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Minerals 2018, 8(12), 593; https://doi.org/10.3390/min8120593

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

1
National Research Council, Institute for Dynamics of Environmental Processes (IDPA), Section of Milan, 20133 Milan, Italy
2
Natural History Museum, 20121 Milan, Italy
3
Department of Earth Sciences “Ardito Desio”, University of Milan, 20133 Milan, Italy
4
National Research Council, Institute of Geosciences and Earth Resources (IGG), Section of Pavia, 27100 Pavia, Italy
5
Italian Gemmological Institute (IGI), 20123 Milan, Italy
6
Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, 34149 Trieste, Italy
*
Author to whom correspondence should be addressed.
Received: 12 November 2018 / Revised: 1 December 2018 / Accepted: 7 December 2018 / Published: 13 December 2018
(This article belongs to the Special Issue Mineralogy and Geochemistry of Gems)
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Abstract

In the early 2000s, an exceptional discovery of gem-quality multi-coloured tourmalines, hosted in Litium-Cesium-Tantalum (LCT) pegmatites, was made in the Adamello Massif, Italy. Gem-quality tourmalines had never been found before in the Alps, and this new pegmatitic deposit is of particular interest and worthy of a detailed characterization. We studied a suite of faceted samples by classical gemmological methods, and fragments were studied with Synchrotron X-ray computed micro-tomography, which 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. Rare Earth Elements (REE) vary from 30 to 130 ppm with steep Light Rare Earth Elemts (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+ in equal proportions, confirming previous findings. In addition to the gemmological value, the crystal-chemical studies of tourmalines are unanimously considered to be a sensitive recorder of the geological processes leading to their formation, and therefore, this study may contribute to understanding the evolution of the pegmatites related to the intrusion of the Adamello pluton. View Full-Text
Keywords: granitic pegmatite; gem-quality tourmaline; Adamello Massif; Central Alps; Italy granitic pegmatite; gem-quality tourmaline; Adamello Massif; Central Alps; Italy
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Diella, V.; Pezzotta, F.; Bocchio, R.; Marinoni, N.; Cámara, F.; Langone, A.; Adamo, I.; Lanzafame, G. Gem-Quality Tourmaline from LCT Pegmatite in Adamello Massif, Central Southern Alps, Italy: An Investigation of Its Mineralogy, Crystallography and 3D Inclusions. Minerals 2018, 8, 593.

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