LA-ICP-MS Imaging Analysis Of Gem-Quality Tourmaline: A Novel Method For Direct Identification Of Chromophore In Gemstone Samples

Abstract

The color of gemstones is a vital factor in the determination of their value in the gem market. The elemental content and local distribution of chromophores within gem samples play an important role in their coloration. In this study, the chemical compositions and two-dimensional (2D) element distributions within a gem-quality color-zoned tourmaline were investigated using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The elemental distribution results indicated that major elements (i.e., B, Na, Mg, Al, Si, and Ca) were uniformly distributed in different color regions. Moreover, the contents of the transition elements Cr and V correlated well with the colors of the zoned tourmaline crystal. The highest concentrations of Cr and V were found in the intense green zones of the crystal, whereas the pale green regions contained lower concentrations of these elements. Furthermore, a strong correlation between the Cr distribution and complex color zones was observed. The visual results obtained using LA-ICP-MS imaging analysis clearly demonstrated that the presence of Cr and V contributed to the observed green color. In addition, Cr was determined to be the principal chromophore in the green-colored zoned tourmaline. The ultravioletvisible (UV–Vis) and near infrared (NIR) absorption spectra also indicated that the mixing of Cr as well as minor V caused the green color. The uniformity of the elemental distributions obtained using LA-ICP-MS imaging provides clear evidence that Cr can dominate the chromophore content in tourmaline. The results of this study clearly highlight the advantages of LA-ICP-MS imaging analysis as a novel and effective visual method to identify the chromophore contents in gemstone samples.