minerals Article An Attractive Blue Diopside from Sissone Valley, Western Alps, Italy Franca Caucia 1,*, Maurizio Scacchetti 1, Luigi Marinoni 1, Mattia Gilio 1 , Antonio Langone 2, Omar Bartoli 3, Marco Vanotti 1 and Ivano Foianini 1 1 Earth and Environmental Sciences Department, University of Pavia, Via Ferrata 1, 27100 Pavia, Italy; [email protected] (M.S.); [email protected] (L.M.); [email protected] (M.G.); [email protected] (M.V.); [email protected] (I.F.) 2 IGG-Geosciences and Earth ResourcesInstitute CNR, Via Ferrata 1, 27100 Pavia, Italy; [email protected] 3 Geosciences Department, University of Padova, Via Gradenigo 6, 35100 Padova, Italy; [email protected] * Correspondence: [email protected] 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 Citation: Caucia, F.; Scacchetti, M.; 3 Marinoni, L.; Gilio, M.; Langone, A.; radiation, and their specific density varies between 3.24 and 3.30 g/cm while medium refraction Bartoli, O.; Vanotti, M.; Foianini, I. between 1.680–1.683. The diopside shows a polycrystalline texture with interstitial Mg-calcite which An Attractive Blue Diopside from acts as binder. The characteristic blue–turquoise color is mainly determined by traces of V and Sissone Valley, Western Alps, Italy. subordinately of Fe, Mn, Cr and Ti. The contents of V and Ti show a good positive correlation. Minerals 2021, 11, 837. https:// The minerals associated with diopside in the lenticular veins also show enrichments in V. The blue doi.org/10.3390/min11080837 diopside of the Sissone valley could certainly present a good commercial value, but unfortunately it is difficult to reach the outcrop sites. Academic Editors: Thomas Hainschwang and Frederick Keywords: diopside; LA-ICP-MS; Sissone valley; blue gemstone; chromophore elements; Saman spec- Lin Sutherland troscopy Received: 31 May 2021 Accepted: 26 July 2021 Published: 1 August 2021 1. Introduction Publisher’s Note: MDPI stays neutral Diopside is a Ca and Mg silicate with formula CaMg(Si2O6) belonging to the pyroxene with regard to jurisdictional claims in group.This mineral is generally found in the form of prismatic crystals with a square or published maps and institutional affil- octagonal rectangular section or also as granular or fibrous-radial aggregates of light green, iations. 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]. Diopside generally occurs in several contact-metamorphic rocks such as metamorphosed siliceous limestones, dolomites and skarns, in ultramafic igneous rocks such as kimberlites, Copyright: © 2021 by the authors. peridotites, and also rodingites [1–3]. Diopside also occurs in basic-ultrabasic rocks as Licensee MDPI, Basel, Switzerland. picritic lava [4] and basaltic lava [5]. This article is an open access article Diopside has been already considered a gem since ancient times; its name derives distributed under the terms and from the Greek words “di” (for “two” or “double”) and “opsis” (“look”), probably referring conditions of the Creative Commons to its properties of birefringence or pleochroism. In the gem-quality varieties, diopside Attribution (CC BY) license (https:// shows a diaphaneity varying from transparent to translucent and vitreous luster. The most creativecommons.org/licenses/by/ important chromophore elements of the gem-quality diopside are Cr and Fe, which give 4.0/). Minerals 2021, 11, 837. https://doi.org/10.3390/min11080837 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 23 Minerals 2021, 11, 837 2 of 23 ter. The most important chromophore elements of the gem‐quality diopside are Cr and Fe, which givethe the mineral mineral different different shades shades of of green, green, and and V, V, Mn Mn and and Ti, Ti, which which are are generally gen‐ responsible erally responsiblefor the for blue the blue varieties varieties [6]. [6]. Gem‐quality diopsideGem-quality is mined diopside in several is mined countries; in several the countries; most famous the mostvarieties famous are varieties are the “black star”,the the “black “Cr‐ star”,diopside” the “Cr-diopside” and the “violane”. and the “Black “violane”. star” diopside “Black star” comes diopside from comes from India; its nameIndia; is due its to name the black is due or to greenish the black‐black or greenish-black color and to the color optical and phenomenon to the optical phenomenon of “asterism”, ofwhich “asterism”, is a four which‐rayed is star a four-rayed visible on star the visiblesurface on of thethe surfacecabochon of thegem. cabochon This gem. This effect is due toeffect the presence is due to