Tourmaline is a chemically complex semi-precious gem ((Na,Ca)(Fe, Mg, Mn, Li, Ti, Cr, Cu)3Al6(BO3)3Si6O18(OH,F)4). Numerous ionic substitutions cause the large variation in color and within crystal zoning patterns that have given tourmaline its popularity in the gem industry. Tourmaline is relatively abundant and found world-wide. The determination of country of origin is an important factor in gem pricing; our earlier work demonstrates that multivariate analysis of Laser-Induced Breakdown Spectroscopy (LIBS) spectra can determine provenance on less chemically variable minerals such as ruby, sapphire, and emerald. The purpose of this study is to determine the provenance of green tourmaline as a test of LIBS capability for analysis of one of the most chemically complex gems.

Green tourmaline samples from five countries (Afghanistan, Brazil, Madagascar, Nigeria, USA) were analyzed at Materialytics. M2S hardware consists of dual Nd:YAG lasers, an intensified CCD camera, an Echelle spectrometer, and computer software designed to maximize the signal obtained from a geomaterial while minimizing the traces of testing. A minimum of 30 tourmaline samples from each locality were analyzed; individual spectra consist of the intensities of 40,000 wavelengths between 200 and 900 nm. The M2S software generates a model of spectral sequences to which three spectra from each unknown tourmaline are compared. Success rates, calculated as the percent of correct identifications, are as follows: Afghanistan: 100%; Brazil: 94%; Madagascar: 99%; Nigeria: 100%; USA: 98%. Provenance determinations using traditional gemological methods (UV-VIS-NIR absorbance/transmission, inclusions, gemological features, and LA-ICP-MS) are reported as professional opinions. The M2S system has the potential to quantify gem provenance identification.

Date Presented:  July, 2011
Conference Presented: NASLIBS 2011

Authored By:
1. McManus, Catherine E., Likes, Tristan M.
2. Buckley, Steven G., Lithgow, Greg
3. McMillian, Nancy J.

Author Affiliations:
1.  Materialytics, LLC, P.O. Box 10988, Killeen, TX 76547
2.  Photon Machines, Inc. Redmond, WA 98052
3.  Geological Sciences, New Mexico State University, Las Cruces, NM 88003