Mixed-habit diamonds: evidence of a specific mantle fluid chemistry?

Mixed-habit diamonds are a relatively understudied subset of samples, considering the amount of research that has been carried out on these vessels from the mantle. They exhibit periods of growth during which they were bound by two surface forms [1]; smooth octahedral facets, along with hummocky, non-facetted "cuboid" surfaces whose mean orientation is {100} [2]. This type of diamond is commonly referred to as star or centre-cross, because the cuboid sectors have a much darker appearance due to abundant graphite inclusions (Figure 1). However, for unknown reasons, cuboid sectors do not always contain these graphite inclusions. This means that occurrence of this type of diamond is likely under-reported simply as it is not always visible. Mixed-habit diamonds have a number of unique charac-teristics (i) high nitrogen concentrations in both O and C sectors (higher in O than C), (ii) high hydrogen concentra-tions in the C sectors only (iii) low levels of nitrogen aggreg-ation (iv) anomalously low platelet concentrations in the C sectors (v) vivid green CL that highlights an extremely complex growth history. We present a study that utilizes a holistic analytical approach (IR mapping, CL imaging, CL spectroscopy, SIMS C-isotope, Raman, EBSD) to identify all of these unique characteristics and understand them within the context of diamond growth. This will provide insights into the chemistry of the mantle fluids involved in this type of diamond growth. [1] Frank (1967) Proc. Int. Industrial Diamond Conf., 119-135. [2] Lang (1974) Proc. R. Soc. Lond., A. 340, 233-248.