ALBERT

Beschreibung: Microanalysis of Fe 3+ /Fe 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 %Fe 3+ 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 R 2 correlation with Fe 3+ /Fe, (3) the ratio of spectral intensities at two energies determined by highest R 2 correlation with Fe 3+ /Fe, (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 Fe 3+ /Fe show significant improvements in accuracy over the former four types of univariate models. Fe 3+ /Fe 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 Fe 3+ vs. Fe 2+ have a pronounced (and diagnostic) effect on the XAS spectra that can be reliably used to precisely constrain Fe 3+ /Fe.

Beschreibung: 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 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 Fe 3+ /Fe. This paper addresses these two deficiencies, focusing specifically on a set of garnet group minerals. Variable-temperature Mössbauer 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. Mössbauer 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.

Beschreibung: 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 Fe 2+ and Fe 3+ 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 (), quadrupole splitting (), area, recoil-free fraction ( f ), Mössbauer temperature ( M ), and intrinsic isomer shift ( I ). The most systematic variations with composition are seen for and f , while small variations are seen for , M , and I . 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.