Unit-cell dimensions of pure and trace metal-associated goethites

Abstract

Synthetic goethite prepared under highly alkaline conditions incorporates Co 3+, Ni 2+, Cu 2+, Zn 2+, Cd 2+, and Pb 4+ 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 Cu 2+ (Jahn-Teller-effect), Zn 2+, and Cd 2+. 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.