The enigmatic structure of cobaltite revisited

The structure of the mineral cobaltite (CoAsS) has puzzled scientists ever since the first investigations were conducted in the early 20th century, with various space groups having been proposed to describe its internal symmetry. In this study, we present and analyse new X-ray and electron diffraction data from a natural cobaltite sample. Based on single-crystal X-ray diffraction (SCXRD) data collected over the temperature range 85-500 K, we assign the space group Pca21 and accurately solve the crystal structure of cobaltite. We also find that the cobaltite structure shows complete As-S ordering throughout the investigated temperature interval, with unit-cell parameters varying by ∼0.33%. Moreover, while powder X-ray diffraction diffractograms are consistent with our cobaltite structure solution derived from SCXRD data, electron diffraction patterns of cobaltite crystals instead display “forbidden” Bragg reflections for the Pca21 space group. This discrepancy between X-ray diffraction patterns and electron diffraction patterns reflects the frequent occurrence of micro- or nanoscale twinning in cobaltite crystals and indicates a prolonged mineralogical evolution of this sulfoarsenide derived from high-temperature polymorphs.