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Microanalysis of Fe3+/ΣFe in geological samples using synchrotron-based X-ray absorption spectroscopy has become routine since the introduction of standards and model compounds. Existing calibrations commonly use least-squares linear combinations of pre-edge data from standard reference spectra with known coordination number and valence state acquired on powdered samples to avoid preferred orientation. However, application of these methods to single mineral grains is appropriate only for isometric minerals and limits their application to analysis of in situ grains in thin sections. In this work, a calibration suite developed by acquiring X-ray absorption near-edge spectroscopy (XANES) data from amphibole single crystals with the beam polarized along the major optical directions (X, Y, and Z) is employed. Seven different methods for predicting %Fe3+were employed based on (1) area-normalized pre-edge peak centroid, (2) the energy of the main absorption edge at the location where the normalized edge intensity has the highest R2correlation with Fe3+/ΣFe, (3) the ratio of spectral intensities at two energies determined by highest R2correlation with Fe3+/ΣFe, (4) use of the slope (first derivative) at every channel to select the best predictor channel, (5 and 6) partial least-squares models with variable and constant numbers of components, and (7) least absolute shrinkage and selection operator models. The latter three sophisticated multivariate analysis techniques for predicting Fe3+/ΣFe show significant improvements in accuracy over the former four types of univariate models. Fe3+/ΣFe can be measured in randomly oriented amphibole single crystals with an accuracy of ±5.5–6.2% absolute. Multivariate approaches demonstrate that for amphiboles main edge and EXAFS regions contain important features for predicting valence state. This suggests that in this mineral group, local structural changes accommodating site occupancy by Fe3+vs. Fe2+have a pronounced (and diagnostic) effect on the XAS spectra that can be reliably used to precisely constrain Fe3+/ΣFe.

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.

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 fO₂has been estimated using coexisting Fe-Ti oxide compositions or Fe3+/Fe2+ratios in silicate melts, for which the relationship between fO₂and 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 fO₂in 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%Fe3+Pyx/wt%Fe3+ melt) 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 crystallization oxygen fugacity.

A new method has been developed to orient 50-100 μm size single crystals to obtain XANES or EXAFS spectra along any crystallographic orientation relative to a polarized synchrotron X-ray beam. The method uses a modified spindle stage and an X-ray goniometer head to hold a crystal and the glass fiber on which it is mounted. The orientation of the crystallographic axes within the crystal can be determined by X-ray diffraction in all cases or with the use of the polarizing light microscope if the minerals are biaxial. Once an optical orientation direction has been located, it can be oriented parallel to the polarization direction of the synchrotron source for spectral acquisition.

A new method has been developed to orient 50-100 μm size single crystals to obtain XANES or EXAFS spectra along any crystallographic orientation relative to a polarized synchrotron X-ray beam. The method uses a modified spindle stage and an X-ray goniometer head to hold a crystal and the glass fiber on which it is mounted. The orientation of the crystallographic axes within the crystal can be determined by X-ray diffraction in all cases or with the use of the polarizing light microscope if the minerals are biaxial. Once an optical orientation direction has been located, it can be oriented parallel to the polarization direction of the synchrotron source for spectral acquisition.

Synchrotron micro-XANES (X-ray absorption near-edge structure) spectroscopy (SmX) was used to measure Fe3+and Fe2+distribution among minerals in standard thin sections with an X-ray beam size of 10 ¥ 15 mm. Measurements were made at beamline X26a, National Synchrotron Light Source, Brookhaven National Lab. Samples studied included metapelites from garnet to upper sillimanite zone rocks that coexist with graphite, graphite/ilmenite, or varying combinations of ilmenite, magnetite, and hematite. SmX results are compared with Mössbauer spectroscopic measurements of Fe3+on mineral separates from the same rocks. Results show excellent agreement (within ±5-10%) between Mössbauer and SmX for the Fe-rich phases biotite, chlorite, staurolite, and garnet. Mössbauer spectra of muscovite typically show lower values than SmX for Fe3+/SFe (especially at low grades), suggesting contamination of the muscovite separates by fine-grained chlorite. However, heterogeneity of Fe3+and Fe2+is probably the chief source of discrepancy between the bulk and micro-scale measurements. The %Fe3+(relative to total Fe) in these samples ranges from a high of 90% in muscovite to a low of 0-2% in garnet, with Ms > St = Bt = Chl > Tur > Grt in graphite/ilmenitebearing rocks.

