Basal twinning of Greenland corundum

Abstract

Natural corundum shows two types of twins: “basal twin”, by reflection on (0003) pinacoid, very rare, and “rhombohedral twin”, by reflection on ($10\overline{1 }1$), more frequent. The analysis of the structural continuity across the composition plane does not show any reason for 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 seems to confirm a morphological control on the occurrence of the “basal twin”. All analysed twinned samples show macrosteps on their pinacoidal faces and this feature has been related to the high-temperature conditions and intense fluid-rock interactions of Greenland deposit. This clearly suggests a strong relationship between the “basal twin” occurrence, the development of basal faces, and the formation conditions. However, due to the complex geological context and the different features of samples (e.g. two individuals with almost the same size versus several lamellae stacked along c axis), it is not possible to establish with certitude if the “basal twins” observed in Greenland samples are growth or mechanical twins.

Appendix

Investigation of possible epitaxy between corundum and phlogopite

As shown by Royer (1928, 1954), epitaxy requires a common two-dimensional unit cell on the composition plane separating the two minerals. Because of the difference in chemistry and crystal structure, some tolerance has to be included. A common two-dimensional unit cell can be found if the Bravais lattice of the two minerals include pairs of [uvw] directions with close periodicity and interaxial angles. In order for epitaxy to form, these directions should belong to lattice planes corresponding to crystal forms developed in the morphology of the minerals. The most common crystal forms of corundum are the rhombohedra {hh$\overline{2 }$hl}; as discussed above, the basal pinacoid {0003} is rare. Phlogopite is instead dominated by the basal pinacoid {001}; the other forms, much less developed, are the rhombic prisms {hk0}.

In the supplementary material (spreadsheet) we have computed all the possible sets of pairs of directions for the two minerals, up to 20 Å period, with a maximal linear difference of 0.5 Å and angular difference of 3º. Concretely, out of all the possible [uvw] directions up to 20 Å for each mineral, we have selected the pairs [uvw]₁ and [uvw]₂ (ordered so that the period of [uvw]₁ is shorter than or equal to the period of [uvw]₂) which satisfy the following requirements:

• the period of [uvw]₁ for corundum and for phlogopite differ by no more than 0.5 Å;

• the period of [uvw]₂ for corundum and for phlogopite differ by no more than 0.5 Å;

• the angle between [uvw]₁ and [uvw]₂ for corundum and the angle between [uvw]₁ and [uvw]₂ for phlogopite differ by no more than 3 Å.

These conditions allow to identify two-dimensional unit cells reasonably close to each other to be compatible with the occurrence of epitaxy. For phlogopite, we have used the cell parameters reported by Laurora et al. (2007), sample MS9; this is a 1M polytype, which is the most frequent polytype in phlogopite (about polytypism in micas, see Nespolo and Ďurovič, 2002). In red, the planes containing lattice directions satisfying the above conditions which correspond to a crystal form commonly present in the mineral. Yellow background is used when these planes occur in both minerals: these are the only cases when one may reasonably expect epitaxy between corundum and phlogopite. Of these, the {001} basal pinacoid of phlogopite should be considered with particular attention, given its large predominance in the morphology of the mineral. By simple inspection one immediately reckons that epitaxy, if present, would rather occur between a rhombohedral face of corundum and the basal pinacoid of phlogopite, favouring therefore the “rhombohedral” twin rather than the “basal” twin.