structure determination, chemical analyses, and optical spectroscopy. The optimized structural formula is Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed, with a = 15.951(2) and c = 7.138(1) Å. Within a 3σ error there is no evidence for Mn occupancy at the Z site by refinement of Al ↔ Mn, and, thus, no final proof for Mn2+ at the Z site, either. Oxidation of these tourmalines at 700-750 °C and 1 bar for 10-72 h converted Fe2+ to Fe3+ and Mn2+ to Mn3+ with concomitant exchange with Al of the Z site. The refined ZFe content in the Fe-rich tourmaline increased by ~40% relative to its initial occupancy. The refined YFe content was smaller and the distance was significantly reduced relative to the unoxidized sample. A similar effect was observed for the oxidized Mn2+-rich tourmaline. Simultaneously, H and F were expelled from both samples as indicated by structural refinements, and H expulsion was indicated by infrared spectroscopy. The final species after oxidizing the Fe2+-rich tourmaline is buergerite. Its color had changed from blackish to brown-red. After oxidizing the Mn2+-rich tourmaline, the previously dark yellow sample was very dark brown-red, as expected for the oxidation of Mn2+ to Mn3+. The unit-cell parameter a decreased during oxidation whereas the c parameter showed a slight increase. [ABSTRACT FROM AUTHOR]

Microprobe analyses of antigorite show that (Al+Cr) and inferred Fe3+ correlate inversely with Si apfu in a Tschermaks substitution. This observation suggests that the uptake of Fe3+ is not simply related to fO2. For Si = 1.95 apfu estimated Fe3+ = 0.032 apfu (or 0.95 wt% Fe2O3). Such estimates of Fe3+ require high analytical accuracy and precision, and assume a fixed polysomatic formula (e.g., m = 17) and freedom from interlayer sheet-silicate impurities. In many cases the estimates appear to be high. An alternative measure of Fe3+ is provided by the partitioning of total Fe and Mg between antigorite and olivine in well-equilibrated natural antigorite-olivine-magnetite parageneses. Extrapolation of Nernst and Roozeboom partition plots to Fe-free olivine permits an estimate of the Fe3+ content of the average antigorite in this paragenesis, namely 0.42 or 0.64 wt% Fe2O3). The partition estimates are in good agreement with the results of Mössbauer spectroscopy performed here on 14 antigorites from metaperidotites, together with four from the literature. These spectra reveal a range of 0.16 to 1.94 in wt% Fe2O3) in metaperidotite antigorite, with an average of 0.83. In two olivine-bearing rocks, antigorite has Fe3+/∑Fe ratios of 0.13 and 0.15, which corresponds to wt% Fe2O3) = 0.47 and 0.54, respectively. Larger amounts of Fe2O3) occur in some, but not all, vein antigorites. The prograde formation of antigorite in serpentinite from lizardite is accompanied by loss of some cronstedtite component and the precipitation of additional magnetite. The Roozeboom Mg/Fe partition plot is concave down rather than up; in other words the partition coefficient KD is a function of the XMg of olivine. This behavior has been found in other olivine- mineral pairs. It can be interpreted to reflect strongly non-ideal solution behavior of MgFe- olivine at low temperatures, viz. WG ≈ 8.5 kJ assuming a symmetrical solution. MgFe-brucite appears to be similarly non-ideal. [ABSTRACT FROM AUTHOR]

A broad (FWHM ≈ 7300 cm-1) intense band at ∼21 700 cm-1 in the optical absorption spectra of natural Fe2+, Fe3+-rich garnets is attributed to electronic intervalence charge-transfer transitions (IVCT), VIIIFe2+ + VIFe3+ → VIIIFe3+ + VIFe2+. In Fe3+, Fe2+-bearing garnets of predominantly almandine compositions, this band causes yellowish tinges in addition to the pink color, typical of pure Fe3+-free almandines. In garnets from deeper-seated mafic granulites from kimberlite pipes in Siberia with high skiagite (Fe32+Fe23+Si3O12) contents, IVCT causes intense brownish-yellow colors. The relatively high energy of the band (∼21 700 cm-1) compared to diverse minerals showing IVCT between Fe2+ and Fe3+ in adjacent octahedral sites, is attributed to the charge-transfer transition taking place between Fe2+ and Fe3+ in non-equivalent, dodecahedral and octahedral sites of the garnet structure. Band intensity is directly correlated with the product of Fe2+ and Fe3+ as measured by Mössbauer spectroscopy. The energy of the IVCT band is nearly independent of temperature, whereas its intensity decreases slightly with increasing temperature. Pressure induces a weak shift of the band to lower energies, ΔV/ΔP ≈ -75 cm-1/GPa, but intensity of the bands remains practically unchanged. Such temperature and pressure dependencies are quite different from those in other minerals showing IVCT between Fe2+ and Fe3+ in equivalent octahedral positions of structure. [ABSTRACT FROM AUTHOR]

