LOW Z-ELEMENTS (MG, AL, AND SI) K-EDGE X-RAY-ABSORPTION SPECTROSCOPY IN MINERALS AND DISORDERED-SYSTEMS

Soft X-ray absorption near edge spectroscopy (XANES) and extended X-ra y absorption fine structure (EXAFS) spectroscopy have been performed a t the Mg-, Al- and Si-K edges in order to establish the ability of thi s spectroscopy to derive structural information in disordered solids s uch as glasses and gels. Mg- and Al-K XANES are good structural probes to determine the coordination state of these elements in important mi nerals, glasses and gels. In a CaO-MgO-2SiO(2) glass Mg XANES spectra differ from that found in the crystalline equivalent, with a significa nt shift of the edge maxima to lower energy, consistent with a CN lowe r than 6. Mg-EXAFS on the same sample are in agreement and indicate th e presence of 5-coordinated Mg with Mg-O distances of 2.01 Angstrom. I n aluminosilicate gels, Al-K XANES has been used to investigate the Al -[4]/Al-total ratios. These ratios increase as the Al/Si ratios decrea se. Aluminosilicate and ferric-silicate gels were studied by using Si- K edge XANES. XANES spectra differ significantly among the samples stu died. Aluminosilicate gels with Al/Si = 1 present a different Al and S i local environment from that known in clay minerals with the same Al/ Si ratio. The gel-to-mineral transformation thus implies a dissolution -recrystallization mechanism. On the contrary, ferric-silicate gel pre sents a Si local environment close to that found in nontronite which m ay be formed by a long range ordering of the initial gels.