Abstract

Three-dimensional textural observations of inclusion and porosity patterns in a 23-carat carbonado diamond using high-resolution X-ray computed tomography reveal new information bearing on the nature and origin of this enigmatic material. A prominent patinaed surface is texturally linked to a banding and grading of inclusions and pore space beneath, extending several millimeters into the specimen. In situ observation demonstrates that almost all inclusions are polymineralic and show replacement textures, corroborating previous work indicating that the pore network is fully three-dimensionally (3-D) connected, and that virtually all macro-inclusions are secondary. Large metal inclusions are only found immediately adjacent to the margin of the specimen, and are thus also likely to be secondary or even tertiary. However, we also report pseudomorphs after a phase forming pristinely euhedral rhombic dodecahedra, individually and in clusters from 0.3 to 1 mm in diameter; although we could find no evidence of this phase persisting, it nevertheless represents the first “true” macro-inclusion reported in carbonado, which almost certainly formed syngenetically with the diamond material. The pore system is essentially trimodal, consisting of single and clustered pseudomorphs, oblate pores 0.1–0.3 mm in length with a clear preferred orientation, and 20 µm to <1 µm pores that form the connected network. Our observations support recent work suggesting that carbonado crystallized from a carbon-supersaturated fluid and suggest that the second stage may correspond with the creation of the pore alignment fabric. We further postulate that, although the present-day macro-inclusions are certainly secondary, the bulk material that comprises them may not be, and may instead be broken-down remains of the original included phase(s). While further verification is needed, a model built around this hypothesis may provide the simplest explanation to many of the unusual features of carbonado.

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