Characterization of Al-Si Ordering State in Alkali Feldspar Using ALCHEMI

Atom Location by CHanneling-Enhanced MIcroanalysis (ALCHEMI) is used in this study to determine the occupancy of Al atoms in the T1o site (t1o) of alkali feldspar. Combined with the method shown by Taft?and Buseck (1983), analytical electron microscopy proves to be a viable technique for fully characterizing the Al-Si ordering state in the feldspar framework. We have applied this method to both gem orthoclase from Itrongay, Madagascar, and to its heated counterpart. Our preliminary results give 0.737 and 0.372 as 2t1 (= t1o + t1m) and t1o respectively for the original orthoclase, versus 0.517 and 0.256 for heated orthoclase. Standard X-ray powder diffraction experiments will be done to verify these measurements. Furthermore, we are seeking a quantitative relationship between the incident electron intensity based on the Bloch-wave formulation and the positions of interstitial sites with respect to the reference channeling plane. This approach could give a more precise interpretation of ALCHEMI data in our case and in similar applications.