ARGYLE TYPE Ia BROWN DIAMONDS: Gemmological Properties, FTIR, UV-Vis, CL and ESR Features.

Thirty-five light brown rough diamonds from the Argyle mine (Australia) were examined, taking into consideration their surface and internal characteristics, their gemmological properties and Raman, FTIR, UV-Vis, CL and ESR features. Hexagonal depressions and etch patterns were the most common surface features observed. Internal characteristics identified included irregular cracks, dislocation planes, minerals of the eclogitic series and graphite. Anomalous birefringence due to strain effects was detected in all specimens. Absorption features in the mid infra-red (MIR) revealed a moderately low nitrogen concentration (about 700 ppm). All diamonds were found to be type IaAB and showed dominant B>A composition. Hydrogen was always present. Near infra-red (NIR) absorption spectra showed weak features ascribed to tha AC1 amber centre. The most common UV-Vis absorption features were a weak to moderate N3 peak, a broad unidentified and non-assigned peak at 482 nm and a broad 550 nm band (between 500 and 600 nm). The cathodoluminescent (CL) imaging technique revealed a general bright green emission. CL spectra highlighted two broad components, centred at 450 and 512-520 nm, with superimposed specific emission features. The electron spin resonance (ESR) spectra evidenced a weak triplet signal due to a N defect consistent with the W7 centre, closely related to the amber centre, and a more intense signal attributed to dangling C-C sp³ hybridised bonds. Taking into account the microscopic and spectroscopic features, it was considered that the examined brown diamonds developed in a moderately N-impoverished eclogitic environment. They experienced dissolution and resorption processes, suffered platelet degradation and post-growth plastic deformation.