Pignatelli, I.; Giuliani, G.; Morlot, C.; Pardieu, V.
Journal of Gemmology, 2022, 38, 4, 364-375
Voir en ligne : https://doi.org/10.15506/JoG.2022.38.4.364
Abstract :
A ‘Star of David’ appearance is rarely exhibited on gem corundum crystals due to triangular step patterns in opposing orientations on each pinacoidal face. Such samples were recently found at the Aappaluttoq mine in Greenland, and are characterised by eye-visible triangular macrosteps with micrometre- to millimetre-size edges, an uncommon feature on natural corundum crystals. The edge height, as well as the presence of wide terraces and trench-like defects on the crystals, suggest that the macrosteps result from step bunching and faceting1 processes. The macrosteps likely formed under high-temperature conditions and intense fluid-rock interactions, in a growth medium where impurities and direction of fluid flow played an important role in the step formation. Large macrosteps are also found on corundum crystals from other deposits of both magmatic and metamorphic origin.