ELECTRON-BEAM-INDUCED AMORPHIZATION OF STISHOVITE - SILICON-COORDINATION CHANGE OBSERVED USING SI K-EDGE EXTENDED ELECTRON-ENERGY-LOSS FINE-STRUCTURE

The analysis of the extended energy-loss fine structure (EXELFS) of th e Si K-edge for sixfold-coordinated Si in synthetic stishovite and fou rfold-coordinated Si in natural alpha-quartz is reported by using elec tron energy-loss spectroscopy (EELS) in combination with trans mission electron microscopy (TEM). The stishovite Si K-edge EXELFS spectra we re measured as a time-dependent series to document irradiation-induced amorphization. The amorphization was also investigated through the ch ange in Si K- and O K-edge energy-loss near edge structure (ELNES). Fo r alpha-quartz, in contrast to stishovite, electron irradiation-induce d vitrification, verified by selected area electron diffraction (SAED) , produced no detectable changes of the EXELFS. The Si K-edge EXELFS w ere analysed with the classical extended X-ray absorption fine structu re (EXAFS) treatment and compared to ab initio curve-waved multiple-sc attering (MS) calculations of EXAFS spectra for stishovite and alpha-q uartz. Highly accurate information on the local atomic environment of the silicon atoms during the irradiation-induced amorphization of stis hovite is obtained from the EXELFS structure parameters (Si-O bond dis tances, coordination numbers and Debye-Waller factors). The mean Si-O bond distance R and mean Si coordination number N changes from R=0.177 5nm and N=6 for stishovite through a disordered intermediate state (R approximate to 0.172nm and N approximate to 5) to R approximate to 0.1 67 nm and N approximate to 4.5 for a nearly amorphous state similar to alpha-quartz (R=0.1609 nn and N=4). During the amorphization process, the Debye-Waller factor (DWF) passes through a maximum value of sigma (N)(2) approximate to 83.8pm(2) as it changes from sigma(st)(2)= 51.8p m(2) for sixfold to sigma(qu)(2) = 18.4pm(2) for fourfold coordination of Si. This Increase in Debye-Waller factor indicates an increase in mean-square relative displacement (MSRD) between the central silicon a tom and its oxygen neighbours that is consistent with the presence of an intermediate structural state with fivefold coordination of Si. The distribution of coordination states can be estimated by modelling the amorphization as a decay process. Using the EXELFS data for amorphiza tion, a new method is developed to derive the relative amounts of Si c oordinations in high-pressure minerals with mixed coordination. For th e radiation-induced amorphization process of stishovite the formation of a transitory structure with Si largely in fivefold coordination is deduced.