Clay Minerals (1996) 31, 465-476 PROPERTIES OF SYNTHETIC GOETHITES WITH Co FOR FE SUBSTITUTION U. G. GASSER*'**, E. JEANROY*, C. MUSTIN*, O. BARRES t, R. Nf2ESCH ~, J. BERTHELIN* AND A. J. HERBILLON* * Centre de P~dologie Biologique, U.P.R. 6831 du C.N.R.S. associ~e gt l'Universit~ Henri Poincar~, Nancy, 17, rue Notre-Dame des Pauvres, B.P. 5, F 54501 Vancloeuvre-lbs-Nancy, France, t Laboratoire "Environnement et Miru~ralurgie", U.R.A. 235 du C.N.R.S., B.P. 40, F 54501 Vandeeuvre-lks-Nancy, France, and ~Labor fiir Tonmineralogie, Institut fiir Geotechnik ETHZ, CH 8092 Ziirich, Switzerland (Received 2 February 1996; revised 24 May 1996) ABSTRACT: Isomorphic substitution in goethites is common in nature and the properties of goethites generally change as a function of the degree of substitution (e.g. Al-goethites). In synthetic goethites, substitution by other elements such as Co is also known. Recent literature indicates that the influence of AI and Co on the unit-cell dimensions of goethite is similar. In contrast to Al-goethites, however, little is known about other properties of Co-goethites and in this study some properties of synthetic Co-goethites were investigated by XRD, IR, TEM, TGA and reductive dissolution techniques. Eight goethite samples (S1 to $8) with varying Co concentrations were synthesized from mixed alkaline solutions of Fe(III) nitrate and Co(II) nitrate, aged at 63~ and ambient pressure. The goethites contained up to 9.5 mol.% Co. Their redness increased with Co concentration, e.g. 0.5 Y 6,0/6.4 for S1 and 6.4 YR 3.3/3.2 for $8. Surface area ranged from 46 to 88 m2/g. Unit-cell parameters a, b, c and v all showed a negative linear dependency on the Co concentration of the goethites. Transmission and diffuse reflectance IR spectrometry showed the presence of strong bands which were interpreted as v-OH, g-OH and "/-OH vibrations. The &OH and T-OH band positions showed a positive linear dependency on the Co concentration of the samples. Dehydroxylation occurred between 280 and 315~ and dehydroxylation peak positions tended to decrease with increasing Co concentrations. As with Al-goethites, Co-goethite reductive dissolution rates decreased parabolically with increasing substitution. X-ray diffraction and IR analyses, TGA and congruent reductive dissolution suggest the existence of single phases, i.e. Co-goethites of varying degrees of isomorphic substitution. Iron oxides such as goethite and hematite are common minerals in many weathering environ- ments of the earth; thus, the properties of Fe oxides are of interest to scholars in various fields (e.g. agronomy, chemistry, economics, environmental engineering, forestry and mining). Iron oxides often contain non-iron metals (NIM) such as AI, Cr, Mn and Ni (Schwertmann & Taylor, 1989). With the exception of A1, the structural incorpora- tion of NIM in natural Fe oxides is difficult to ** Present address: College of Engineering ISW, Im Grtintal, P.O. Box 335, CH 8820 Wtidenswil, Switzerland prove, because of low NIM concentrations (generally <2 mol.%); it is often necessary, therefore, to study synthetic samples. Synthetic goethite may structurally incorporate NIM such as A1, Cd, Co, Cr, Cu, Mn, Ni, and Pb (Cornell, 1991; Gerth, 1990). Substituted goethites often have different properties from those of pure goethites (e.g. unit-cell dimensions, position of IR bands, dissolution rates and thermal properties). The unit- cell dimension b of substituted goethites generally follows the Vegard rule (linear interpolation between end-members), but some goethites may also follow the Vegard rule for a (e.g. Cd, Co, Cr and Pb) and c (e.g. A1, Co and Cr; Gerth, 1990; Schulze, 1982; Schwertmann et al., 1989). The 9 1996 The Mineralogical Society