New solid solution with the garnet structure: the berzeliite–schaferite isomorphous series from Fumarole of Tolbachik

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Marco E. Ciriotti
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New solid solution with the garnet structure: the berzeliite–schaferite isomorphous series from Fumarole of Tolbachik

Messaggio da Marco E. Ciriotti » sab 10 apr, 2021 14:48

Referenza:
▪ Koshlyakova, N.N., Pekov, I.V., Zubkova, N.V., Agakhanov, A.A., Turchkova, A.G., Kartashov, P.M., Sidorov, E.G., Pushcharovsky, D.Yu. (2020): A new solid solution with the garnet structure: the berzeliite–schaferite isomorphous series from Fumarole exhalations of the Tolbachik volcano, Kamchatka. Zapiski RMO (Proceedings of the Russian Mineralogical Society), 149(1), 69-84.

Abstract:
An extended solid solution (isomorphous series) between arsenate and vanadate minerals of the garnet supergroup is first described. It is the berzeliite (NaCa2)Mg2(AsO4)3–schaferite (NaCa2)Mg2(VO4)3 series found in exhalations of the Arsenatnaya fumarole, Tolbachik volcano, Kamchatka, Russia. These minerals occur here as aggregates of transparent yellow or orange crystals up to 1 mm and are associated with anhydrite, forsterite, diopside, andradite, hauyne, K-Na feldspars, hematite, magnesioferrite, spinel, baryte, aphthitalite-like sulfates, minerals of the powellite-scheelite series, ludwigite, calciojohillerite, paraberzeliite, members of rhabdoborite-(V)–rhabdoborite-(W)–rhabdoborite-(Mo), apatite–svabite–pliniusite, tilasite– isokite, udinaite–arsenudinaite, and wagnerite–arsenowagnerite solid-solution systems. These minerals were formed at temperatures not lower than 550 °C under high oxygen fugacity. The tetraherdal site composition in the schaferite–berzeliite series from the Arsenatnaya fumarole ranges continuously from (V2.54As0.48P0.04Si0.01) to (As2.77V0.22Si0.03P0.01). Crystal structures were studied for three samples (space group Ia–3d) with different As : V ratios: (1) (Ca2.15Na0.85)Σ3(Mg2.0)(V1.95As0.90Si0.15)Σ3O12, a = 12.39737(7) Å, R = 0.0210; (2) (Ca2.00Na1.00)Σ3(Mg2.0)(As1.90V0.90P0.20)Σ3O12, a= 12.35366(10) Å, R= 0.0181; (3) (Ca2.05Na0.95)Σ3(Mg2.0)(As2.35V0.60Si0.05)Σ3O12, a = 12.36093(10) Å, R = 0.0251. The dimorphism between arsenates belonging to the garnet and alluaudite structure types is discussed.
Marco E. Ciriotti

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