X-ray absorption spectroscopy of silicates for in situ, sub-micrometer mineral identification

Abstract

We present X-ray absorption near-edge structure (XANES) spectroscopy of 11 silicate and aluminosilicate minerals and two glasses at the SiK and SiL2,3, and OK edges. The similar nearestneighbor environments lead to similar spectral lineshapes at each edge, but the fine-structure differences allow individual and groups of structurally similar minerals to be distinguished. By combining spectra and their first energy derivative from three absorption edges, we show that every mineral studied is distinguishable with XANES. This background work, combined with X-ray PhotoElectron Emission spectroMicroscopy (X-PEEM), allows non-destructive in situ, sub-micrometer (to 35 nm) X-ray analysis of materials, including silicate inclusions, which has not been possible previously. Images and spectra from a 7 μm ∞ 3.5 μm quartz inclusion in zircon are presented as a test of this novel technique in geology. tive to cubic-hexagonal polymorphism in II-VI semiconductors, for which the nearest-neighbor positions are unchanged, and that this modification is not detectable with K-edge absorption (Gilbert et al. 2002). Si occurs within SiO4 tetrahedra in all the minerals analyzed in this study. Evidence already exists that XANES spectroscopy of silicates may allow species identification. Using large cluster molecular orbital calculations, other authors have concluded that the unoccupied states are more sensitive than the valence band to the details of SiO4 tetrahedral linkage in silicates (Tanaka et al. 1995). A recent large study of SiL2,3 EELS showed excellent discrimination between and within nesosilicate, inosilicate, and sorosilicate classes (Garvie and Buseck 1999). By contrast, lineshapes within the tectosilicate and phyllosilicate classes were far less distinct. The present study addresses this problem. We use X-ray absorption spectra at the SiK and SiL2,3 edges, and the OK edge of selected silicates to observe the relative sensitivity of each absorption edge to different silicate structures. The minerals in this study were selected to distinguish feldspar, quartz, or silicate glass inclusions in zircons (Peck et al. 2001).