CONSTRAINS ON HIGHLY STRAINED PINK DIAMONDS BY HIGH SPATIAL RESOLUTION FTIR AND RAMAN MAPPING

Pink diamonds are among the rarest and most valuable of gems, yet the origin of the pink color is still not fully understood. The pink color is restricted to micrometer-thick lamellae or bands oriented along <111>, which are created by plastic deformation, during a post growth event. Studies showed that plastic deformation is accommodated by twinning for some pink diamonds.

The aim of this study is to better understand the physical properties of pink diamonds and how the color is generated. For that purpose, we used high spatial resolution Fourier Transform Infrared spectroscopy (FTIR) and Raman spectroscopy which gave us information about the repartition of the defects and of the remaining strain in the diamond structure, respectively.

Samples came from all around the world and are classified into 2 groups: group 1 diamonds are from Argyle in Australia and Santa Elena in Venezuela and present pink bandings; group 2 diamonds are from other localities and have very distinctive thin (~1µm) pink lamellae. All diamonds are type Ia diamonds. FTIR mapping mainly showed zoning of nitrogen defects following growth sectors, when compared to cathodoluminescence (CL) images; no FTIR features are segregated in the pink or colorless areas.

Raman spectroscopy shows that the pink areas are defective zones, with a high density of photoluminescent (PL) defects. Raman mapping revealed that the strain is mostly localized at the pink lamellae for group 2 diamonds, while is ubiquitous in group 1 diamonds. The highest amount of strain recorded was 3 GPa over 1µm in a group 2 pink diamond. At the intersection of two pink lamellae, the strain is so intense that the diamond Raman band displays 4 lines, indicating at least two highly stressed regions in the probed volume, which is the first time such a phenomenon is reported in a natural sample.

Natural diamond shows that it can accommodate a large amount of stress during plastic deformation in mantle conditions by mechanical twinning. Still, large amount of strain remains in the diamond structure, which does not seem to affect the diamond integrity. Plastic deformation creates new PL (and CL) centers and most likely also the center responsible for the pink color, which is still unidentified.