Pink color in Type I diamonds: Is deformation twinning the cause?

Abstract

Plastic deformation of diamond has long been associated with the generation of color, specifically brown and pink. Extensive previous optical and spectroscopic characterization of natural pink Type I (nitrogen containing) diamonds has revealed two clear groupings, with distinct geographical origins. Group 1 pinks, which have low concentrations of nitrogen and are relatively highly aggregated (IaA ≤ B), have only been found in the Argyle lamproite pipe (Australia) and Santa Elena alluvial deposits (Venezuela). Group 2 pinks, which have much higher nitrogen concentrations and exhibit low levels of aggregation, have been found in deposits from southern Africa, Canada, and Russia. Pink color is intimately associated with deformation lamellae on the {111} crystal planes, and understanding their formation and structure has been a priority with respect to defining the source of this gemologically valuable color center. In group 2 pinks, these {111} lamellae have been characterized as deformation microtwins by both transmission electron microscopy and X-ray diffraction. Subsequently the {111} lamellae in group 1 pinks have been assumed to also be deformation microtwins. In this paper we report electron backscatter diffraction (EBSD) studies of three brown and six pink naturally deformed diamonds with varying nitrogen concentrations and aggregation states. The results show that there are no deformation microtwins in the group 1 pink or brown diamonds. The study also highlights the usefulness of orientation contrast imaging as a simple and rapid method for determining the presence of microtwins. Our results suggest that the color in the group 1 pink diamonds is not directly related to the presence of deformation twins. However, we propose that twins may have been present but subsequently removed by de-twinning, a process that utilizes the same Shockley partial dislocations involved in the original twinning event. Therefore, it may be the process of twinning (and de-twinning) that creates the defect responsible for pink color, as opposed to the actual structure of microtwins themselves. In addition, a large laboratory data set of pink diamond analyses reveals the occurrence of group 1 pink diamonds in the Namibian marine (secondary) deposits. This would appear to suggest an additional source of group 1 pink diamonds in southern Africa, but the antiquity of these diamonds means that a common source on the former Pangaea supercontinent cannot be ruled out.

The Unique Nature of Argyle Fancy Diamonds: Internal Structure, Paragenesis, and Reasons for Color

“…An electron backscatter diffraction study of group 1 pink diamonds, however, found no evidence of microtwins (Howell et al, 2015). Howell et al (2015) suggest that twins were originally present and that they were removed by a subsequent, almost identical detwinning process. This would imply that it is the deformation process (twinning/detwinning) that creates the defect responsible for the pink color, not the structure of the microtwins themselves.…”

“…Extensive global study of the natural pink type I (N-containing) diamonds reveals two clear groupings related to their localities of origin. Group 1 pink stones have low concentrations of highly aggregated N (IaA < B), and they have only been found in the Argyle pipe, the Santa Elena alluvial deposits, Venezuela (Gaillou et al, 2010), and the Namibian marine deposits (Howell et al, 2015). Group 2 pink diamonds, having much higher N concentrations and lower levels of N aggregation, occur in deposits from southern Africa, Canada, and Russia (Gaillou et al, 2010;Fedorova et al, 2013;Howell et al, 2015).…”

“…Group 2 pink diamonds accommodate plastic deformation by microtwinning, as detected by electron paramagnetic resonance, transmission electron microscopy, or X-ray diffraction (Mineeva et al, 2007(Mineeva et al, , 2009Gaillou et al, 2010;Titkov et al, 2012). An electron backscatter diffraction study of group 1 pink diamonds, however, found no evidence of microtwins (Howell et al, 2015). Howell et al (2015) suggest that twins were originally present and that they were removed by a subsequent, almost identical detwinning process.…”

Natural-Color Pink, Purple, Red, and Brown Diamonds: Band of Many Colors

“…Left ing a diamond's extended residency at high temperature (>900°C; DeVries, 1975;Brookes and Daniel, 2001), either in the mantle or during kimberlite eruption to the earth's surface. These deformation lamellae have been attributed to slip/glide planes (dislocation movement) and to micro-twins (Mineeva et al, 2009: Gaillou et al, 2010Titkov et al, 2012;Howell et al, 2015). In many diamonds these deformation lamellae are closely associated with brown, pink, or purple color; these colors can also appear evenly through the entire diamond or irregularly as distinct patchy areas (figure 5, C and D).…”

“…Previously, several aspects of gem-quality "pink" diamonds have been addressed, such as their gemological characteristics (e.g., King et al, 2002); their defects and associated spectroscopic features (e.g., Titkov et al, 2008;Deljanin et al, 2008;Fisher et al, 2009;Gaillou et al, 2010;Stepanov et al, 2011;Byrne et al, 2012a,b;Titkov et al, 2012;Gaillou et al, 2012;Howell et al, 2015); and reports on notable "pink" diamonds (Smith and Bosshart, 2002;King et al, 2014). Here we review the published literature and supplement it by compiling results obtained from 90,000+ natural diamonds recorded in the GIA internal database.…”