Indium and germanium in the structure of sphalerite: an example of coupled substitution with Copper

Abstract

The distribution of indium in sphalerite from hydrothermal veins in the Saint-Martinla-Sauvete district (Loire, France) was studied by microprobe. Several hundred analyses were carried out. Sphalerite, devoid of chalcopyrite inclusions, exhibits an oscillatory zoning which can be related to the variation of minor-element concentrations. Indium (up to 1.05 wt. %) is positively correlated with copper, and Cd with Fe. The distribution of these two pairs of elements is antithetic within (110) growth zones. In and Cu show a very complex distribution within some twinned sphalerite crystals and are preferentially concentrated in {111} growth zones. A variation of diffusion/growth velocity ratio is evoked to explain this behaviour. From core to rim of sphalerite crystals the Cd concentration decreases and that of In (and Cu) increases. This increase corresponds to the development of {111} faces, which become progressively more prominent. The slope of the regression line calculated from Cu and In values corresponding to In < 0.4 wt. % is subparallel to (Cu/In)at = 1. This indicates the substitution mechanism Cu(I) + In(III) ⇄ 2 Zn(II). However, the Cu concentration is generally higher than required by this equation. Cuex rises toward the rim of sphalerite crystals; Cu-rich and In-free zones exist in some crystals. Cuex concentration is generally above the CuS solubility limit in sphalerite. It is suggested that Fe(III) could assume the local charge balance of Cu(I). This hypothesis is compatible with the rise of Cuex in late growth zones and with the late crystallization of chalcopyrite with respect to sphalerite observed in these deposits. Consequently, the incorporation of In and Cu in sphalerite from the Saint-Martin-la-Sauvete district is governed by the following equation: M(I) + M(III) ⇄ 2 Zn(II), where M(I) = Cu; M(III) = In, Fe.