Abstract
The Tres Marias carbonate-hosted Zn-Ge deposit in Chihuahua, Mexico contains willemite [Zn2SiO4] with unusually high concentrations of minor and trace elements (e.g., Pb, Ge, As, P, V); Pb concentrations are as high as 2 wt %, and Ge may reach 4000 ppm (average 900 ppm). Electron microprobe analyses and synchrotron X-ray fluorescence maps show that Zn and Ge, as well as Zn and Pb are negatively correlated, whereas Ge and Pb are positively correlated across zoned willemite crystals. In cathodoluminescence (CL) images, those areas of willemite having high trace element concentrations have no, or low CL intensities, whereas zones low in trace elements (except for P) display bright blue CL colors. X-ray absorption fine structure (XAFS) spectroscopy was used to characterize the chemical nature of Ge and Pb in willemite. Comparisons to reference spectra of natural and artificial substances points to the presence of Ge4+ and Pb2+ in Tres Marias willemite. No evidence for Pb4+ was detected. Oscillatory zonation reflects trace element incorporation into willemite from the oxidation of primary Ge-bearing sphalerite and galena (PbS) by siliceous aqueous fluids.
Highlights
Germanium (Ge) is a high technology metal in great demand for use in the manufacture of optics and sensors, computer chips, solar cells, and as a catalyst in plastics manufacturing
Investigating the chemical behavior of trace elements in willemite is of interest because of its use as an ore of non-sulfide zinc [4] and for understanding chemical substitution mechanisms, and the controls on its luminescence and other electronic properties [5,6]
Willemite from the Bou Arhous deposit [13] in Morocco contains over 1000 ppm Ge [14]
Summary
Germanium (Ge) is a high technology metal in great demand for use in the manufacture of optics and sensors, computer chips, solar cells, and as a catalyst in plastics manufacturing. Investigating the chemical behavior of trace elements in willemite is of interest because of its use as an ore of non-sulfide zinc [4] and for understanding chemical substitution mechanisms, and the controls on its luminescence and other electronic properties [5,6]. Tetravalent, tetrahedrally coordinated lead, IV Pb4+ , has an ionic radius of 0.65 Å, similar to 0.60 Å for IV Zn2+ [15] In this case, Pb would exist in a sp hybridized form, and following arguments concerning Fe [16] and Mn [17] substitution into willemite, it might preferentially enter the slightly larger Zn(2) position. Understanding the oxidation state of these elements in the willemite structure is critical for understanding trace element substitution mechanisms for geochemical and technological applications
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