Cathodoluminescence of Natural, Plastically Deformed Pink Diamonds

Abstract

The 49 type I natural pink diamonds examined exhibit color restricted to lamellae or bands oriented along {111} that are created by plastic deformation. Pink diamonds fall into two groups: (1) diamonds from Argyle in Australia and Santa Elena in Venezuela are heavily strained throughout and exhibit pink bands alternating with colorless areas, and (2) diamonds from other localities have strain localized near the discrete pink lamellae. Growth zones are highlighted by a blue cathodoluminescence (CL) and crosscut by the pink lamellae that emit yellowish-green CL that originates from the H3 center. This center probably forms by the recombination of nitrogen-related centers (A-aggregates) and vacancies mobilized by natural annealing in the Earth's mantle. Twinning is the most likely mechanism through which plastic deformation is accommodated for the two groups of diamonds. The plastic deformation creates new centers visible through spectroscopic methods, including the one responsible for the pink color, which remains unidentified. The differences in the plastic deformation features, and resulting CL properties, for the two groups might correlate to the particular geologic conditions under which the diamonds formed; those from Argyle and Santa Elena are deposits located within Proterozoic cratons, whereas most diamonds originate from Archean cratons.

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

“…The higher-aggregation population is dominated by stones from Argyle, with a few examples from Namibia's combined onshore alluvial deposits and offshore dredging operations. The characteristics of these stones are consistent with those already defined as group 1 pink diamonds by Gaillou et al (2010Gaillou et al ( , 2012. The lower-aggregation category contains diamonds from several localities including South African productions (Finsch and De Beers Pool are kimberlite deposits; Koingnass and Tweepad are alluvial), Canada's Victor mine (kimberlite), and other stones from the Namibian productions.…”

“…Based on the FTIR data and the color distribution in the six pink diamonds, it is clear that they can be separated into the two groups defined by Gaillou et al (2010Gaillou et al ( , 2012. Samples A62-06, A167-P1, and A62-12 have the characteristics of group 1 pink diamonds: low nitrogen concentrations with high aggregation states; less well-defined color graining that correlates poorly with features observed in the grainy CL; two of them come from the Argyle mine.…”

“…However, Gaillou et al (2012) did note a significant connection though that will probably form the basis of future investigations into the geographical source and conditions of formation for group 1 pink diamonds. A comprehensive study of more than 5000 diamond crystals and fragments from the Guaniamo area (which contains the Santa Elena alluvial deposit) by Kaminsky et al (2000) noted three significant similarities with those from Argyle: (1) a high proportion of eclogitic inclusions (2) very high mean temperatures of diamond formation (based on P-T estimates from the inclusions), and (3) high levels of Ti and Na in garnet, and Ti and K in clinopyroxene inclusions in the diamonds.…”

“…To do this we report FTIR and EBSD data from a collection of naturally deformed Type I diamonds of both pink and brown color with varying nitrogen contents and aggregation states. We also provide a large, previously unpublished, FTIR data set for pink diamonds from various geographical sources to supplement the existing data in the literature and to test the groupings of Type I pinks by Gaillou et al (2010Gaillou et al ( , 2012.…”

“…Recently, extensive categorization of Type I (i.e., nitrogen containing) gem-quality pink diamonds by Gaillou et al (2010Gaillou et al ( , 2012 identified two distinct groups. The primary crystallographic and spectroscopic features of these two groups of diamonds are listed in Table 1.…”