The gem-quality tourmaline occurring in the Adamello Park natural reservation, Central Southern Alps, Italy

An exceptional discovery of gem-quality multi coloured tourmalines hosted in Litium-Cesium-Tantalum (LCT) pegmatites, has been made in the Adamello Park natural reservation (Adamello Massif, Italy), in the early 2000s. The area of the finding (Adamé valley) is included within the limits of a special natural reservation of the Adamello Park, where minerals collecting is forbidden; therefore, in consideration of the scientific importance of the discovery, a joint project of benefit to science and to museum preservation was organized involving the Museum of Natural History of Milan and the regional natural Park (Pezzotta and Guastoni, 2002). Gem quality tourmalines were never found before in the Alps and this new pegmatitic deposit is of particular interest for the better understanding of the petrologic and geochemical evolution of the Adamello tonalitic intrusion and its hosting rocks. Recent literature pointed out how tourmaline, in view of the large numbers of interchangeable elements in its crystal structure, is exceptionally sensible to chemical and physical changings during crystallization, allowing detailed petrologic investigations about the formation environment. Field investigations showed the presence of polychrome tourmaline crystals occurring in pegmatite dikes hosted in the hornfels derived by the contact metamorphism between the Adamello tonalite and the Mesozoic sedimentary sequence. In order to study the original conditions of formation and the paragenesis, accurate sampling and survey of such pegmatites were performed. The Natural History Museum in collaboration with the National Research Council and the Department of Earth Sciences of the University of Milan, started a series of mineralogical, petrological and gemmological investigations of the collected material. The preliminary results on a suite of gem-quality faceted tourmaline and raw material are here presented. Classical gemmological studies have been performed on faceted stones (0.77-1.22 ct) of green to brown colour. Electron microprobe analyses combined with laser ablation-inductively coupled plasma-mass analyses allowed the determination of the contents of the major, minor and trace elements, showing the occurrence of mainly elbaite compositions, with lesser amounts of rossmanite, foitite and fluor-liddicoatite. This work is included in the Adamello Park activities related to the management of geoheritage and focused on the protection and conservation of minerals. An exhibit of part of the recovered material is now open to public at the Casa-Museo of the natural Park in Cevo (BS). The remaining mineral and rock specimens are preserved in the collections of the Natural History Museum of Milan.