A COMPOSITIONAL ANALYSIS OF ZONED WATERMELON TOURMALINES USING LASER-INDUCED BREAKDOWN SPECTROSCOPY (LIBS)

The term watermelon tourmaline describes a variety of tourmaline that is primarily elbaite and/or liddicoatite with the distinctive zoning pattern of a pink core and green rim. This project examines compositional variations in these zoning patterns by using laser-induced breakdown spectroscopy (LIBS). LIBS is a laser ablation method that diffracts light emitted by atoms as excited electrons decay to lower-energy orbitals during cooling of a laser ablation plasma. Because all elements emit light during the LIBS process, it is possible to obtain a detailed record of changes in tourmaline composition related to changes in the environment of crystallization.

The pink and green zones of watermelon tourmalines from the Otjua Mine in Namibia were analyzed using LIBS. Entire spectra were analyzed using the multivariate technique Principal Component Analysis (PCA). PCA discerns relationships within a complex data set and reduces the variability to a manageable number by calculating linear regressions through the data set, called principal components. A PCA score plot is a scatter plot in which data points are plotted against two user-selected principal components (PC); spectra that cluster together are compositionally similar and those that are far apart are dissimilar. PCA loading plots show the relative influence of each variable (i.e., wavelength) on the direction of the PC through the dataset. Loading plots can be data-mined to learn which elements are more abundant in the groups of minerals in PCA.

Thirty spectra were acquired from each of the pink and green zones of the sample; these spectra were averaged by five to create six spectra for each zone for use in PCA. Pink zones have higher concentrations of Li, Si, Na, Al and Ca; green zones are enriched in Mg, Fe, Mn, and H. Mn is commonly purported to be the chromophore in pink watermelon tourmaline zones; however, it appears here to be concentrated in the green zone. Similar results were observed by Tollefson and Ihinger (2018 GSA Abstracts with Prog.) with the possible explanation that the oxidation state of Mn, rather than concentration, plays a large role in producing color. Also, the observation that H has higher abundance in the outer green rim is interesting. Further analysis is required to determine whether these compositional trends hold true for various localities.