Work

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

Year: 2023

Type: article

Abstract: Vivianite, a hydrated ferrous phosphate [Fe II 3 (PO 4 ) 2 • 8 H 2 O] that forms in oxygenpoor, but Fe 2þ -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 (As V O 4 3-) substitution has never been documented in natural vivianites.Only partial substitution has been reported in synthetic analogues, and parasymplesite [Fe II 3 (AsO 4 ) 2 • 8 H 2 O], 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 (Fe II 3 [(PO 4 ) 1-x (AsO 4 ) x ] 2 • 8 H 2 O) 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 As V O 4 3-and PO 4 3-.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 PO 4 3-, provides a potentially stable sink for arsenic in anoxic soils and sediments, and may have implications in ferruginous early Earth oceans.

Source: Geochemical Perspectives Letters

DOI: 10.7185/geochemlet.2325