Systematics in the Structure and Xanes Spectra of Pyroxenes, Amphiboles, and Micas as Derived From Oriented Single Crystals

Abstract

In order to assess the extent of X-ray anisotropy in biaxial minerals, Fe XANES spectra were acquired on single crystals of pyroxenes, amphiboles, and micas with the X-ray beam polarized along the X, Y, and Z optical orientations. Specifically, single crystals of enstatite (En), augite (Aug), aegirine (Ae), kaersutite (Krs; three different compositions), dioctahedral (Ms and Ill), and trioctahedral (Ann and Phl) micas were selected. Each crystal was oriented morphologically and with the aid of EXCALIBR using a spindle stage equipped polarized light microscope, and their XANES spectra were acquired at beamline X26a, NSLS, using a special geometry of the beam to accommodate the goniometer head and a 20 ϫ 30 m X-ray beam. As would be expected from visible and IR spectra of these minerals, the intensity of both pre-edge and main edge peaks is variable as a function of orientation. Structural similarities among these mineral groups result in similarities among their XANES spectra. Spectra acquired along the length of the chains (c for pyroxene, c for amphibole, and a for sheet silicates) are similar, with corresponding changes in the optical directions, such that Y dioct ≈ Z trioct ≈ Z Krs ≈ Z En ≈ X Ae (clinopyroxene is intermediate). Spectra taken along the b crystallographic axes (along the layer of octahedra, across the I-beam) are similar for all groups (Z dioct ≈ Y trioct ≈ Y Krs ≈ X En ≈ Y Aug ), and more similar for the amphiboles and micas. Lastly, spectra acquired along the stacking direction, which is the a crystallographic direction for pyroxene and amphiboles and the c crystallographic direction in sheet silicates, are analogous. From these relationships, assignment of specific transitions to individual features in XANES spectra are facilitated. Furthermore, errors on use of pre-edge positions for determination of Fe 3+ /⌺Fe in minerals can be better constrained.Keywords: XANES, XAS, single crystal, enstatite, augite, aegirine, kaersutite, annite, phlogopite, illite, muscovite, estimate of ferric iron. pourrait s'y attendre d'après les spectres de ces minéraux en lumière visible et infra-rouge, l'intensité des pics pré-seuil et seuil principal est variable selon l'orientation. Les ressemblances structurales parmi ces groupes de minéraux résultent de ressemblances dans les spectres XANES. Les spectres acquis le long des chaînes (c dans le cas de pyroxènes et des amphiboles, et a dans le cas de silicates en feuillets) sont semblables, compte tenu des changements des directions optiques, de sorte que Y dioct ≈ Z trioct ≈ Z Krs ≈ Z En ≈ X Ae (le clinopyroxène est intermédiaire). Les spectres prélevés le long de l'axe b (le long de la couche d'octaèdres, transversalement aux modules en I) sont semblables pour tous les groupes (Z dioct ≈ Y trioct ≈ Y Krs ≈ X En ≈ Y Aug ), et le sont davantage pour les amphiboles et les micas. Enfin, les spectres acquis le long de la direction d'empilement, le long de l'axe a dans le cas des pyroxènes et des amphiboles et le long de l'axe c dans le cas des silicates en ...

Determination of the Fe oxidation state of the Chassigny kaersutite: A microXANES spectroscopic study

“…This pre-edge position shifts toward higher energy side with increasing Fe 3+ /ΣFe ratios of minerals by indicating a linear relationship as a function of the Fe 3+ /ΣFe ratio (Bajt et al 1994). A number of research groups employed the XANES techniques and demonstrated its availability for studying the valence state of Fe of terrestrial minerals (Delaney et al 1998;Dyar et al 2001Dyar et al , 2002Wilke et al 2001). In particular, Wilke et al (2001) reported a XANES study of a series of Fe 2+ -and Fe 3+ -bearing minerals with a wide range of coordination environments and concluded that the Fe 3+ contents can be determined with an accuracy of ±10 mol% when the coordination environment is given.…”

“…The XANES pre-edge position of a single crystalline mineral is known to shift slightly by the effect of X-ray anisotropy (Dyar et al 2002). This anisotropic phenomena appears to produce a limited accuracy in the determination of the Fe 3+ /ΣFe ratio of Chassigny kaersutite, because it is practically difficult to measure an spectra of Chassigny kaersutite with the X-ray beam polarized along the crystalline orientation similar to standard samples.…”

Amorphous silicate nanoparticles with controlled Fe-Mg pyroxene compositions

“…This method has been applied to a wide range of minerals and glasses [35,36,37], although it can potentially be subject to error if the difference in site geometry between the reference standards and the unknowns is too large [35,36]. The centroid positions however do allow systematic trends to be identified and Fig.…”

Redox states of thirteen shergottites as inferred from iron micro X-ray absorption near edge structure of maskelynite

“…Delaney et al (1998) reported that the Fe 3+ /∑Fe ratios estimated using XANES analysis are consistent with those Mössbauer estimated using wet chemical analysis. Dyar et al (2001Dyar et al ( , 2002 and Wilke et al (2001) also used Fe XANES spectroscopy to quantitatively estimate the Fe 3+ / ∑Fe ratios for silicate and oxide minerals. Dyar et al (2002) polarized the X-ray beam along the X, Y, and Z optical orientations to obtain Fe XANES spectra from single crystals of pyroxenes, amphiboles, and micas.…”

“…Dyar et al (2001Dyar et al ( , 2002 and Wilke et al (2001) also used Fe XANES spectroscopy to quantitatively estimate the Fe 3+ / ∑Fe ratios for silicate and oxide minerals. Dyar et al (2002) polarized the X-ray beam along the X, Y, and Z optical orientations to obtain Fe XANES spectra from single crystals of pyroxenes, amphiboles, and micas. They showed that the pre-edge positions of sheet silicates depend on their crystallographic orientations.…”