WHAT LASER-INDUCED BREAKDOWN SPECTROSCOPY CAN TELL US ABOUT MINERAL COMPOSITION: THE BERYL CASE STUDY

Laser-induced breakdown spectroscopy (LIBS) is a rapid, field-ready analytical technique that provides immediate and minimally-destructive chemical analyses of solids, liquids, or gases. Broadband LIBS of minerals captures the spectrum (200 to 980 nm) of photons emitted by a plasma produced during laser ablation of the mineral surface. The spectrum contains information about essentially the entire periodic table, although LIBS is more sensitive for the light elements.

In this study, LIBS analyses of 135 beryl samples (inclusion-bearing and gem-quality) from 20 countries (11 US states) and Antarctica are used to investigate geologically significant applications of LIBS technology. Although LIBS spectra can be calibrated to determine certain elements of interest, specifically the very light elements B, Be, and Li, at ppm levels, this study focuses on applications that use the broadband spectra: stoichiometric substitutions, gem provenance, and identification of individual gem stones.

The complex stoichiometry of beryl is reflected in LIBS analyses. In beryl (Be₃Al₂Si₆O₁₈), substitution of Li⁺ for Be²⁺ and divalent ions for Al³⁺ is compensated in part by the presence of water, hydroxyl, and alkali ions (Na⁺, K⁺, Rb⁺, and Cs⁺) in open channels parallel to the c-axis. These variations are seen in the beryl broadband spectra, where positive correlations exist between Li, Mn, and Fe peak intensities and those of Na and K.

Broadband spectra are perhaps most useful for comparison of the overall chemical signatures of samples, with applications in the unique identification of individual stones for insurance purposes and determining the provenance of individual samples. Regression of single-shot broadband spectra of gem-quality beryls against beryl spectra in our database indicates that LIBS analysis can uniquely identify individual stones with a 86% success rate. The question of provenance is complicated by complex processes in pegmatites; however, beryls in our collection show a broad affinity to their craton (Laurentia, Gondwana, Baltica, and the mobile belts of western North America and South America).