NEW CLUES ON THE ORIGIN OF CARBONADO DIAMOND FROM THREE DIMENSIONAL TEXTURAL ANALYSIS

Three-dimensional imaging of a large, 27-carat carbonado from the Central African Republic, using high-resolution X-ray computed tomography, provides fresh observations that bear directly on the origin of this enigmatic diamond variety. One side of the diamond has a silvery patina that has been interpreted in similar specimens as a fusion crust. However, this crust is spatially associated with a series of features extending several millimeters into the diamond interior, such as banding and grading of inclusion and pore sizes, indicating it is not strictly a surface trait. Large mm-scale open pores are only observed toward the edge of the specimen, although micropores are observed throughout. Although large inclusions are observed throughout the specimen, smaller inclusions grade from absent toward the patinated side to abundant away from it. Inclusions have a preferred orientation that is not parallel to the apparent grading direction. Almost all inclusions observed are polymineralic, and display various disequilibrium textures, such as mottling and irregular, interweaving grain boundaries. Small inclusions tend to have indistinct shape, but large ones are sometimes observed to be pseudomorphs after a phase with perfect dodecahedral habit delineated by sharp boundaries against the diamond matrix. These textures imply that the vast majority of inclusions are secondary, and thus that the pore network is almost entirely interconnected. Multi-energy imagery on one of the few monomineralic inclusions with prefect dodecahedral habit shows that it has a similar density and only slightly higher mean atomic number than diamond, and thus has a minimal to absent ferric component. The most likely mineral to match these criteria is pyrope, a common mantle inclusion that would strongly imply a mantle origin for carbonado. We are currently attempting to use the CT data to guide laser sectioning of the specimen to confirm or invalidate this interpretation. Three-dimensional imaging is a powerful but still-underutilized tool that can open the door to not only sophisticated new types of analysis, but extension of classical petrographic practice as well, especially when applied to rare and/or difficult to analyze specimens.