Determination of growth rates of (100) and (110) faces of synthetic goethite by scanning force microscopy

For the first time, the growth rate of the (100) and (110) faces of goethit e has been measured in situ with scanning force microscopy. Submicron sized goethite particles were immersed in aerated aqueous Fe(II)solutions, where by Fe(III) was formed by oxidation of Fe(II) by oxygen. Oxidation of Fe(II) is an important and ubiquitous geochemical process in soils and sediments exposed to changing redox conditions. The SFM measurements confirmed that F e(II) oxidation is catalyzed by goethite and showed that Fe(III) is incorpo rated on the existing crystal faces. The growth velocity of the (100) face exceeded the one of the (110) faces by about a factor of 1.5 at the experim ental conditions of this study (10 mM FeSO4 and KCl, 5mM acetate, pH 5). Th e different growth rates result in a predominance of (110) faces, which is also observed when goethite is formed in oversaturated Fe(III) solutions at pH 9, and explains the generally observed morphology of goethite particles . The growth behavior appears to be reaction controlled rather than transpo rt controlled. The preferential growth on the (100) faces could be driven b y steric factors, in that the grooves on the (100) faces formed by rows of missing oxygens provide preferred sites for Fe incorporation. The surface p roperties on the different crystal faces are discussed in the frame of CD-M USIC model. Copyright (C) 1998 Elsevier Science Ltd.