Gem-Quality Tourmaline from LCT Pegmatite in Adamello Massif, Central Southern Alps, Italy: An Investigation of Its Mineralogy, Crystallography and 3D Inclusions

In the early 2000s, an exceptional discovery of gem-quality multi-coloured tourmalines, hosted in Litium-Cesium-Tantalum (LCT) pegmatites, has been made in the Adamello Massif, Italy. Gem quality tourmalines were never found before in the Alps and this new pegmatitic deposit is of particular interest and worthy of a detailed characterization. We have studied a suite of faceted samples by classical gemmological methods and fragments were studied with Synchrotron X-ray computed micro-tomography that evidenced the occurrence of inclusions, cracks and voids. Electron Microprobe combined with Laser Ablation analyses were performed to determine major, minor and trace element contents. Selected samples were analysed by single crystal X-ray diffraction method. The specimens range in colour from colourless to yellow, pink, orange, light blue, green, amber, brownish-pink, purple and black. Chemically, the tourmalines range from fluor-elbaite to fluor-liddicoatite and rossmanite: these chemical changes occur in the same sample and affect the colour. RE elements vary from 30 to 130 ppm with steep LREE- enriched patterns and a negative Eu-anomaly. Structural data confirmed the elbaitic composition and showed that high manganese content may induce the local static disorder at the O(1) anion site, coordinating the Y cation sites occupied on average by Li, Al and Mn2+ on equal proportions, confirming previous findings. In addition to the gemmological value, the crystal-chemical studies of tourmalines are unanimously considered as a sensitive recorder of the geological processes leading to their formation, and, therefore, this study may contribute to understand the evolution of the pegmatites related to the intrusion of the Adamello pluton.