Abstract: Olivine (high-Fe forsterite; Fo# 69-76) and associated minerals have been studied in a coesite-bearing diamondiferous eclogite from the Udachnaya-East kimberlite pipe. Olivine was identified in the rock interstices together with K-feldspar, sodalite, phlogopite, and djerfisherite. The composition of individual olivine grains varies within the xenolith and is characterized by a consistent presence of P2O5, MnO, CaO, and Al2O3. The coexistence of olivine and coesite is uncommon for mantle xenoliths. The occurrence of olivine and K, Na, and Сl-bearing minerals in the studied sample is suggested to have been caused by the interaction of the eclogite with a protokimberlite melt enriched in alkali components and chlorine.