New columnar texture of carbonado: Cathodoluminescence study

We investigated a very unusual sample of carbonado by optical and electron microscopy, and cathodoluminescence imaging and spectroscopy. We observed two textures: a microporphyritic texture, and columnar transparent diamonds growing perpendicularly.We propose that this sample grew in three stages: the slow growth of large porphyrocrysts (step 1), followed by a fast growth period that produced the fine-grained cement (step 2), and ultimate growth of columnar diamond perpendicular to the initial material (step 3). The first two steps growth have been extensively documented in the literature, and the third one is described for the first time in carbonado. The classical texture suggests a growth from flowing fluids while the columnar texture evidences diamond development in an open fracture.The orange (N–V0 center) and green (N2V center) cathodoluminescence observed around pores, grain boundaries and fractures throughout the sample is due to irradiation of uranium-rich fluids. As uranium was not mobile at the time of carbonado formation (between 3.8 and 2.6 Ga), we propose that irradiation occurred most likely long after diamond growth. The blue CL of the porphyrocrysts and of the columnar diamond, preserved from late transient irradiation, is due to N3V centers (and, to a lesser extent, N2V centers). It is a primary feature which reveals a certain degree of nitrogen aggregation. This is more likely related to some high-temperature events that remain to be determined.