Polarized EXAFS, distance-valence least-squares modeling (DVLS), and quantitative texture analysis approaches to the structural refinement of Garfield nontronite

Abstract

This paper reports on the new application of polarized extended X-ray absorption fine structure (P-EXAFS) spectroscopy to fine-grained layer silicates taking the Garfield nontronite as a case study. Up to now application of P-EXAFS to structural studies of layer silicates has been restricted to single phyllosilicate crystals Manceau et al. 1990), but we show here that P-EXAFS can rigorously be applied to self-supporting clay films without loss of spatial resolution. The quantitative analysis of P-EXAFS requires however the preparation of highly oriented clay films, the orientation distribution of which can be assessed by texture goniometry. The Fe K-edge linear dichroism measurements were simulated by ab initio EXAFS modeling performed on a nontronite cluster whose structure was refined by distance-valence least-squares calculations. It is shown that ab initio modeling quantitatively accounts for the angular dependence of experimental EXAFS spectra. These calculations allowed for the identification of the fundamental character of single-and multiple-scattering paths of the photoelectron, and the structural interpretation of all spectral features observed up to 6.5 for the in-plane and outof-plane radial structure functions of nontronite. In practice, P-EXAFS measurements allow the determination of the flattening angle of Fe(O,OH) 6 octahedra, cations distribution in the octahedral sheet with an enhanced sensitivity, and differentiation between dioctahedral and trioctahedral structures.

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

“…Although spectra of precisely oriented samples have not yet been published, orientation has been shown to have a strong effect on peak intensity in micas (Manceau et al 1998, Dyar et al 2001. Unpublished data reported in the latter paper show that the best agreement between Fe XANES spectra for single crystal (at various angles) and homogeneous powder (grain size of ca.…”

“…Thus, the goal of this study is to compare and contrast the structural similarities among these mineral groups and to relate them to their XANES spectra and optical orientations. From these relationships, a better understanding of the transitions represented by these spectra may ultimately be achieved, along with an improvement in use by Manceau et al (1998), Brigatti et al (2001), Giuli et al (2001), and Benfatto et al (2001), and Tombolini et al (2002), among others.…”

The effect of phytostabilization on Zn speciation in a dredged contaminated sediment using scanning electron microscopy, X-ray fluorescence, EXAFS spectroscopy, and principal components analysis

“…This assignment is supported experimentally in P-EXAFS by the observation that the resonance smears out and resembles that in hydrotalcite when the contribution from the tetrahedral sheet is minimized (almost cancelled) and, alternatively, the contribution from the octahedral sheet maximized, in the in-plane orientation. In addition, the next-nearest Si shell (Si2 shell) at 4.4 -4.5 Å and higher oxygen shells generate the third peak observed on the RSF of Zn-rich phyllosilicate at R + ∆R ~ 3.5 -4.0 Å (Manceau et al, 1998). This peak is obviously absent in (Zn-Al)-hydrotalcite (Manceau et al, 1999) (Fig.…”

Hydration of methane intercalated in Na-smectites with distinct layer charge: Insights from molecular simulations

“…[22]. c [20]. The self-diffusion coefficients D of the interlayer species are calculated with the Einstein formula [33,34] 1 N…”

“…Garfield-type nontronite only bears tetrahedral charges [20], Arizona-type montmollonite only has octahedral charges [21], and Wyoming-type montmollonite has both types [22] (Table 1). In these clay models, the isomorphic substitutions obey Loewenstein's rule [23].…”