Abstract
In this work a rare and attractive blue diopside present in Sissone valley in the Western Alps was investigated through different methodologies: geological survey; standard gemological methods; X-Ray Powder Diffraction; SEM observations; Raman spectrometry; EMP analyses of major elements; and LA-ICP-MS analyses for minor and trace elements. The host rock of investigated gems is represented by a Mg-calcite bearing marble, belonging to the Suretta nappe and composed of blue diopside, lizardite, phlogopite, forsterite, Ca-Mg-amphibole, and thomsonite; the rock was metamorphosed by the intrusion of Masino-Bregaglia pluton. The diopside is generally found in the core of veins in contact with green–blue tremolite and, more externally, with green–yellowish lizardite. The diopside samples show opaque diaphaneity, are inert to long and short-waves UV radiation, and their specific density varies between 3.24 and 3.30 g/cm3 while medium refraction between 1.680–1.683. The diopside shows a polycrystalline texture with interstitial Mg-calcite which acts as binder. The characteristic blue–turquoise color is mainly determined by traces of V and subordinately 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 blue diopside of the Sissone valley could certainly present a good commercial value, but unfortunately it is difficult to reach the outcrop sites.
Highlights
Diopside is a Ca and Mg silicate with formula CaMg(Si2 O6 ) belonging to the pyroxene group.This mineral is generally found in the form of prismatic crystals with a square or octagonal rectangular section or as granular or fibrous-radial aggregates of light green, blue, whitish, yellowish or dark brown color
Cabochon 2 is totally made up of diopside and shows a very nice blue–turquoise color; the other gems consist of a more complex association made up by blue diopside with amphibole, Mg-calcite, lizardite (Table 2) which determines a more variable color from light blue to blue/greenish blue, and from light grey to grey.The diaphaneity is always opaque; all the samples are inert to long- and short-wave UV
The various modal proportions of the minerals associated with diopside contribute to the variation of the specific density and refraction indices
Summary
Diopside is a Ca and Mg silicate with formula CaMg(Si2 O6 ) belonging to the pyroxene group.This mineral is generally found in the form of prismatic crystals with a square or octagonal rectangular section or as granular or fibrous-radial aggregates of light green, blue, whitish, yellowish or dark brown color. It has a Mohs hardness between 5.5 and 6.5; it is fragile and perfectly flaked according to the two orthogonal planes [100] and [010]. The most important chromophore elements of the gem-quality diopside are Cr and Fe, which give
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