Paper No. 26-10
Presentation Time: 1:30 PM-5:30 PM
THE COLOR AND CLARITY OF FELDSPARS: EXPERIMENTS IN HEAT TREATING
Heat treating is a common practice in the gemstone industry to improve color and clarity of many different types of minerals. Feldspars, which are common in the Earth’s crust, are sometimes used as gemstones if they are high enough quality. The purpose of this study is twofold: (1) to determine the effects of heat treating on feldspars and, thus, the effects on aesthetic value; and (2) to determine if the hues of amazonite correlate to the concentrations of Pb and water in the mineral. Four different types of feldspars were heat treated: heliolite (sunstone), hecatolite (moonstone), bytownite, and peach hecatolite (peach moonstone). In each experiment, part of the original sample was left unheated for comparison. Samples of the four minerals were heated in a muffle furnace to different temperatures (400-600 °C) and for different durations (30min.-1hr.). The color changes in the samples were subsequently analyzed using a VIS/NIR GL gem spectrometer. The four samples that were heat treated all lost some of their original color. The heat treated samples ended up having a lighter color compared to the non-heat treated samples. When the samples were heated at higher temperatures for a longer duration, the samples experienced loss in color, and cloudiness increased. Although many gemstones can experience improved color and clarity through heat treating, I found no specific evidence indicating that heat treating can improve feldspars. The second part of this study focused on amazonite, a variety of microcline with a characteristic blue/green color. The amazonite samples were crushed up and analyzed using XRF spectroscopy. Results from the colorization of microcline have yielded similar outcomes to previous research by others (Rein et al, 2006), with no apparent correlation between hue and Pb content. Therefore, the lead content in the samples had no effect on the hue. Rather, samples with high concentrations of water in them showed an apparent correlation between color intensity and the amount of lead present. The Pb concentration shows no correlation to hue, but the Pb content does correlate with color intensity if there are also high concentrations of water.