Abstract
Anhedral quartz slabs from a vein deposit in Madagascar showing smooth cleavage planes parallel to positive rhombohedra r {1011} are described. Only two of the three equivalent pairs of the form {1011} show cleavage, although of different quality (smoothness). The best developed planes lie parallel to Brazil-twin lamellae; the less well developed planes do not. We presume small technically induced thermal shocks to be the releasing mechanism for rhombohedral cleavage fractures. In combination with hydrolytic weakening and directed stress, large smooth cleavage planes were formed. Since the r-faces of quartz are F-faces, they have a minimum of free surface energy. We assume this to be the reason for cleavage parallel to positive rhombohedral faces.