theof magnetite presence of needles. magnetite The needles. beautiful The star beautiful diopsides star inspired diopsides inspired the imagination of the ancient Greeks, who believed they were small bright stars that turned the imagination of the ancient Greeks, who believed they were small bright stars that to stone when they fell to Earth (Figure1). turned to stone when they fell to Earth (Figure 1). Figure 1. FigurePhotographs 1. Photographs of somegem-quality of somegem‐quality diopsides: diopsides: (a )(a “violane”) “violane” (Praborna(Praborna mine, mine, Valle Valle d’Aosta, d’Aosta, Italy; 16 mm × 9 mmItaly;× 716 mm mm); (×b )9 “blackmm × star”7 mm); (Queensland, (b) “black Australia;star” (Queensland, 10 mm × Australia;7 mm); (c )10 Cr-diopside mm × 7 mm); (Yakutia, (c) Russia; 11 mm × 6 mmCr‐diopside× 4 mm). (Yakutia, Private collection Russia; 11 of mm the author× 6 mm F.C. × 4 Photomm). FrancaPrivate Caucia. collection of the author F.C. Photo Franca Caucia. Cr-diopside is characterized by an intense green color, sometimes with bluish shades, Cr‐diopsidedue is to characterized the presence ofby Cr. an It intense is commonly green minedcolor, insometimes the Yakutia with region bluish of Russia, while shades, due tothe the light presence green of variety, Cr. It is with commonly the trade mined name in of the “tashmarine”, Yakutia region comes of Russia, from the Tien Shan while the lightmountain green variety, range with in the the Xinjian trade name region of of “tashmarine”, China. “Violane” comes is afrom variety the ofTien diopside rich in Shan mountainmanganese, range in the extracted Xinjian mainly region from of China. the Praborna “Violane” manganese is a variety mine of near diopside St. Marcel in Aosta rich in manganese,Valley extracted (Italy). It mainly is a very from rare the mineral Praborna with manganese a color ranging mine from near violet St. Marcel to bluish purple to in Aosta Valleylilac, (Italy). depending It is a very on the rare content mineral of with the chromophore a color ranging elements, from violet which to are bluish mainly the Mn2+, purple to lilac,Mn depending3+, Fe2+,V on4+ theand content Ti4+ ions of [ the1–3 ,chromophore7]. elements, which are main‐ ly the Mn2+, Mn3+, FeIn2+ this, V4+ work,and Ti4+ some ions diopside[1–3,7]. samples from the upper Sissone valley, showing an In this work,unusual some blue diopside color were samples analyzed from tothe evaluate upper Sissone their gemological valley, showing relevance, an to identify unusual blue colorthe chromophores were analyzed responsible to evaluate for their color gemological and to investigate relevance, the to identify genetic the conditions. This chromophoresdiopside responsible is highly for color appreciated and to investigate by collectors the and genetic numerous conditions. specimens This are diop exhibited‐ in local side is highly museums.appreciated The by Sissone collectors valley and has numerous been the subjectspecimens of various are exhibited scientific in research local projects due museums. Theto Sissone the presence valley of has numerous been the sites subject of petrological, of various scientific mineralogical research and projects gemological interest. due to the presence of numerous sites of petrological, mineralogical and gemological in‐ terest. 2. Geological Background The Sissone valley is located in the north-western part of Malenco valley, which is considered one of the most interesting areas in Italy for studying the structure of the Alpine Minerals 2021, 11, x FOR PEER REVIEW 3 of 23 2. Geological Background Minerals 2021, 11, 837 3 of 23 The Sissone valley is located in the north‐western part of Malenco valley, which is considered one of the most interesting areas in Italy for studying the structure of the Al‐ pine chain and its geodynamic evolution [8–10]. The overall geostructural framework of the region is characterizedchain and its by geodynamic a series of thrust evolution nappes [8–10 with]. The a sub overall‐horizontal geostructural orientation framework of the and thicknessesregion ranging is characterized from a few hundred by a series to a of few thrust thousand nappes meters. with a From sub-horizontal the bottom orientation and to the top, thesethicknesses nappes overlap ranging according from a few to hundredthe following to a few sequence: thousand Suretta, meters. Forno From and the bottom to the Malenco, Margna,top, these Sella, nappes Bernina, overlap medium–upper according to the Austroalpine following sequence: units Suretta,[11].