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

Geochemical Perspectives Letters, vol. 26, pp. 50–56

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.

Authors 7

1. Jeffrey Paulo H. Perez corresponding

   GFZ Helmholtz Centre for Geosciences

   Affiliation as printed

   . GFZ German Research Center for Geosciences , Telegrafenberg , 14473 Potsdam , Germany

   . GFZ German Research Center for Geosciences, Telegrafenberg, 14473 Potsdam, Germany

2. Marharyta Okhrymenko Aachen Current affiliation: Department of Chemistry

   GFZ Helmholtz Centre for Geosciences · RWTH Aachen University

   Affiliation as printed

   . GFZ German Research Center for Geosciences , Telegrafenberg , 14473 Potsdam , Germany

   Current affiliation: Department of Chemistry , RWTH Aachen University , 52056 Aachen , Germany

   . GFZ German Research Center for Geosciences, Telegrafenberg, 14473 Potsdam, Germany

   Current affiliation: Department of Chemistry, RWTH Aachen University, 52056 Aachen, Germany

3. Roberts Blukis

   GFZ Helmholtz Centre for Geosciences · Leibniz Institute for Crystal Growth

   Affiliation as printed

   . GFZ German Research Center for Geosciences , Telegrafenberg , 14473 Potsdam , Germany

   Current affiliation : Leibniz-Institut für Kristallzüchtung (IKZ) , Berlin 12489 , Germany

   . GFZ German Research Center for Geosciences, Telegrafenberg, 14473 Potsdam, Germany

   Current affiliation: Leibniz-Institut für Kristallzüchtung (IKZ), Berlin 12489, Germany

4. Vladimir V. Roddatis

   GFZ Helmholtz Centre for Geosciences

   Affiliation as printed

   . GFZ German Research Center for Geosciences , Telegrafenberg , 14473 Potsdam , Germany

   . GFZ German Research Center for Geosciences, Telegrafenberg, 14473 Potsdam, Germany

5. Sathish Mayanna

   Carl Zeiss (Germany) · GFZ Helmholtz Centre for Geosciences

   Affiliation as printed

   . GFZ German Research Center for Geosciences , Telegrafenberg , 14473 Potsdam , Germany

   Current affiliation: Carl Zeiss Microscopy GmbH , 73447 Oberkochen , Germany

   . GFZ German Research Center for Geosciences, Telegrafenberg, 14473 Potsdam, Germany

   Current affiliation: Carl Zeiss Microscopy GmbH, 73447 Oberkochen, Germany

6. J. Frederick W. Mosselmans

   Diamond Light Source

   Affiliation as printed

   . Diamond Light Source , Harwell Science and Innovation Campus , Didcot , Oxfordshire , OX11 0DE , United Kingdom

   . Diamond Light Source, Harwell Science and Innovation Campus, Didcot, Oxfordshire, OX11 0DE, United Kingdom

7. Liane G. Benning

   GFZ Helmholtz Centre for Geosciences · Freie Universität Berlin

   Affiliation as printed

   . Department of Earth Sciences , Freie Universität Berlin , Malteserstrasse 74-100 , 12249 Berlin , Germany

   . GFZ German Research Center for Geosciences , Telegrafenberg , 14473 Potsdam , Germany

   . Department of Earth Sciences, Freie Universität Berlin, Malteserstrasse 74-100, 12249 Berlin, Germany

   . GFZ German Research Center for Geosciences, Telegrafenberg, 14473 Potsdam, Germany