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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) areanormalized pre-edge peak centroid, (2) the energy of the main absorption edge at the location where the normalized edge intensity has the highest R2 correlation with Fe3+/ΣFe, (3) the ratio of spectral intensities at two energies determined by highest R2 correlation 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. [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]

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]

Presents a study that developed a method to orient single crystals to obtain X-ray absorption near edge structure or extended X-ray absorption fine structure spectra. Methodology; Results; Discussion.

Deals with a study which measured iron distribution among minerals in metapelites. Background to the study; Theoretical framework for sample selection; Results and discussion.

Examines the compositional variation in igneous biotite through analyses of a suite of biotites of variable composition from volcanic and xenolith parageneses. Effect of dehydrogenation on the original composition of biotites; Examination of biotite data from mantle samples; Analysis of mantle rocks from Mars.

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.