Abstract
We report the results of a high-pressure high-temperature (HPHT) experimental investigation into the deformation of diamonds using the D-DIA apparatus. Electron backscatter diffraction (EBSD) data confirm that well-defined 300–700nm wide {111} slip lamellae are in fact deformation micro-twins with a 60° rotation around a <111> axis. Such twins formed at high confining pressures even without any apparatus-induced differential stress; mechanical anisotropy within the cell assembly was sufficient for their formation with very little subsequent lattice bending (<1° per 100μm). When apparatus-induced differential stresses were applied to diamonds under HPHT conditions, deformation twin lamellae were generated, and continuous and discontinuous crystal lattice bending occurred (4–18° per 100μm), including bending of the {111} twin lamellae. The {111} <011> slip system dominates as expected for the face-centred cubic (FCC) structure of diamond. Slip occurs on multiple {111} planes resulting in rotation around <112> axes. Deformation microstructure characteristics depend on the orientation of the principal stress axes and finite strain but are independent of confining pressure and nitrogen content. All of the uniaxially deformed samples took on a brown colour, irrespective of their initial nitrogen characteristics. This is in contrast to the two quasi-hydrostatic experiments, which retained their original colour (colourless for nitrogen free diamond, yellow for single substitutional nitrogen, Type Ib diamond) despite the formation of {111} twin lamellae. Comparison of our experimental data with those from two natural brown diamonds from Finsch mine (South Africa) shows the same activation of the dominant slip system. However, no deformation twin lamellae are present in the natural samples. This difference may be due to the lower strain rates experienced by the natural samples investigated. Our study shows the applicability and potential of this type of analysis to the investigation of plastic deformation of diamonds under mantle conditions.
Highlights
Diamonds, along with the mineral and fluid inclusions that they may carry, represent the deepest samples of the Earth's mantle that can be found at the surface [1]
The results show that the deformation DIA (D-DIA) technique can deform diamonds at temperatures and pressures relevant to nature, and demonstrate the usefulness of Electron backscatter diffraction (EBSD) analysis for the quantitative investigation of plastic deformation in diamonds
We suggest that the higher strain rate resulted in proportionally higher dislocation density, while the rapid recovery from the high pressure and high temperature (HPHT) conditions resulted in the retention of the high-angle boundaries
Summary
Along with the mineral and fluid inclusions that they may carry, represent the deepest samples of the Earth's mantle that can be found at the surface [1] Their economic and scientific importance means that they are one of the most intensively studied minerals. Diamond crystals show a range of different morphologies, growth mechanisms, colours and textures, all of which can affect their economic value. Some of these characteristics are interdependent, for example brown. As observed in other face-centred cubic (FCC) materials, the main slip system in diamond is {111} b110> This means that the {111} planes are the active slip planes with movement in the b 110> directions. In deformation experiments performed on diamonds under vacuum at high temperature [8] as well as at high pressure and high temperature (HPHT; [9]), the {111} lamellae produced
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