An EXAFS study of glassy and crystalline phases of compositions CaAl2Si2O8 (anorthite) and CaMgSi2O6 (diopside)

Abstract

EXAFS spectroscopy has been used to study the local environment of calcium in glasses and crystals of anorthite (CaAl2Si2O8) and diopside (CaMgSi2O6) compositions. The glass results reveal significant local order at the calcium sites, as has been demonstrated for sodium in soda silicate glasses (Greaves et al. 1981). Comparison of calcium spectra in the present study reveal considerable similarities in the distribution of nearest neighbour oxygens for the two glasses. For the crystals the calcium environments are different and agreement with crystallographic data is good. Comparing glasses and crystals, however, the calcium-oxygen pair distribution function for anorthite glass is very close to that found in the crystal, whereas diopside glass and diopside crystal are quite different. The similarity in the calcium sites for anorthite glass and anorthite crystal is consistent with the small differences observed between the X-ray diffraction data for the two phases (Taylor and Brown 1979) and with the similarity in their densities. Although the anionic units of diopside glass may contain chains like diopside crystal, as suggested by Raman spectroscopy (Sharma and Yoder 1978), EXAFS reveals that the calcium environment involves significantly longer calcium-oxygen distances than in diopside crystal. This difference explains the lower density of the glass and rules out a quasi-crystalline description for the glass structure.