Growth history and textures of quartz twinned in accordance with the Japan law

Abstract

The growth history of quartz twinned according to the Japan law and the development of fine textures near Japan twin boundaries were analysed using optical microscopy, cathodoluminescence, and computational simulations. Cathodoluminescence images of the samples show two distinct growth stages. In the early growth stage, growth sectors of rhombohedral faces bordered by the {1122} composition plane serve as preferential growth sites that grow approximately 1.5–1.8 times faster than the other symmetrically equivalent faces. Thus, the samples initially exhibit a “fan-shape”. The composition plane is quite straight and parallel to {1122} in the growth sectors of rhombohedral faces but undulates in the growth sectors of prism faces. In the later growth stage, the twinned crystals grow rather isotropically and change into “V-shape”. Prism faces start to develop at the composition plane, which is no longer parallel to {1122}. Observations of etched figures show that fine Brazil twin lamellae are concentrated almost exclusively near the Japan twin boundary. In the early growth stage, polysynthetic Brazil twin lamellae are formed in an hourglass-shaped sector, and the lamellae develop progressively as the Japan twin grows. However, there are considerably fewer lamellae in the later growth stage. Structures at the intersection of Japan and Brazil twin boundaries were modelled using molecular dynamics simulations. The intersections of Brazil and Japan twin boundaries were found to be structurally coherent when they have certain composition planes. The results imply that Japan twin boundaries only have structural diversity, allowing either right-handed or left-handed quartz to grow coherently on the substrate crystals, when the composition plane of Japan twin is parallel to {1122} and rhombohedral faces are bordered by the {1122} composition plane. This structural diversity of the {1122} Japan twin boundaries causes development of fine Brazil twin lamellae and triggers faster growth rate of rhombohedral faces bordered by the straight {1122} composition plane.