UNIT-CELL DIMENSIONS OF PURE AND TRACE METAL-ASSOCIATED GOETHITES

Synthetic goethite prepared under highly alkaline conditions incorporates Co3+, Ni2+, Cu2+, Zn2+, Cd2+, and Pb4+ into its structure by isomorphous substitution. Systematic changes of the unit-cell b-dimension with increasing substitution can be related to the ionic radii of incorporated metals. The octahedra of the goethite structure become distorted along the crystallographic a-axis by an incorporation of Cu2+ (Jahn-Teller-effect), Zn2+, and Cd2+. For Zn- and Cd-goethite, the distortion along a can be explained by a smaller ionic size which both foreign metals exhibit in tetrahedral coordination. Thorium is postulated to be incorporated by non-structural incorporation while U is not incorporated. Non-substituted as well as Co-, Ni-, Cd-, and Pb-goethites can have a slightly variable a-dimension depending on the temperature during crystal formation. A higher a-dimension is believed to be the result of structural defects and does not indicate a separate mineral phase. © 1990.