A xenolith made of exceptionally Cr-rich minerals (mostly omphacite, kyanite and minor rutile) has been found in the diamondiferous Mbuji-Mayi kimberlite province, Democratic Republic of Congo (DRC). Chromium contents (Cr2O3) are hetero-geneously distributed in the bulk sample, as well as in a single crystal: 3.34–5.70 wt% in omphacite, 4.45–11.81 wt% in kyanite and 2.68–4.65 wt% in rutile. Such Cr-rich kyanite is extremely rare; only grospydite xenoliths from the Zagadochnaya kimberlite (Yakutia) have been reported with kyanite containing up to 12.86 wt% Cr2O3. However, unlike typical grospydites, no garnet is observed in the Mbuji-Mayi Cr-rich kyanite-bearing clinopyroxenite.
The textural features of this rock and the tentative thermobarometric equilibration conditions (25–35 kbar, 700–800 °C) argue for an origin related to that of eclogites, which are the predominant xenoliths in the Mbuji-Mayi kimberlites. The calculated whole-rock composition suggests an Al-, Ca- and Cr-rich protolith that resembles Cr-rich leucogabbros or anorthosites commonly found in Archean anorthosite massifs. The peak P-T conditions registered by this unusual rock can be reached either by Archean subduction or by mafic magma underplating at the crust-to-mantle boundary with subsequent delamination into the upper mantle.