The structure of haplobasaltic glasses investigated using X-ray absorption near edge structure (XANES) spectroscopy at the Si, Al, Mg, and O K-edges and Ca, Si, and Al L2,3-edges

Abstract

X-ray absorption near-edge structure (XANES) spectroscopy has been used to investigate the local structural environments around each of the elements within the haplobasaltic system. The XANES results reveal both, linear changes in Ca, O and Si whereas non-linear changes are also observed for Si as well as Al and Mg. Unexpectedly, both Si and Al spectra show complex edge structure that varies non-linearly across the join. The Si L2,3-edge results suggest that the Si environment in An100 is very similar to that in crystalline anorthite whereas the Di100 and diopside spectra show large offsets. The L3 and L2 peaks display a linear behavior whereas peak B of the Si L2,3 spectra displays a non-linear behavior displaying a maximum position at the Di40 and Di100 compositions and decreasing in between. We estimate the SiO bond length from shift in the peak B position which suggests a change in bond length between 1.58 and 1.66Å across the join. The Al K-edge spectra suggests that there is a constant decrease in the inter-tetrahedral angle as Al is added. The local Mg environment displays non-linear changes in intensity and for compositions with An content greater than Di40 it appears that [4]Mg plays an important role whereas [5]Mg is dominant in diopside-rich melts, though octahedral coordinated Mg cannot be ruled out. The oxygen environment appears similar to crystalline anorthite in the haplobasaltic system. Calcium is present in highly distorted sites that are dominantly [7]Ca in anorthite-rich glasses whereas [8]Ca is more important in diopside-rich glasses. These results provide additional independent constraints on the atomic structure of haplobasaltic silicate melts and highlight the need for further investigation into the relationships between network formers, modifiers and the oxygen to which they are bound.