Alchemi Analysis of Cation Distributions in Spinel-Structured Compounds Using Oxygen as a Marker Element

Abstract

The establishment of ALCHEMI (atom location by channeling-enhanced microanalysis) as a viable microanalytical technique owes much to its early success in determining cation distributions in crystals having the structure of the mineral spinel, MgAl2O4. The first application of ALCHEMI was to a synthetic chromite spinel, and geological applications to natural mineral spinels containing multiple 3d transition elements provided many of the early applications of the technique. Spinel has three distinct regular lattice sites, two cation sites and one anion site, occupying the 8a, 16d and 32e Wyckoff positions of the Fd3m space group. In projection along (800), tetrahedrally coordinated (8a) cations occupy one set of alternate planes, whereas octahedrally coordinated (16d) cations and, to a good approximation, the anions occupy the other set. In principle, therefore, ALCHEMI performed at an 800 planar channeling condition could be used to analyze site distributions in spinel, using oxygen as a marker for the planes containing the octahedral cations, as was early noted.