Vivianite-parasymplesite solid solution: A sink for arsenic in ferruginous environments?

[Marco E. Ciriotti]

Referenza:
▪ Perez, J.B.H., Okhrymenko, M., Blukis, R., Roddatis, V., Mayanna, S., Mosselmans, J.F.W., Benning, L.G. (2023): Vivianite-parasymplesite solid solution: A sink for arsenic in ferruginous environments? Geochemical Perspectives Letters, 26, 50–56.

Abstract:
Vivianite, a hydrated ferrous phosphate [FeII3(PO4)2·8H2O] that forms in oxygen-poor, but Fe2+-rich conditions is important in nutrient cycling in anoxic environments.In natural vivianites, isomorphic substitution of divalent cations for structural Fe(II) are typical. However, anion substitution is rare; in particular, arsenate (AsVO43−) substitution has never been documented in natural vivianites. Only partial substitution has been reported in synthetic analogues, and parasymplesite [FeII3(AsO4)2·8OH2], the arsenic end member of the vivianite mineral group, is found in hydrothermal deposits. In this study, we detail structural changes in synthesised As-vivianites (FeII3[(PO4)1−x(AsO4)x]2·8H2O) with systematically increased degrees of As(V) substitution (0.22 ≤ x ≤ 0.95). As(V) was successfully incorporated into the vivianite crystal structure, creating a homogenous, solid solution between AsVO43− and PO43−. Like both end members, the intermediate As-vivianites crystallised in the monoclinic system (C2m space group), and retained the platelet crystal habit of As-free vivianite, even at the highest As(V) substitution. This uniform incorporation of As(V), and its replacement of PO43−, provides a potentially stable sink for arsenic in anoxic soils and sediments, and may have implications in ferruginous early Earth oceans.

NdR: Vedasi anche:
▪ Yoshiasa, A., Miyano, Y., Isobe, H., Sugiyama, K., Arima, H., Nakatsuka, A., Momma, K., Miyawaki, R. (2016): Structural refinement of köttigite–parasymplesite solid solution: Unique cation site occupancy and chemical bonding with water molecules. Journal of Mineralogical and Petrological Sciences, 111, 363-369.
▪ Hongu, H., Yoshiasa, A., Kitahara, G, Miyano, Y., Han, K., Momma, K., Miyawaki, R., Tokuda, M., Sugiyama, K. (2021): Crystal structure refinement and crystal chemistry of parasymplesite and vivianite. Journal of Mineralogical and Petrological Sciences, 116, 183-192.