Distribution and Substitution Mechanism of Ge in a Ge-(Fe)-Bearing Sphalerite

Abstract

Abstract:The distribution and substitution mechanism of Ge in the Ge-rich sphalerite from the Tres Marias Zn deposit, Mexico, was studied using a combination of techniques at μm-to atomic scales. Trace element mapping by Laser Ablation Inductively Coupled 118Mass Spectrometry shows that Ge is enriched in the same bands as Fe, and that Ge-rich sphalerite also contains measurable levels of several other minor elements, including As, Pb and Tl. Micron-to nanoscale heterogeneity in the sample, both textural and compositional, is revealed by investigation using Focused Ion Beam-Scanning Electron Microscopy (FIB-SEM) combined with Synchrotron X-ray Fluorescence mapping and High-Resolution Transmission Electron Microscopy imaging of FIB-prepared samples. Results show that Ge is preferentially incorporated within Fe-rich sphalerite with textural complexity finer than that of the microbeam used for the X-ray Absorption Near Edge Structure (XANES) measurements. Such heterogeneity, expressed as intergrowths between 3C sphalerite and 2H wurtzite on [11 0] zones, could be the result of either a primary growth process, or alternatively, polystage crystallization, in which early Fe-Ge-rich sphalerite is partially replaced by Fe-Ge-poor wurtzite. FIB-SEM imaging shows evidence for replacement supporting the latter. Transformation of sphalerite into wurtzite is promoted by (111)* twinning or lattice-scale defects, leading to a heterogeneous ZnS sample, in which the dominant component, sphalerite, can host up to ~20% wurtzite. Ge K-edge XANES spectra for this sphalerite are identical to those of the germanite and argyrodite standards and the synthetic chalcogenide glasses GeS2 and GeSe2, indicating the Ge formally exists in the tetravalent form in this sphalerite. Fe K-edge XANES spectra for the same sample indicate that Fe is present mainly as Fe 2+, and Cu K-edge XANES spectra are characteristic for Cu + . Since there is no evidence for coupled substitution involving a monovalent element, we propose that Ge ) with vacancies in the structure to compensate for charge balance. This study shows the utility of synchrotron radiation combined with electron beam micro-analysis in investigating low-level concentrations of minor metals in common sulfides.

Characterization of Germanium Speciation in Sphalerite (ZnS) from Central and Eastern Tennessee, USA, by X-ray Absorption Spectroscopy

“…The GeI 2 standard, which is a mix of GeI 2 , GeI 4 , and GeO 2 , has three characteristic peaks: one at 11,107.5 eV, corresponding to GeI 2 ; another at 11,109 eV, corresponding to GeI 4 ; and one at 11,112 eV, corresponding to GeO 2 . These values differ to those found by References [20,23] (Figure 1); however, even though the energy positions are different, the distances in energy between the configurations remain the same, i.e., 1 eV between Ge(0) and Ge(II)I 2 , 2 eV between Ge(II)I 2 and Ge(IV)I 4 , and 3 eV between Ge(IV)I 4 and Ge(IV)O 2 .…”

“…However, the large beamsize meant that both dark and light growth bands were analysed during each measurement of the sphalerites from Central Tennessee. The conditions used in this study, aimed at increasing the sensitivity, are therefore complementary to those used by [20,21], which were aimed at increasing the spatial resolution.…”

“…Reference [20] also concluded that a large shift in the absorption edge position between two Ge 4+ species can be observed and that this shift was linked to the nature of the ligand (e.g., GeS and GeSe; Figure 1). Thus, the standards for this study were chosen by selecting compounds in which germanium is surrounded by atoms that have an electronegativity close to that of sulphur.…”

Mineralogical Distribution of Germanium, Gallium and Indium at the Mt Carlton High-Sulfidation Epithermal Deposit, NE Australia, and Comparison with Similar Deposits Worldwide

“…In general there is an excess of Cu over Ag in sphalerite, suggesting that Cu is the principal cation involved in the substitution. No statistically significant correlation was observed between Ge and other trace elements in sphalerite, something that has also been noted in previous studies [69,74,75]. This could potentially suggest that Ge substituted for Zn with vacancies in the structure to allow for charge balance, i.e., 2Zn 2+ ↔ Ge 4+ + □ [74,75].…”

“…However, a high activity of Cu or Ag may still promote the formation of Ge-bearing Cu or Ag sulfides at lower sulfur activities [9]. The precise nature of Ge incorporation into sphalerite is less clear, and potentially more complex, than that of In and Ga [10,69,74,75,107,108]. It is, therefore, difficult to assess possible physicochemical controls on Ge enrichment in these ores.…”