An attractive blue diopside from Sissone Valley, western Alps, Italy

In this work a rare and attractive blue diopside present in Sissone valley in the WesternAlps was investigated through different methodologies: geological survey; standard gemologicalmethods; X-Ray Powder Diffraction; SEM observations; Raman spectrometry; EMP analyses of majorelements; and LA-ICP-MS analyses for minor and trace elements. The host rock of investigatedgems is represented by a Mg-calcite bearing marble, belonging to the Suretta nappe and composedof blue diopside, lizardite, phlogopite, forsterite, Ca-Mg-amphibole, and thomsonite; the rock wasmetamorphosed by the intrusion of Masino-Bregaglia pluton. The diopside is generally found inthe core of veins in contact with green-blue tremolite and, more externally, with green-yellowishlizardite. The diopside samples show opaque diaphaneity, are inert to long and short-waves UVradiation, and their specific density varies between 3.24 and 3.30 g/cm3 while medium refractionbetween 1.680-1.683. The diopside shows a polycrystalline texture with interstitial Mg-calcite whichacts as binder. The characteristic blue-turquoise color is mainly determined by traces of V andsubordinately of Fe, Mn, Cr and Ti. The contents of V and Ti show a good positive correlation.The minerals associated with diopside in the lenticular veins also show enrichments in V. The bluediopside of the Sissone valley could certainly present a good commercial value, but unfortunately itis difficult to reach the outcrop sites.