Calaverite - Structure

Structure

Calaverite’s structure has been both an object of fascination and frustration in the scientific community for many years. Goldschmidt et al. indexed calaverite 105 crystals resulting in 92 forms but needed five different lattices to index all of the faces. This led to consideration that calaverite violated Haüy's Law of Rational Indices.

The introduction of X-ray diffraction did not completely solve this problem. Tunell and Ksanda in 1936 and then Tunell and Pauling in 1952 solved the C2/m general structure of calaverite. However, additional diffraction spots which they could not interpret were present in the survey. Later, transmission electron microscopy study suggested that the satellite reflections in calaverite were due to Au in commensurately displacive modulation superimposed on the average C2/m structure. In 1988, Schutte and DeBoer solved the structure by using the 3H super space group C2/m (α O γ)Os. They also showed that these modulations consist mainly of the displacements of tellurium atoms and the observed modulations were interpreted in terms of valence fluctuations between the Au+ and Au3+. According to Schutte and DeBoer, those displacements also affect the coordination number of calaverite.

In 2009, Bindi et al. concluded that the different coordination numbers associated with calaverite were indeed associated with a significant differentiation in the valence sum of Au, and that the random distribution of Ag suppresses the fluctuation of Au+ and Au3+, whereas the ordered distribution reinforces it.