TEM STUDY OF A CHROMIUM-BEARING KYANITE FROM A MANTLE XENOLITH: EVIDENCE FOR AN ALUMINA-RICH EXSOLUTION PRECURSOR PHASE

Abstract

The chromium-bearing kyanite specimen U-158, extracted from a kyanite-eclogite xenolith in a Yakutian kimberlite pipe was studied by transmission electron microscopy TEM. This specimen has an analysed composition (Al-1.989 Cr-0.014 Fe-0.007 Ti-0.001)(Sigma =2.011) [O/Si0.990O4] and has been examined by electronic absorption and fluorescence spectroscopy (Platonov et al., 1998). The (010) oriented foil previously studied by spectroscopy, shows areas of dark diffraction contrast which were found to originate from incipient alumina-type exsolutions forming platelets about 2 nm thick and 5-10 nm wide. They are intergrown with the kyanite matrix and have the orientation relationships [0 (1) over bar(1) over bar](ky) // [01 (1) over bar0](cor) and (100)(ky) // (0001)(cor). The volume fraction of these exsolution precursors is estimated to be less than 0.1 vol. %. Elemental mapping using inelastically scattered electrons in the EELS spectra (Al-L-3 and Si-L-3 edges) demonstrated Si-depletion and Al-enrichment in the exsolution precursors. Furthermore, EDX analysis with a nominal spot size of 4 nm confirmed the areas with dark contrasts to be more Al-rich than the kyanite matrix without these contrasts, while chromium contents were the same. The EDX analysis is evidently a superposition of the chemical composition of both the kyanite matrix and the additional phase identified by TEM. In any case, the chemical analysis confirms the crystallogaphic inferences about composition. The results confirm the presence of a precursor of an Al-rich, chromium-bearing corundum-type exsolution as previously suggested to explain two greatly differing crystal fields for Cr3+ in the studied chromium-bearing kyanite.