Quartz with rhombohedral cleavage from Madagascar

Abstract

Anhedral quartz slabs from a vein deposit in Madagascar showing smooth cleavage planes parallel to positive rhombohedra r {l0ll) are described. Only two of the three equivalent pairs ofthe form {l0Tl} show cleavage, although ofdiflerent quality (smoothness). The best developed planes lie parallel to Brazil-twin lamellae; the less well developed planes do not. We presume small tectonically 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.