International Journal of Astrobiology

Research Article

Electrochemical studies of iron meteorites: phosphorus redox chemistry on the early Earth

David E. Bryanta1, David Greenfielda2, Richard D. Walshawa3, Suzanne M. Evansa1, Alexander E. Nimmoa1, Caroline L. Smitha4, Liming Wanga5, Matthew A. Paseka6 and Terence P. Keea1 c1

a1 School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK

a2 Centre for Corrosion Technology, Materials and Engineering Research Institute, Sheffield Hallam University, Sheffield S1 1WB, UK

a3 Leeds Electron Microscopy and Spectroscopy Centre, University of Leeds, Leeds LS2 9JT, UK

a4 Meteorite Curator, Department of Mineralogy, Natural History Museum, London SW7 5BD, UK

a5 School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China

a6 NASA Astrobiology Institute, University of Arizona, 1629E. University Blvd., Tucson, AZ, 85721, USA

Abstract

The mineral schreibersite, (Fe,Ni)3P, a ubiquitous component of iron meteorites, is known to undergo anoxic hydrolytic modification to afford a range of phosphorus oxyacids. H-phosphonic acid (H3PO3) is the principal hydrolytic product under hydrothermal conditions, as confirmed here by 31P-NMR spectroscopic studies on shavings of the Seymchan pallasite (Magadan, Russia, 1967), but in the presence of photochemical irradiation a more reduced derivative, H-phosphinic (H3PO2) acid, dominates. The significance of such lower oxidation state oxyacids of phosphorus to prebiotic chemistry upon the early Earth lies with the facts that such forms of phosphorus are considerably more soluble and chemically reactive than orthophosphate, the commonly found form of phosphorus on Earth, thus allowing nature a mechanism to circumvent the so-called Phosphate Problem.

This paper describes the Galvanic corrosion of Fe3P, a hydrolytic modification pathway for schreibersite, leading again to H-phosphinic acid as the key P-containing product. We envisage this pathway to be highly significant within a meteoritic context as iron meteorites are polymetallic composites in which dissimilar metals, with different electrochemical potentials, are connected by an electrically conducting matrix. In the presence of a suitable electrolyte medium, i.e., salt water, galvanic corrosion can take place. In addition to model electrochemical studies, we also report the first application of the Kelvin technique to map surface potentials of a meteorite sample that allows the electrochemical differentiation of schreibersite inclusions within an Fe:Ni matrix. Such experiments, coupled with thermodynamic calculations, may allow us to better understand the chemical redox behaviour of meteoritic components with early Earth environments.

(Received July 08 2008)

(Accepted October 21 2008)

(Online publication January 05 2009)

Correspondence:

c1 e-mail: t.p.kee@leeds.ac.uk