Summary
Violet clinopyroxenes (“violan”) from the manganese deposit of Praborna, near St. Marcel (Aosta Valley, Italy) occur in two different varieties: euhedral crystals grown in vugs and massive lamellar to fibrous aggregates. The euhedral crystals are predominantlyP2/n omphacite with a(Di+Hd+Jo)/(Jd+Ac+Mnjd) ratio close to unity. The lamellar aggregates consist mainly of disorderedC2/c impure diopside with alternating patches of pure diopsidic and moderately omphacitic compositions.
The deep violet colour, occasionally shading to blue, is presumably due to the presence of both Mn3+ and Mn2+ ions. Detailed crystal structure determinations andEPR spectra investigations, however, did not clearly solve the question of the oxidation state of manganese because of the low amounts of manganese being present in “violan”. Crystal structure determinations and microprobe analyses nevertheless clearly showed that “violan” is not a mineral species. The name should therefore be reserved for the “ex-colore” violet varieties of both diopside and omphacite.
Zusammenfassung
Violett gefärbte Klinopyroxene (“violan”) von der Manganlagerstätte Praborna bei St. Marcel (Aosta-Tal, Italien) bilden entweder idiomorphe Kristalle in Hohlräumen, oder massive lamellare bis feinfaserige Aggregate. Die idiomorphen Kristalle sind zum überwiegenden TeilP2/n Omphazite mit einem (Di+Hd+Jo):(Jd+Ac+Mnjd) Verhältnis von etwa 1 ∶ 1. Die lamellaren Aggregate bestehen hauptsächlich aus ungeordnetemC2/c Diopsid, wobei die chemische Zusammesetzung partienweise zwischen diopsidisch und leicht omphazitisch wechselt.
Die tief violette Farbe wird wahrscheinlich durch das Vorhandensein von Mn3+ und Mn2+-Ionen verursacht. Eine eindeutige Klärung dieser Frage war jedoch trotz detaillierter Kristallstruktur-Bestimmung und Auswertung vonEPR Spektren nicht möglich, da der Mangangehalt der “Violane” überraschend gering ist.
Die detaillierten Kristallstruktur-Analysen ergaben jedoch eindeutig, daß “Violan” keine selbständige Mineralspecies ist. Dieser Name sollte daher nur für die violetten Farbvarietäten von Omphaziten und Diopsiden verwendet werden.
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Mottana, A., Rossi, G., Kracher, A. et al. Violan revisited: Mn-bearing omphacite and diopside. TMPM Tschermaks Petr. Mitt. 26, 187–201 (1979). https://doi.org/10.1007/BF01081845
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DOI: https://doi.org/10.1007/BF01081845