gems, corundum, chemistry.2, properties.3, Greenland.12

Natural corundum shows two types of twins: “basal twin,” by reflection on the (0003) pinacoid, which is very rare, and “rhombohedral twin,” by reflection on the (10̅11) plane, which is more frequent. The analysis of the structural continuity across the composition plane does not reveal any inherent reason for such a large difference in occurrence frequency, which is likely related to the limited development of the (0003) plane in the characteristic morphology of corundum. “Basal twins” occur with unusually high frequency in samples from Greenland, which also present an atypical platy morphology, where the (0003) face is well developed. This observation appears to confirm a morphological control on the occurrence of the “basal twin.” All analysed twinned samples show macrosteps on their pinacoidal faces, a feature associated with the high-temperature conditions and intense fluid–rock interactions of the Greenland deposit. This clearly suggests a strong relationship between “basal twin” occurrence, the development of basal faces, and the formation conditions. However, due to the complex geological context and differing features of the samples (e.g. two individuals with almost the same size versus several lamellae stacked along the c-axis), it is not possible to determine with certainty whether the “basal twins” observed in Greenland samples are growth twins or mechanical twins.