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Mineralogical Magazine; October 2005; v. 69; no. 5; p. 667-676; DOI: 10.1180/0026461056950278
© 2005 Mineralogical Society of Great Britain and Ireland
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Fe isotope fractionation during the precipitation of ferrihydrite and transformation of ferrihydrite to goethite

R. E. Clayton1,{dagger}, K. A. Hudson-Edwards1,*, D. Malinovsky2 and P. Andersson3

1 Research School of Earth Sciences at UCL-Birkbeck, Gower St., London WC1E 6BT, UK
2 Division of Applied Geology, Luleå University of Technology, S-971 87 Luleå, Sweden
3 Laboratory for Isotope Geology, Swedish Museum of Natural History, Box 50007, 104 05 Stockholm, Sweden

* E-mail: k.hudson-edwards{at}geology.bbk.ac.uk

Ferrihydrite and goethite are amongst the most important substrates for the sorption of contaminants in soil and other environmental media. Isotopic studies of the transition elements, particularly those that exhibit more than one oxidation state and show pH- and/or redox-sensitive behaviour at low temperatures, have been shown to be potentially useful present-day and past proxies for redox (or palaeoredox) conditions. We have made preliminary investigations of Fe isotope fractionation that take place during the formation of FeIII (oxy)hydroxides (FeIIIox) from an aqueous FeIII(NO3)3 solution (FeIIIaq) under laboratory conditions. We have attempted to keep the chemical system simple by excluding ‘vital effects’ and major changes in redox through the maintenance of abiotic conditions and use of FeIIIaq. Isotopic measurements (56Fe/54Fe, 57Fe/54Fe) of the FeIII(NO3)3 stock solution, the original ferrihydrite and the mixed ferrihydrite/goethite-supernatant FeIIIaq ‘pairs’ were carried out using a double focusing multicollector inductively coupled plasma mass spectrometer. The results reveal an apparent systematic variation indicating larger {Delta}FeIIIaq–FeIIIox with decrease in the ferrihydrite:goethite ratio, which reflects the time allowed for isotopic exchange. These values range from virtually zero (0.03{per thousand}) after 24 h to 0.30{per thousand} after 70 h. In each FeIIIox–FeIIIaq ‘pair’ the lighter Fe isotope is partitioned into the FeIIIox, leaving the FeIIIaq isotopically heavier. The observed fractionation reflects isotopic exchange of Fe between the FeIIIox and FeIIIaq upon at least a two step transition of ferrihydrite to goethite.

KEYWORDS: ferrihydrite, goethite, transformation, iron isotope, fractionation, ferric nitrate




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