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

Abstract

Photoelectron total yield Al K-XANES and EXAFS spectra were collected for a series of Na 2OAl 2O 3SiO 2 minerals and glasses. The purpose of this study is to determine if EXAFS can accurately characterize these known Al environments in terms of AlO distances and coordination numbers, and to characterize the unknown Al environment in the glasses. Al XANES spectra were collected for corundum, sillimanite, albite, jadeite and nepheline, as well as jadeite glass, nepheline glass and a 75 mol% silica glass series having R ( = Al/Na) ranging from 0.28 to 1.61. Al EXAFS spectra were collected for corundum, sillimanite, jadeite and albite, as well as jadeite glass, albite glass and a 75 mol% silica glass with an R of 1.61. Al XANES spectra for our model compounds have two edge maxima at 1568 and 1572 eV for minerals having octahedral Al, while one edge maximum is present at 1566 eV for minerals with tetrahedral Al. The glasses all have a single edge maximum near 1566 eV, indicating tetrahedral-like Al sites in the glasses. The results of our Al EXAFS refinements of the model compounds are in agreement to within ±0.03 Å for AlO distances and to within ±23% for coordination numbers of recent crystal structure refinements in the literature. The EXAFS results for the glasses clearly indicate Al in tetrahedral sites, yielding AlO distances near 1.77 Å, well within the range of average AlO distances for aluminate tetrahedra of aluminosilicate minerals. Our XANES and EXAFS results indicate tetrahedral Al throughout our 75 mol% silica glass series, which may be consistent with the existence of tricluster units occurring in the Al-rich glasses in the sodium aluminosilicate system.