Composite hexabromobenzene crystals

Weissenberg photographs of hexabromobenzene crystals have shown that many crystals consist of two or more portions.It has been found that two such portions always have their b axes (i.e. the needle axes) parallel.The difference in orientation between them is always one of two experimentally determined angles, or a linear combination of these two angles.The phenomenon cannot be termed twinning and its relation to true twinning is discussed.It is suggested that the formation of such composite crystals may be due to oriented overgrowth or oriented agglutination of small crystallites, and results from the pseudo-hexagonal symmetry of the lattice.Preliminary attempts to explain why only two orientation laws are observed, and why true twinning has not been found, have not been successful.