Natural Al-saturated chlorite having a wide range of Mg:Fe ratios [Mg/(Mg + Fe) = 0.357 to 0.943] was studied to determine the effect of Mg-Fe substitution on the unit-cell parameters. With a nearly constant Al content [AlT/(AlT + Mg + Fe) = 0.362 to 0.416, where AlT represents total Al contents in both tetrahedral and octahedral sites], the only major variable affecting the unit-cell dimensions of natural chlorite is the Mg:Fe ratio. The value of about 0.4 for the Al content represents apparent Al saturation for chlorites in metamorphosed rocks. Unit-cell parameters were determined by least-squares refinement from 39 chlorite samples obtained over a large range in metamorphic grades, from X-ray data derived from Gandolfi pattern simulations (via single crystals and a single-crystal diffractometer) from this study and from traditional powder diffractometer data from the literature. Nine of the samples were analyzed for their Fe3+ contents (= 0.147 to 0.304 atoms) by Mössbauer spectroscopy; chemical compositions were generally obtained by electron-microprobe analysis. Statistical tests show that the change of the crystallographic a and b axes are linear over the range of Mg:Fe ratios studied, whereas the crystallographic c axis and the β angle have no significant relationship to these Mg:Fe ratios. Equations relating the effect of Mg:Fe ratio on a and b are: a (Å) = -0.092 x + 5.408; x = Mg/(Mg + Fe) and b (Å) = - 0.162 x + 9.370, respectively. Electrostatic models involving Pauling's electrostatic valency principle for Al-saturated chlorite were determined assuming: (1) Al-Al avoidance for Al-substituted tetrahedra, (2) no vacancies, and (3) no Al3+ substitution in the M1 sites. Electrostatic restrictions occur for R3+ → R2+ substitutions in the M2 sites of the 2:1 layer. These restrictions produce an Al saturation in chlorite at AlT/(AlT + Mg + Fe) ratios of near 0.4. This saturation limit requires that 37.5% of the tetrahedral sites (1.5 out of 4 sites) are occupied by Al, and the electrostatic charge (electrostatic valency units, e.v.u.) close to +1 e.v.u. is favored for the interlayer to offset the net negative charge on the 2:1 layer. [ABSTRACT FROM AUTHOR]

Room-temperature 57Fe Mössbauer spectra of synthetic orthopyroxenes equilibrated between 900-1000 °C and spanning the compositional range from ferrosilite to nearly enstatite were acquired. Peak areas corresponding to Fe2+ in the M1 and M2 sites of the structure were corrected for effects of thickness and differential recoil-free fractions (f) on the two sites using a constant value of C = 1.1, the correction parameter. The distribution coefficient, KD, for Fe and Mg in M1 and M2 was calculated for all samples, and results were compared to KD values from previous studies where C was assumed to be unity. Our results demonstrate that choice of C dramatically influences calculated KD values. Thus, the failure of previous studies to consider C corrections may well be responsible for differences between Mössbauer and XRD studies of site occupancies noted by previous workers. Further characterizations of f for pyroxenes of variable compositions are needed. [ABSTRACT FROM AUTHOR]

The field of Mössbauer spectroscopy (MS) has recently enjoyed renewed visibility in the diverse geoscience communities as a result of the inclusion of Mössbauer spectrometers on the Mars Exploration Rovers. Furthermore, new improvements in technology have made possible studies involving very small samples (1-5 mg or less) and samples with very low Fe contents (such as feldspars), in addition to samples measured in situ in thin sections. Because of these advances, use of Mössbauer spectroscopy in Earth science applications is expected to continue to grow, providing information on site occupancies; valence states; magnetic properties; and size distributions of (largely) Fe-bearing geological materials, including minerals, glasses, and rocks. Thus, it is timely to review here the underlying physics behind the technique, with a focus on the study of geological samples. With this background, recent advances in the field, including (a) changes in instrumentation that have allowed analysis of very small samples and of surface properties, (b) new models for fitting and interpreting spectra, and (c) new calculations of recoil-free fraction, are discussed. These results have made possible increasingly sophisticated studies of minerals, which are summarized here and organized by major mineral groups. They are also facilitating processing and interpretation of data from Mars. [ABSTRACT FROM AUTHOR]

