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Coesite (or pseudomorphic quartz) is more common than previously recognized in the eclogite xenolith suite from the Roberts Victor Mine, South Africa. All coesite eclogites in this study (18 samples) are classified as group I, and have mineral compositions that span virtually the entire compositional range of group I eclogites described from this locality. Most of the Roberts Victor group I eclogite suite may be in equilibrium with free silica, thus weakening the case for these rocks representing residues of partial melting. Oxygen isotope compositions of garnets from 15 samples are in the range 5.32–6.95‰, similar to other Roberts Victor group I eclogites. These observations are consistent with a model in which protoliths of these rocks were oceanic basalts and intrusive rocks that underwent exchange with seawater at relatively low temperatures, to variable extents, prior to subduction and metamorphism to eclogite …