HEAT TREATMENT AND IRRADIATION OF YELLOW TOURMALINE FROM EAST AFRICA

The heat treatment of tourmaline has been widely studied, as the exposure to high temperatures results in an improvement in the clarity, color, and value of the mineral (Rossman 2007, Ertl 2007, Nassau 1975). Many attribute the color of yellow tourmaline to manganese 2+ dispersed throughout the crystal matrix. When this manganese is exposed to high temperatures, oxidation occurs and the Mn 2+ is converted into Mn 3+. This change in valence is generally also associated with a color change from yellow to a deep red. In order to assess potential improvements in the yellow tourmaline from a new East African deposit, heat treatments were performed at a range of temperatures. The tourmaline was also irradiated by being placed in the presence of natural uranium-bearing minerals for several months to test whether the gamma radiation from the uranium would induce the oxidation of the Mn, by displacing an electron. The composition of the tourmaline was measured using an X-ray fluorescent spectrometer (XRF), as well as a scanning electron microscope (SEM). The color change was characterized using a VIS/NIR GL Gem spectrometer. Results from these heat treatments have yielded similar outcomes to others’ previous experiments, with greater color change coming from higher temperatures for extended periods of time. Little to no color change was recorded in the radiation experiments. Only small amounts of manganese, roughly 40-200 ppm, were found in the XRF analysis of the stones, in contrast to the Mn-rich tourmaline in others’ experiments which contained 3-8 wt.% MnO. The XRF was calibrated and programmed to specifically measure Mn, and counting took place for 300 seconds at the peak, and for 30 seconds each on two backgrounds. Ten points were included on the calibration curve with standards varying from 0-4300 ppm Mn. The low amounts of Mn that were found could imply that much less than 3 wt.% MnO is needed to color these tourmaline gemstones, or that another element is the cause of the coloration.