CRYSTAL CHEMISTRY OF SCANDIUM AS A TRACE ELEMENT IN MINERALS

A dominant theme in the geochemistry of scandium, originally suggested by Goldschmidt and Peters has been the supposed diadochy of the Se3 ion with the Fe2 and Mg ions. Studies of the crystal chemistry of synthetic Sc compounds, especially with regard to the solid-solution relations of Se as a major component in synthetic systems, now indicate that the distribution of Se3 as a trace element in the lithosphère is based on a diadochic relation to 6-coordinated (Fe3,Al). This is supported by the isostructural relation between many oxycompounds of Sc3 and of (Fe3, Al), and by recent measurements of the Sc-O distances in numerous compounds. The radius of Sc3 in 6-coordination with oxygen, although variable depending on the ionic-covalent proportions in the bond and other factors, is much smaller than the traditional value 0.83 Â of Zachariasen and averages about 0.73 Â or less. The close crystallo-chemical relation of Sc3 to In3 is not displayed in nature, but there are limited solid solutions with Y3, Er3-Yb3, Ti4, Sn4 and Zr4 in certain geochemical environments. © 1968, Walter de Gruyter. All rights reserved.