Tales from diamond surface features – A tribute to Derek Robinson

Diamonds entrained by kimberlites and olivine lamproites formed predominantly in peridotite and eclogite substrates within the lithospheric mantle. The main growth forms of monocrystalline diamonds are octahedral and cubic. However, many diamonds also exhibit a range of surface features derived during mantle residence and/or entrainment to surface. In pioneering research, Derek Robinson developed an interpretative catalogue of diamond morphologies and surface features, improving understanding of diamond growth, plastic deformation, oxidative etching and resorption processes, and the impacts of sedimentary transport and diamond recovery practices. He also established the sequence of events reflected in diamond physical characteristics. The etching and resorption surface features developed on diamonds provide important constraints on their exposure to oxidizing fluids (mainly CO₂, H₂O) in the mantle and/or kimberlite melt. There is broad consensus that common resorption features such as tetrahexahedroid (THH) forms result from interaction with H₂O-bearing kimberlite fluids. However, other surface features on trigonal octahedral faces (e.g., deep hexagonal pits, triangular plates) have been attributed to either pre-kimberlite mantle metasomatism or variations in kimberlite melt/fluid conditions. Evidence supporting mantle resorption includes cathodoluminescence (CL) imaging of internal diamond growth layers and rounded diamonds in some mantle xenoliths. As most diamonds in mantle xenoliths are typically sharp-edged with few etch features, the formation of specific surface etch features by pre- (or syn-) kimberlite mantle metasomatism is equivocal. Alternative explanations include limited ingress of kimberlitic fluids into host xenoliths during entrainment, ascent and/or emplacement, sampling of multiple diamond resorption groups from different pulses of kimberlite magma with distinct volatile compositions eruption/degassing histories.