HPHT treatment of CO 2 containing and CO 2-related brown diamonds

Abstract

Ten type I diamonds containing CO 2 and three diamonds related to these (referred to as “Pseudo CO 2” diamonds) were treated by the HPHT process to observe changes in colour, colour distribution, inclusions, luminescence and spectral features in the visible to the mid-infrared regions. All samples were of predominantly brown colour before the treatment and had rather inhomogeneous colour distribution not related to strain like in classic type Ia brown diamonds. Upon HPHT treatment most of them changed to a more yellow hue, with a greenish modifying colour, but the dominant colour was still brown. The modification of colour was found to be much more discreet than it is the case for deformation-related type Ia brown diamond and the colour was not at all related to known centers such as H3. The colour modifications were caused by the destruction of broad absorption bands in the Vis–NIR spectrum by the HPHT process and generally increased transmission from 400 to 700 nm, likely related to a slight increase in single nitrogen, but no typical HPHT annealing related colour centers such as H3 and H2 were produced in the spectra of the samples. This could be confirmed by the luminescence when excited by shortwave and longwave UV light, under which all samples showed a yellow emission that was practically not modified by the treatment. The infrared spectra of the 10 CO 2 diamonds, defined by the presence of at least the CO 2 stretching ( ν 3) and bending ( ν 2) absorptions, were unusually modified by the HPHT treatment: the treatment caused the CO 2 bands to increase in intensity in diamonds with low to medium CO 2 content, and the apparently diminished the intensity of the nitrogen-related one phonon absorptions (when present). In the spectra of the three “Pseudo CO 2” samples, which lack by definition the CO 2 peaks, the CO 2 peaks were created and the one phonon absorption reduced by the treatment. The photoluminescence spectra exhibiting several unpublished emission peaks remained practically unchanged by HPHT processing: none of the emissions were annealed out and no new centers created. The data obtained by HPHT treatment leads us to propose, that the theory of solid CO 2 inclusions being responsible for the CO 2 absorptions in diamond may be erroneous, and that the CO 2 may be present structurally. Thus it is proposed that structurally bonded oxygen may play an important role in such diamonds. The present study indicates that treated CO 2 and “Pseudo CO 2” diamonds can in most cases not be identified by the methods available in well-equipped gemmological laboratories, since no detectable defect centers appear to be induced by the treatment.