Beginning in August 2004, raw data acquired by the Mössbauer spectrometers on the Mars Exploration Rovers (MERs) have been released to the science community as Experimental Data Records (EDRs) for each Martian day (sol) on which measurements were made. To provide convenient direct access to the EDR data, to enable independent assessment and analysis, and to allow confirmation of MER-team scientific conclusions, a new Windows-based computer program, MERView, has been developed. Direct inspection of the binary structure of an EDR file, conversion of the binary data to decimal column format for display or export, and full-screen graphical displays are included, allowing, in a highly transparent and user-friendly manner, immediate and thorough overview of the entire EDR data set. MERView also includes automated procedures for velocity calibration. Correction for non-linearity is done using the EDR-provided drive error signal, a phase shift, and the requirement that the two halves of a reference spectrum must overlap exactly when plotted on a correct velocity scale. Absolute velocity calibration (mm/s) is then accomplished by comparing each reference spectrum with known peak positions of standard reference materials. [ABSTRACT FROM AUTHOR]

Oxygen fugacity (fo2) is an important parameter because it influences the crystallization sequences of magmas, as well as the composition of the resultant minerals. Traditionally fo2 has been estimated using coexisting Fe-Ti oxide compositions or Fe3+/Fe2+ ratios in silicate melts, for which the relationship between fo2 and Fe3+ has been calibrated. Problems arise in systems where oxides and/or glasses (melts) are absent, or when oxides are suspected to have been re-equilibrated. Therefore, it is important to develop alternate ways to look at fo2 in volcanic rocks. In this work, we describe a set of experiments to establish the relationship between the Fe3+ content of clinopyroxene and a basaltic melt as a function of oxygen fugacity. The starting bulk compositions were chosen to be representative of martian parental melts, with pyroxene (either augite or pigeonite) on their liquidi, in order for the results to be applicable to the martian meteorite magma source regions. Experimental conditions ranged from fo2 = IW to ∼IW+8.6 at 1165 °C for pigeonite/melt pairs and at 1235 °C for melts alone, and fo2 = IW-1 to IW+4 at ∼1190 °C for augite/melt pairs. Results show that Fe3+ varies from 0-65% of the total Fe over this range of experimental conditions, and mass D (wt%FePyx3+/wt%Femelt3+) ranged from 0.75-1.44 and from 0.00-0.77, for pigeonite and augite respectively. XANES and Mössbauer analytical methods give similar results within known error bars of the two techniques, with the best agreement in compositions with >10% Fe3+. Our data show that only at high values of fo2 (>IW + 3.5, or ≈QFM) can Fe3+ distribution in pyroxene be used to estimate... [ABSTRACT FROM AUTHOR]

Provides information on a study which examined the amount and distribution of iron, Fe2+ and Fe3+, in five samples of fayalite previously studied by Mossbauer spectroscopy. Review of related literature; Experimental methods; Results and discussion.

The acidic sulfate-rich waters of the Meridiani Planum region were potentially a habitable environment for iron-oxidizing bacteria on ancient Mars. If life existed in this ancient martian environment, jarosite minerals precipitating in these waters may record evidence of this biological activity. Since the Meridiani jarosite is thermodynamically stable at the martian surface, any biosignatures preserved in the jarosites may be readily available for analysis in the current surface sediments during the ongoing robotic exploration of Mars. However, thermal decomposition experiments indicate that organic compound detection of sediments containing jarosite may be challenging when using pyrolysis experiments; the instrument commonly used to assess organic matter in martian samples. So, here, we assess if the biogenicity of the Meridiani-type jarosites can be determined using complimentary spectroscopic techniques also utilized during the robotic exploration of Mars, including the upcoming ExoMars2020 rover mission. An abiotic jarosite, synthesized following established protocols, and a biological jarosite counterpart, derived from a microbial enrichment culture of Rio Tinto river sediments, were used to compare four spectroscopy techniques employed in the robotic exploration of Mars (Raman spectroscopy, mid-infrared (IR) spectroscopy, visible near-infrared reflectance (VNIR) spectroscopy and Mössbauer spectroscopy) to determine if the complimentary information obtained using these instruments can help elucidate the biological influence of Meridiani-type jarosites. Raman spectral differences might be due to the presence of unreacted reagents in the synthetic spectra and not biological contributions. Reflectance (IR/VNIR) spectra might exhibit minor organic absorption contributions, but are observed in both sample spectra, and do not represent a biosignature. Mössbauer spectra show minor differences in fit parameters that are related to crystal morphology and are unrelated to the biological (i.e., organic) component of the system. Results of this study suggest that the identification of biosignatures in Meridiani-type jarosites using the in situ robotic exploration on Mars may be possible but will be challenging. Our work provides additional insight into extraterrestrial biosignature detection and data interpretation for Mars exploration and indicates that sample return missions are likely required to unequivocally resolve the possible biogenicity of the Meridiani sediments or other jarosite-containing sediments. [ABSTRACT FROM AUTHOR]

