A new solid solution with the garnet structure: the berzeliite–schaferite isomorphous series from Fumarole exhalations of the Tolbachik volcano, Kamchatka

Koshlyakova N. N., Pekov I. V., Zubkova N. V., Agakhanov A. A., Turchkova A. G., Kartashov P. M., Sidorov E. G., Pushcharovsky D. Yu.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2020. V. 149. N 6. P. 69-84

https://doi.org/10.31857/S0869605520060040

Language: Russian

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 (V_2.54As_0.48P_0.04Si_0.01) to (As_2.77V_0.22Si_0.03P_0.01). Crystal structures were studied for three samples (space group Ia–3d) with different As : V ratios: (1) (Ca_2.15Na_0.85)_Σ3(Mg_2.0)(V_1.95As_0.90Si_0.15)_Σ3O₁₂, a= 12.39737(7) Å, R= 0.0210; (2) (Ca_2.00Na_1.00)_Σ3(Mg_2.0)(As_1.90V_0.90P_0.20)_Σ3O₁₂, a= 12.35366(10) Å, R= 0.0181; (3) (Ca_2.05Na_0.95)_Σ3(Mg_2.0)(As_2.35V_0.60Si_0.05)_Σ3O₁₂, a= 12.36093(10) Å, R= 0.0251. The dimorphism between arsenates belonging to the garnet and alluaudite structure types is discussed.

Keywords: schaferite, berzeliite, garnet supergroup, berzeliite group, arsenate, vanadate, solid- solution in garnet, Arsenatnaya fumarole, Tolbachik volcano, Kamchatka