THE GEOLOGIC NARRATIVES OF GEM TOURMALINES: IMPRINTS OF CRUSTAL EVOLUTION (Invited Presentation)

The romance and adventure of gemstones is of great historic and economic importance. An additional compelling narrative is the geologic history embedded in the stones prior to mining and transformation into a gem. Many gemstones acquire chemical and textural features that, when analyzed and interpreted, reveal their unique story of geologic evolution. Tourmalines are particularly sensitive to the geologic environment in which they grow by incorporating and retaining chemical and textural signatures reflecting their geologic evolution. In addition, crystallographic features such as surface energetics of specific tourmaline faces overlay an additional chemical complexity. Three examples highlight the distinctive narratives embedded in gem tourmalines. (1) The “neon-blue” Paraiba and Paraiba-like tourmalines, the most expensive of the tourmaline gemstones, are elbaitic and liddicoatitic species, and are characterized by significant amounts of Cu (0.5-4.5 wt% CuO) that serves as a chromophore. Development of such tourmalines requires an unusual scenario in which a highly fractionated (Li- and B-rich) pegmatite is emplaced into Fe-poor host rocks. In addition, these tourmalines can only develop when the zones are devoid of chalcophile minerals (e.g. sulfides) that would typically incorporate Cu. Consequently, significant Cu is available for substitution in tourmaline. (2) Slabs of liddicoatites from Madagascar typically exhibit oscillatory- and sector-zoning observed as tens to hundreds of highly repetitive color bands that change in character where developed on different faces. In these tourmalines, the geologic narrative begins with tourmaline crystallization in an anomalous Ca- and Li-rich pegmatite i.e. Ca was not removed from the melt by other calcic minerals. Coupled with the general monotonic changes in chemistry with growth, distinct oscillatory bands develop in a non-random (fractal) manner, likely related to surface properties of individual faces as well as the local melt. (3) The gemmy deep-green “chrome” tourmalines from Tanzania are typically V-bearing uvites and dravites with trace amounts of Cr. They are hosted in high-grade metamorphosed calcareous rocks and probably develop from B sourced in the original sediment. Each gem tourmaline embeds signatures of crustal evolution.