Carbonado revisited: Insights from neutron diffraction, high resolution orientation mapping and numerical simulations

Abstract

One of the most controversial diamond types is carbonado, as its origin and geological history are still under debate. Here, we investigate selected carbonado samples using neutron diffraction and high resolution orientation mapping in combination with numerical simulations. Neutron diffraction analyses show that fine grained carbonado samples exhibit a distinct lack of crystallographic preferred orientation. Quantitative crystallographic orientation analyses performed on transmission electron microscope (TEM) sections reveal that the 2–10μm grains exhibit locally significant internal deformation. Such features are consistent with crystal plastic deformation of a grain aggregate that initially formed by rapid nucleation, characterized by a high number of nucleation sites and no crystallographic preferred orientation. Crystal plastic deformation resulted in high stress heterogeneities close to grain boundaries, even at low bulk strains, inducing a high degree of lattice distortion without significant grain size reduction and the development of a crystallographic preferred orientation. Observed differences in the character of the grain boundary network and internal deformation structures can be explained by significant post-deformation annealing occurring to variable degrees in the carbonado samples. Differences in intensity of crystal bending and subgrain boundary sharpness can be explained by dislocation annihilation and rearrangement, respectively. During annealing grain energy is reduced resulting in distinct changes to the grain boundary geometry. Grain scale numerical modelling shows that anisotropic grain growth, where grain boundary energy is determined by the orientation of a boundary segment relative to the crystallographic orientation of adjacent grains results in straight boundary segments with abrupt changes in orientation even if the boundary is occurring between two triple junctions forming a “zigzag” pattern. In addition, in diamond anisotropic grain growth results in triple junctions that rarely show 120° angles.Our results support the interpretation that carbonados may have undergone at least 2 or 3 stages of development with rapid nucleation, crystal plastic deformation to low strains and variable degrees of post-deformation annealing. Such a history is commonly observed in Earth's crustal or mantle rocks. Hence, for carbonados it is not necessary to invoke an extraordinary and/or extraterrestrial origin and history.The combination of methods utilized here, promises to help advance our understanding of diamond and diamond aggregates in the future.