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New Insights for Gem-Quality Mn-Bearing Diopside-Omphacite

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New Insights for Gem-Quality Mn-Bearing Diopside-Omphacite minerals Article New Insights for Gem-Quality Mn-Bearing Diopside-Omphacite, Violane Variety, from Saint Marcel (Val D’Aosta, Italy): Its Trace Elements and Spectroscopic Characterization Valeria Diella 1,* , Rosangela Bocchio 2, Franca Caucia 3, Nicoletta Marinoni 2, Antonio Langone 4 and Elena Possenti 5 1 National Research Council, Institute of Environmental Geology and Geoengineering (IGAG), Section of Milan, 20133 Milan, Italy 2 Department of Earth Sciences “Ardito Desio”, University of Milan, 20133 Milan, Italy; [email protected] (R.B.); [email protected] (N.M.) 3 Department of Earth and Environmental Sciences, University of Pavia, 27100 Pavia, Italy; [email protected] 4 National Research Council, Institute of Geosciences and Earth Resources (IGG), Section of Pavia, 27100 Pavia, Italy; [email protected] 5 National Research Council, Institute of Heritage Science (ISPC), Section of Milan, 20125 Milan, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-02-50315621 Abstract: This study proposes new data on the rare pyroxene, variety violane, sampled from its Citation: Diella, V.; Bocchio, R.; type locality, Praborna manganese deposit, near Saint Marcel (Val d’Aosta, Italy). Violane is very Caucia, F.; Marinoni, N.; Langone, A.; Possenti, E. New Insights for appreciated as a gemstone for its different hues of violet-blue color and is characterized by its diop- Gem-Quality Mn-Bearing sidic or omphacitic composition. To assess the possible causes of color, electron-probe microanalysis Diopside-Omphacite, Violane Variety, (EMPA) and laser-ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) were used from Saint Marcel (Val D’Aosta, Italy): to establish the chemical composition. Raman and Fourier transform infrared (FTIR) spectroscopy, Its Trace Elements and ideal for the non-destructive analysis, identified the different present phases directly on gemstone. Spectroscopic Characterization. Raman and FTIR spectra highlighted the presence, in the same sample, of diopside and omphacite Minerals 2021, 11, 171. https:// showing almost inappreciable violet hue difference. The two minerals were easily differentiated by doi.org/10.3390/min11020171 microprobe analyses and showed a compositional heterogeneity not linked to the different colors. The best way to detect the color-causing elements in microcrystalline violanes resulted in the analysis Academic Editor: of trace elements and their quantification. An enrichment of Ti and Li characterizes darker violet Thomas Hainschwang omphacite and that of V and rare-earth elements (REE) the lilac-lavender or light violet-blue diopside. Received: 11 January 2021 Accepted: 1 February 2021 In general, our results led us to say that the color changes, previously proposed as due to Mn both Published: 7 February 2021 in divalent and trivalent oxidation state, may be controlled by trace elements or by concentration of minor elements, such as Fe, and their oxidation state. REE patterns showed a negative anomaly Publisher’s Note: MDPI stays neutral of Ce that could be ascribed to the variation of the oxygen chemical potential occurring in the ore. with regard to jurisdictional claims in The new data, combined with previous results, may provide new constrains on the processes that published maps and institutional affil- generated the Mn-rich deposit of Praborna. iations. Keywords: violane-pyroxene; gemology; Val d’Aosta; Western Alps; Italy Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. 1. Introduction This article is an open access article Pyroxene, variety violane (or violan) from the Mn-ore deposit of Praborna in Saint distributed under the terms and Marcel Valley (Valle d’Aosta Region, North Western Italy; WGS84 system: lat. 45◦40045” conditions of the Creative Commons N; long. 7◦26057” E) is a rare mineral in nature and has been considered as gem-quality Attribution (CC BY) license (https:// material since last century due to its beautiful color varying from purple-blue to violet. creativecommons.org/licenses/by/ In the catalogue of “Italian Type Minerals”, the violane of Praborna mine is quoted as “a 4.0/). Minerals 2021, 11, 171. https://doi.org/10.3390/min11020171 https://www.mdpi.com/journal/minerals Minerals 2021, 11, x FOR PEER REVIEW 2 of 21 first discovered in Italy [1]. However, this definition is not at all complete because many Minerals 2021, 11, 171 years ago, it was proved that the violet-blue clinopyroxenes from Praborna belong to2 oftwo 21 separate species: diopside, CaMg(Si2O6), and omphacite, (Ca,Na)(Mg,Fe,Al)(Si2O6), differ- ing both in composition and space group symmetry [2–5]. The Saint Marcel area is the most significant locality (type