Crystal chemistry of trace elements in natural and synthetic goethite

The crystal chemistry of Cr (0.73%), Mn (0.88%), Co (0.166%), Ni (0.898%), Cu (0.263%), and Zn (0.0264%) in natural goethite (alpha FeOOH) from an ear thy saprolite formation in West-Africa was investigated by fluorescence-yie ld extended X-ray absorption fine structure (EXAFS) spectroscopy. Synthetic goethites and phyllomanganates were used as model compounds for structural determinations. The measurement of EXAFS spectra at energies higher than t he Fe K-edge (Co, Ni, Cu, and Zn) is notoriously difficult because the fluo rescence signal from trace elements is typically two orders of magnitude le ss intense than the Fe fluorescence from the matrix. This difficulty was ci rcumvented by measuring total intensity (signal + background) with high pre cision on the ID26 undulator EXAFS spectrometer of the European Synchrotron Radiation Facility (ESRF) with a fast, highly linear, and low-noise diode detector. Cr, Cu, and Zn have the same local structure in natural and synth etic goethites. With the exception of the Cu polyhedron, which is distorted by the Jahn-Teller effect, Me-O and Me-Fe distances are similar to Fe-O an d Fe-Fe distances in goethite. No significant steric effect was observed ar ound Cu and Zn in spite of the similar to 14% increase in ionic radii compa red to Fe3+. The compliance of the Fe site to these substitutional impuriti es probably is accomplished by displacement of nearest O and Fe shells (rel axation effect) and modification of interpolyhedral angles (compliance effe ct) owing to the corner-sharing topology of the goethite structure. X-ray a bsorption near-edge structure analysis reveals an average valence of simila r to3.7 to 3.8 for Mn, and EXAFS indicates that manganese is present as a p hyllomanganate impurity having a hexagonal layer symmetry like asbolane, li thiophorite, and hexagonal birnessite. Cobalt is trivalent and located in t he phyllomanganate layer, and also possibly in the interlayer, substituting for Mn. Selective uptake of cobalt by the Mn oxide impurity results from t he oxidation of soluble Co2+ to, insoluble Co3+ by Mn-3+/Mn4+, this mineral -surface redox reaction accounting for the well-known geochemical affinity of Co for Mn at the earth's surface. No more than similar to 20% of the amo unts of Mn and Co in natural goethite substitute for Fe in the structure, i f present at all. About 75% of total Ni is substituted for Fe in natural go ethite and similar to 25% is associated with the phyllomanganate impurity a s a Ni(OH)(2)-MnO2 mixed-layer phase (Ni-asbolane). The Ni site in syntheti c goethite is strongly relaxed with a contraction of the goethite structure in the plane of edge-sharing double chains (be plane), and an expansion in the direction of corner linkages (approximately the ab plane). This anisot ropic relaxation of the Ni site locally reduces the distortion of the goeth ite structure, and could be due to a clustering of Ni atoms. Copyright (C) 2000 Elsevier Science Ltd.