LOW-TEMPERATURE ANNEALING AND KINETICS OF RADIATION STAINS IN NATURAL DIAMOND

Radiation stains on diamonds can often be used as indicators of radiation exposure for natural diamonds while residing within the earth and to help substantiate natural color origin. These green or brown spots provide evidence that diamonds were exposed to radioactive fluids or mineral grains. The green color is caused by GR1 absorption (lattice vacancies) and exposure to high temperatures cause the GR1 absorption to decrease thus allowing the underlying brown-colored absorption of the radiation stain to be observed. The cause of the brown color remains unclear although it is presumed to be related to vacancy clusters.

In addition to the color mechanism, little is known about the rate of color change for the stains. Although they change within a few minutes at 600^oC, we know this change occurs at lower temperatures when exposed for extended time periods. Knowledge of the rates of change at different temperatures is critical for proper use of the stain color as an indicator of heat exposure.

In an effort to better understand the color change of radiation stains, several green-stained diamonds were heated from room temperature to various peak temperatures (400, 425, 450, 475, 500, 525, 550^oC) and isothermally heated for several hours.

The numerical values of lightness, chroma, and hue (LCH) within color space were determined from the collected images and used to chronicle the changes of the selected radiation stains throughout the annealing time. The data show the decrease in hue angle as the diamonds change from green or bluish green and then as they were heated that hue angle decreased and changed to the yellow and orange range.

Those values for k that were determined for all of the different diamonds; these were then plotted against the inverse of the absolute temperature. From these we then calculated the activation energy and a best fit line and also calculations for upper and lower bounds to calculate an estimate for the error.

Green stains can turn brown at realistic depths over geologic time; therefore, no major geological or temperature changes are necessary to have green and brown stains on same diamond. The green-to-brown transition of radiation stains show an experimental activation energy of 1.6 ± 0.1 eV. This result has good correlation with the theoretically determined activation energy of interstitial migration.