Evidence for Al/Si tetrahedral network is aluminosilicate glasses from AlK-edge x-ray-absorption spectroscopy

The structure of aluminosilicate melts and/or glasses plays a key role in t he earth sciences for the understanding of rock-forming igneous processes, as well as in the materials sciences for their technical applications. In p articular, the alkaline-earth aluminosilicate glasses are an extremely impo rtant group of materials, with a wide range of commercial application, as w ell as serving as an analog for natural basaltic melts. However, definition of their structure and properties is still controversial, and in particula r the role and effect of Al has long been a subject of debate. Here we repo rt a series of experimental x-ray absorption near-edge structure spectra at the Al K edge on a series of synthetic glasses of peralkaline composition in the CaO-Al2O3-SiO2 system, together with a general theoretical framework for data analysis based on an nb initio full multiple-scattering theory. W e propose an Al/Si tetrahedral network model for aluminosilicate glasses ba sed on distorted polyhedra, with varying both the T-O (T=Al or Si) bond len gths and the T-O-T angles, and with different Al/Si composition. This model achieves a significant agreement between experiments and simulations. In t hese glasses, experimental data and theoretical results concur to support a model in which Al is network former with a comparatively well ordered loca l medium-range order (up to 5 Angstrom). [S0163-1829(99)09833-1].