Anisotropic absorption in crystals is routinely observed in many spectroscopic methods and is recognized in visible light optics as pleochroism in crystalline materials. As with other spectroscopies, anisotropy in Fe K-edge X-ray absorption spectroscopy (XAS) can serve both as an indicator of the general structural arrangement and as a conundrum in quantifying the proportions of absorbers in crystals. In materials containing multiple absorbers, observed anisotropies can typically be represented by a linear relationship between measured spectroscopic peak intensities and relative absorber concentrations. In this study, oriented XAS analysis of pyroxenes demonstrates that the macroscopic theory that describes visible light absorption anisotropy of triaxially anisotropic materials can also be applied to X-ray absorption in pyroxenes, as long as the orientation and magnitude of the characteristic absorption vectors are known for each energy. Oriented single crystal XAS analysis of pyroxenes also shows that the measured magnitude of characteristic absorption axes at a given orientation are energydependent and cannot be reproduced by linear combination of intermediate spectra. Although the macroscopic model describes a majority of the anisotropy, there is distinct discordance between the observed and interpolated spectra in the pre-edge between 7109 and 7115 eV, which is marked by spikes in RMSE/mean intensity ratio. Absorption indicatrices for samples analyzed in the visible and at X-ray wavelengths are modeled with a 3-dimensional (3D) pedal surface which functions as an empirical way of interpolating between the observed absorption data. This surface only requires a maximum of three coefficients, and results from the summation of 3Dlemniscates. An absorption indicatrix model can be used to characterize anisotropic absorption in crystals and provides a way of comparing XAS spectra from randomly oriented crystals, such as those from polished sections, to a database of characterized crystals.
The late Mesozoic accretionary boundary in west-central Idaho has played a critical role in tectonic models proposed for the northwestern U.S. Cordillera. From west-to-east, major elements include the Permian to Jurassic Wallowa island-arc terrane, a poorly understood transition zone consisting of the Riggins Group assemblage and deformation belt along the west side of the island arc-continent boundary, Late Jurassic to Cretaceous arc-continent boundary, and Precambrian North American margin intruded by the Cretaceous–Paleogene Idaho batholith. We focus on the transition zone in the area between White Bird and Riggins, Idaho, which includes a contractional belt in variously deformed and metamorphosed rocks of island-arc affinity. We propose that the rocks of the entire transition zone, including those originally defined as the Riggins Group, are likely of Wallowa terrane origin and/or related basinal assemblages. Ultramafic rocks in the transition zone are possibly related to a Jurassic or Cretaceous basinal assemblage that includes the Squaw Creek Schist of the Riggins Group. Our recent work addresses the kinematic history of structures in the contractional belt. The belt was reactivated in the Neogene to accommodate mostly brittle normal faulting that strongly influenced preservation of the Miocene Columbia River Basalt Group at this location along the eastern margin of the flood basalt province. This field guide provides a road log for examining the geology between Moscow and New Meadows, Idaho, along U.S. Highway 95.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.