A suite of basaltic glasses were examined to determine how subtle compositional changes affect mid-infrared spectra (650 to 5400 cm-1). Glasses with different SiO2, FeOtotal, Fe3+/Fe2+, and alkali contents were synthesized in a gas-mixing furnace and analyzed using electron probe microanalysis, Mössbauer spectroscopy, and micro-reflectance Fourier transform infrared spectroscopy. The major mid-infrared spectral feature in silicate glasses is a broad peak located at ~900 to 1100 cm-1 arising from Si-(Al-)O asymmetric stretching vibrational modes. To accurately compare spectra of different glass compositions, we have applied the Kramers-Kronig (KK) transform to our spectra and examined the resulting absorption peak (KK abs. peak). The location of the KK abs. peak shifts to higher wavenumbers as SiO2 content increases (1031-1054 cm-1 with SiO2 from 47.18 to 55.57 wt%). For basaltic glasses with near-constant Al/(Al+Si), the full-width half maximum of the KK abs. peak decreases as alkali content increases (235-188 cm-1 with Na2O+K2O contents from 0.07 to 3.74 wt%). In contrast, the location and shape of the KK abs. peak are not affected by variations in total FeO (6.06-16.30 wt%) and Fe3+/Fe2+ (0.05-1.17). Our results show that KK transformed mid-infrared spectra of basaltic glasses may be used to determine the SiO2 contents in basaltic glasses, irrespective of FeOtotal and Fe3+/Fe2+, and the alkali contents if Al/(Al+Si) is known. These observations will aid in the interpretation of laboratory and remotely sensed IR spectra. [ABSTRACT FROM AUTHOR]

Mineral transformations by two hyperthermophilic Fe(III)-reducing crenarchaea, Pyrodictium delaneyi and Pyrobaculum islandicum, were examined using synthetic nanophase ferrihydrite, lepidocrocite, and akaganeite separately as terminal electron acceptors and compared with abiotic mineral transformations under similar conditions. Spectral analyses using visible–near-infrared, Fourier-transform infrared attenuated total reflectance (FTIR-ATR), Raman, and Mössbauer spectroscopies were complementary and revealed formation of various biomineral assemblages distinguishable from abiotic phases. The most extensive biogenic mineral transformation occurred with ferrihydrite, which formed primarily magnetite with spectral features similar to biomagnetite relative to a synthetic magnetite standard. The FTIR-ATR spectra of ferrihydrite bioreduced by P. delaneyi also showed possible cell-associated organics such as exopolysaccharides. Such combined detections of biomineral assemblages and organics might serve as biomarkers for hyperthermophilic Fe(III) reduction. With lepidocrocite, P. delaneyi produced primarily a ferrous carbonate phase reminiscent of siderite, and with akaganeite, magnetite and a ferrous phosphate phase similar to vivianite were formed. P. islandicum showed minor biogenic production of a ferrous phosphate similar to vivianite when grown on lepidocrocite, and a mixed valent phosphate or sulfate mineral when grown on akaganeite. These results expand the range of biogenic mineral transformations at high temperatures and identify spacecraft-relevant spectroscopies suitable for discriminating mineral biogenicity. [ABSTRACT FROM AUTHOR]

Abstract: Reactivity of a red sandstone under conditions relevant to geosequestration was studied experimentally. Crushed Moenkopi sandstone from the lower Triassic was exposed to supercritical CO2 containing commingled aqueous sulfite or sulfide solutions as proxies for SO2 or H2S co-injected along with CO2. Flow-through experiments simulating conditions near the point of injection and batch experiments simulating conditions at the water-side of the edge of the CO2 plume were conducted. Mössbauer spectroscopy, X-ray diffraction, and Fourier transform infra-red spectroscopy were used to determine changes in iron mineralogy in both types of experiments. However, Mössbauer spectroscopy, which solely derives its signal from the iron content in the sandstone, provides the most information on the changes in the iron mineralogy. The Moenkopi sandstone showed no change in iron mineralogy in any of the flow-through experiments. Pyrite was formed in batch experiments with supercritical CO2 and aqueous sulfide solutions, while siderite formed in batch experiments with super critical CO2 and an aqueous solution containing both sulfide and sulfite. The lack of reactivity in the flow-through experiments is probably due to the preferential wetting of mineral surfaces by the supercritical CO2 phase. In the batch experiments, the mineral surfaces are submerged in the aqueous phase throughout the experiment, which allows dissolved sulfur species to react with the minerals. [Copyright &y& Elsevier]