A study of repeated twinning, lattice imperfections and impurity distribution in amethyst

Abstract

A basal-plane slice of a crystal possessing the amethyst structure of laminated Brazilian twinning in the major rhombohedron growth sectors has been studied by x-ray diffraction topography and absorption topography. The twin boundaries lie in the [0001] zone. Parallel with them lie strongly diffracting lamellae of imperfect crystal containing a high density of decorated dislocations and the highest iron impurity concentration in the slice. The decorated dislocations generally make fairly small angles with the basal plane and appear to pass through the lamellae. X-ray absorption shows that the major rhombohedron growth sectors contain on average 0·04 weight per cent more iron than the minor rhombohedron growth sectors in the specimen. The imperfect lamellae contain about 0·025% more iron than the regions between them.