Exafs and xanes study of the local coordination environment of sodium in a series of silica-rich glasses and selected minerals within the Na 2OAl 2O 3SiO 2 system

Abstract

Photoelectron total-yield Na XANES and EXAFS spectra were gathered to characterize the Na coordination for a collection of Na 2OAl 2O 3SiO 2 glasses and minerals. Included in the collection are the crystalline model compounds albite, jadeite and nepheline as well as glasses at these compositions, Na 2Si 2O 5 glass, and a series of glasses containing 75 mol% silica with R (= Al/Na) values ranging from 0.03 to 1.61. High resolution Na K-XANES spectra are reported for the first time and are of good enough quality for the specimens studied to make qualitative comparisons of Na edge features possible. Due to the variable quality of the Na EXAFS data, only Na 2Si 2O 5 glass, three out of five of the glasses in the glass series, nepheline and nepheline glass were worthy of analysis; these samples are the more soda-rich materials used for this study. Comparisons of Na XANES and EXAFS spectra for the glass series, nepheline, nepheline glass and Na 2Si 2O 5 glass indicate little if any change in the Na environment among these materials. Nepheline glass and crystal show reversal of intensity of the 1055 eV and 1058 eV Na edge features. The similarity of the Na coordination environment is also apparent in the EXAFS spectra, which yield NaO distances ranging from 2.56 to 2.62 Å. This overall similarity of the Na coordination for samples in this study is not surprising since Na in crystalline compounds within this system is usually coordinated by seven to ten oxygens. The physical property variations for our glass series are not explained by the differences in the Na environments depicted by our Na XANES and EXAFS results. The Na EXAFS for nepheline appears to sense only the closest set of oxygens of those in the entire known Na coordination, and this seems to be true for nepheline glass as well, using high temperature crystal structure studies of nepheline for distance comparisons.