Synchrotron micro-XANES (X-ray absorption near-edge structure) spectroscopy (SmX) was used to measure Fe3+and Fe2+distribution among minerals in standard thin sections with an X-ray beam size of 10 ¥ 15 mm. Measurements were made at beamline X26a, National Synchrotron Light Source, Brookhaven National Lab. Samples studied included metapelites from garnet to upper sillimanite zone rocks that coexist with graphite, graphite/ilmenite, or varying combinations of ilmenite, magnetite, and hematite. SmX results are compared with Mössbauer spectroscopic measurements of Fe3+on mineral separates from the same rocks. Results show excellent agreement (within ±5-10%) between Mössbauer and SmX for the Fe-rich phases biotite, chlorite, staurolite, and garnet. Mössbauer spectra of muscovite typically show lower values than SmX for Fe3+/SFe (especially at low grades), suggesting contamination of the muscovite separates by fine-grained chlorite. However, heterogeneity of Fe3+and Fe2+is probably the chief source of discrepancy between the bulk and micro-scale measurements. The %Fe3+(relative to total Fe) in these samples ranges from a high of 90% in muscovite to a low of 0-2% in garnet, with Ms > St = Bt = Chl > Tur > Grt in graphite/ilmenitebearing rocks.

To understand compositional variation in igneous biotite, full analyses of a suite of biotites of variable composition from volcanic and xenolith parageneses have been completed. Major and minor elements were determined by electron microprobe analysis, water was determined by manometry and SIMS analysis, and Fe3+/Fe2+was determined by microXANES and Mössbauer spectroscopy. Our new data, together with previous biotite analyses (total of 52 analyses), reveal correlations between O2-(2-F-Cl-OH) and the sum of the octahedral cations Al + Ti + Fe3++ Cr. This correlation allows estimation of either OH-or Fe3+/Fe2+as long as one or the other has been determined. The hydroxyl site in most mantle micas contains at least 1.0 O2-atoms per formula unit (apfu), indicating that the oxy-component cannot be ignored. The large oxy-component in melt inclusion micas from the martian meteorite Chassigny does not necessarily indicate oxidized or hydrous magmatic conditions because dehydrogenation may have occurred and/or because the oxy-component may be stable at low oxygen fugacity. The large variation in Ti, Al, and Fe3+in natural igneous micas is most likely dependent upon bulk compositional differences in each specific system such as variation of aTiO₂and aAl₂O₃in silicate melts.

To understand compositional variation in igneous biotite, full analyses of a suite of biotites of variable composition from volcanic and xenolith parageneses have been completed. Major and minor elements were determined by electron microprobe analysis, water was determined by manometry and SIMS analysis, and Fe3+/Fe2+was determined by microXANES and Mössbauer spectroscopy. Our new data, together with previous biotite analyses (total of 52 analyses), reveal correlations between O2-(2-F-Cl-OH) and the sum of the octahedral cations Al + Ti + Fe3++ Cr. This correlation allows estimation of either OH-or Fe3+/Fe2+as long as one or the other has been determined. The hydroxyl site in most mantle micas contains at least 1.0 O2-atoms per formula unit (apfu), indicating that the oxy-component cannot be ignored. The large oxy-component in melt inclusion micas from the martian meteorite Chassigny does not necessarily indicate oxidized or hydrous magmatic conditions because dehydrogenation may have occurred and/or because the oxy-component may be stable at low oxygen fugacity. The large variation in Ti, Al, and Fe3+in natural igneous micas is most likely dependent upon bulk compositional differences in each specific system such as variation of aTiO₂and aAl₂O₃in silicate melts.

Synchrotron micro-XANES spectroscopy (SmX) is used to examine the amount and distribution of Fe2+and Fe3+in five samples of fayalite previously studied by Mössbauer spectroscopy. Rockport fayalite is homogeneous and the Fe in it is completely reduced. Olivine from Qianan is almost completely oxidized, and probably contains finely intergrown silica, laihunite, and hematite with hematite predominating. Pantelleria olivine has an oxidized rim around a reduced core, suggesting either a post-crystallization reaction with fayalite and oxygen going to laihunite and hematite or a change in prevailing oxidation during growth. The texture of olivine from the Mourne Mountains suggests exsolution from a fayalite-laihunite solid solution, based on the substitution of three Fe3+atoms for two Fe2+and one vacancy, that was stable at high temperature. Laihunite from the type locality (Lai-He village) shows 1-3 mm mottling between light and dark areas in backscatter images, but these areas cannot correspond to pure fayalite and laihunite exsolution because the bulk Fe3+/ΣFe of such a combination would not correspond to the SmX value of 67% Fe3+. Rather, the texture observed in backscatter is interpreted to represent alternating areas of magnetite-rich and laihunite-rich compositions with a similar (i.e., 67% Fe3+/ΣFe) extent of oxidation.