The twinning microstructure and growth of amethyst quartz

Abstract

The characteristic lamellar-twinning of the right-handed (R) and left-handed (L) structures in the major rhombohedral growth sectors of amethyst quartz has been studied by optical techniques, X-ray topography and transmission electron microscopy (TEM).

The TEM observations show that the region of each Brewster fringe consists of fine-scale Brazil twin lamellae parallel to one of the r, z{10$\overline {\text{1}}$1} planes, and structural considerations suggest that it is one of the r-planes. The twin boundary corresponding to a Brewster fringe has the form of a zig-zag structure consisting of Brazil twin boundaries on two r{10$\overline {\text{1}}$1} planes, with one predominating. The Brewster fringes appear black between crossed polarizers because light travelling along the optic axis [001] passes through almost equal distances of R and L quartz, giving essentially zero optical rotation. From the visibility of the Brazil twin boundaries in electron micrographs and the visibility of the Brewster fringes in X-ray topographs, the fault vector R and the corresponding composition plane of the major Brazil twin associated with each Brewster fringe has been determined. The streaking of the Brewster fringes observed optically and in the X-ray topographs appears to be due to the stair-rod dislocations at the intersections of the Brazil twin boundaries.

Experiments in which synthetic quartz was grown hydrothermally on untwinned seeds and on twinned amethyst seeds showed that the initiation of Brazil twins and the development of Brewster fringes was dependent upon the presence of iron in the growth solution.