locality) for gem-quality rare light blue to purple, manganese-rich variety” of diopside, which is itself a mineral violane in the world, and actually, only few other occurrences are known, e.g., as orna- species first discovered in Italy [1]. However, this definition is not at all complete because mental blue diopside from different localities in Russia ([6] and references therein) or as many years ago, it was proved that the violet-blue clinopyroxenes from Praborna belong to massive aggregates from southern Baffin Island Nunavut, Canada [7] and from Vitali, two separate species: diopside, CaMg(Si2O6), and omphacite, (Ca,Na)(Mg,Fe,Al)(Si2O6), differingAndros Island, both in Greece composition [8]. and space group symmetry [2–5]. TheFine Saint material Marcel has areabeen is found the most in a significantlimited amount locality in a (type restricted locality) part for of gem-qualitythe Praborna violanemine (Figure in the 1), world, and andonly actually,a few pieces only have few otherbeen occurrencesset into jewelry. are known,In particular, e.g., as among orna- mentalcollectors blue and diopside jewelers, from there different are also localities known colorless in Russia quartz ([6] and cabochons references with therein) violane or asin- massiveclusion sampled aggregates in the from quartz-rich southern veins Baffin [9] Island (Figure Nunavut, 2). However, Canada both [7] the and veins from and Vitali, in- Androsclusions Island, of violane Greece are [rather8]. narrow (from millimetric to decimetric in size), therefore the materialFine materiallends itself has to been small found cabochons in a limited and amountcarvings. in To a restricted date, local part mineral of the Prabornacollectors minecontinue (Figure to work1), and the only deposit, a few and pieces it is havelikely been that setsmall into amounts jewelry. of In fine particular, violane will among con- collectorstinue to be and produced jewelers, in the there future. are also known colorless quartz cabochons with violane inclusionBecause sampled a study in theon the quartz-rich gemological veins properties [9] (Figure of2 this). However, very rare bothvariety the of veins pyroxene and inclusionsis lacking, ofand violane the chemical are rather and narrow physical (from data, millimetric except for the to decimetric more recent in work size), of therefore [10], are themore material than 40 lends years itself old, towe small prepared cabochons this review and carvings. and update To date, including local minerala complete collectors gemo- continuelogical characterization to work the deposit, together and itwith is likely newthat information small amounts concerning of fine trace violane element will continue profiles toand be Raman produced and in FTIR the future.spectroscopy. FigureFigure 1.1.Mine Mine andand landfilllandfill ofof PrabonaPrabona ( a(a)) and and collected collected samples samples ( b(b).). Because a study on the gemological properties of this very rare variety of pyroxene is lacking, and the chemical and physical data, except for the more recent work of [10], are more than 40 years old, we prepared this review and update including a complete gemological characterization together with new information concerning trace element profiles and Raman and FTIR spectroscopy. MineralsMinerals 20212021, 11, 11, x, FOR 171 PEER REVIEW 3 of3 of21 21 FigureFigure 2. 2. PendantPendant of of violane, violane, 3.5 3.5 cm cm × ×2.52.5 cm, cm, photo photo by by “Lutezia “Lutezia Gioielli, Gioielli, Stradella, Stradella, Pavia, Pavia, Italy”( Italy”a ()a and) and quartz quartz trillion trillion cut cut containingcontaining violet violet inclus inclusionsions of of violane, violane, 2.5 2.5 ct, ct, photo photo by by F. F. Caucia Caucia (b (b).). 2. Background Information 2. BackgroundThe Praborna Information mine occurs in the Zermatt-Saas meta-ophiolite unit of the Western Alps andThe formed Praborna as minehydrothermal occurs in thesediment Zermatt-Saas at the oceanic meta-ophiolite spreading unit center. of the In Western this area, Alps theand oceanic formed crust as hydrothermalwas subducted sediment and metamorpho at the oceanicsed upspreading to eclogite center.facies conditions In this area, dur- the ingoceanic the Eoalpine crust was high-pressure–low-temperatur subducted and metamorphosede phase up toof eclogitethe alpine facies orogenesis conditions [11–13]. during theThe Eoalpine deposit high-pressure–low-temperature is considered as the most important phase of and the famous alpine orogenesis Mn occurrence [11–13 of]. the PiemonteThe nappe deposit and is is considered constituted as by the boudinag most importanted quartz-rich and famous layers Mnwith occurrence Mn oxides ofand the silicates.Piemonte It also nappe contains and is a constituted few intercalations by boudinaged of metapelites
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