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

Abstract

Vivianite, a hydrated ferrous phosphate [FeII3(PO4)2 · 8 H2O] 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 · 8 H2O], 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 · 8 H2O) 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 (C2/m 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.