T20 Gemological Research in the 21st Century—Characterization, Exploration, and Geological Significance of Diamonds and other Gem Minerals

Sunday, 22 October 2017
8:00 AM - 12:00 PM
Skagit 3

SPATIAL MAPPING OF TEMPERATURE AND IRRADIATION EFFECTS IN NATURAL TYPE IIB DIAMOND

Paper number: T20-4
Program: Technical Sessions
Time: 8:40 AM – 8:55 AM

Authors

Abstract

Blue diamonds are among the rarest and most valuable of naturally occurring gemstones. While Vis-NIR absorption spectra for these diamonds are relatively featureless, PL spectroscopy can provide a wealth of information, particularly on defect centers that are only detected in type IIb diamonds. In this study, two adjacent sections were laser cut from a rough naturally-sourced type IIb diamond. One was subjected to electron irradiation along one edge. Subsequently, both were stepwise annealed from 300oC to 1200oC and the optical defects were documented by changes in IR absorption and photoluminescence spectroscopic mapping.

In FTIR absorption, the principal absorption of uncompensated boron (B0; single, substitutional boron atoms acting as acceptors) is at 2802 cm-1. The total boron concentration within diamond (as determined by a different measurement such as SIMS) should not be misinterpreted with the B0concentration as determined by IR absorption. Donors within diamonds (such as nitrogen-related defects) can compensate other boron atoms and render that portion of the total boron concentration as optically and/or electrically inactive. In these experiments, the irradiation and subsequent annealing of one of these samples created a gradient in B0 across the diamond sample that was chronicled with the spatial mapping. The gradient in B0 changed dramatically as the sample was annealed due to changes in compensating defects.

PL spectroscopic mapping provided some insight into some of those compensating defects as several interstitial-related centers (TR12, 3H, and 648.2 nm) and vacancy-related centers (NV0, GR1, and 776.4 nm among others) were detected and chronicled. Spatially, the intensity gradient for these defects corresponded with the non-uniform irradiation.

The unirradiated sample showed comparatively minor changes in B0 with annealing. Additionally, the PL optical defects, if detected, showed lower intensities with annealing than its irradiated counterpart. The combination irradiation, annealing, and spatial mapping provides insights into the optical centers affecting this valuable material.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-298538

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