Germanium crystal chemistry in hematite and goethite from the Apex Mine, Utah, and some new data on germanium in aqueous solution and in stottite

Abstract

At the Apex Mine in southwest Utah, fine-grained hematite contains as much as 1.0 wt. percent Ge, and fine-grained goethite contains as much as 0.7 wt. percent Ge. The mode of Ge incorporation in these minerals was investigated by high-resolution K-edge fluorescence spectroscopy using synchrotron radiation. Analysis of extended fine structure (EXAFS) K-edge data for Ge and Fe shows that Ge substitutes for Fe in the octahedral metal sites of the studied hematite and goethite, with average Ge-ligand bond lengths of 1.88 Å. The solid solution in hematite probably occurs through the coupled substitution 2 Fe( III) = Ge( IV) + Fe( II), similar to the coupled substitution 2 Fe( III) = Ti( IV) + Fe( II) that occurs in the solid solution series hematite-ilmenite. The solid solution in goethite probably occurs by the loss of an H atom from an OH group, through the coupled substitution Fe( III) + H( I) = Ge( IV). In related experiments, EXAFS data indicate that in a neutral aqueous solution containing 790 ppm Ge, the Ge occurs predominately as Ge(OH) 4, with tetrahedral Ge-OH bond lengths of 1.74 Å. In stottite, FeGe(OH) 6, Fe(II) and Ge(IV) occur in octahedral sites with average Fe-OH and Ge-OH bond lengths of about 2.20 Å and 1.88 Å.