Growth twins in chalcopyrite

Abstract In the three twins known in chalcopyrite, 110 occurs only as mechanical penetration twin. In contrast, whereas 112 and 204 both occur as mechanical and growth twins, respectively. In both cases, the twins are contact twins, and the composition plane is parallel to the twin plane. 204 has been reported as occurring much more frequently than 112 as growth twin. We show that the structure at the composition plane, obtained through the analysis of the pseudo-eigensymmetry of the crystallographic orbits, justifies this different occurrence frequency because the coordination of iron and copper atoms is severely affected by 112 twinning, whereas it is only slightly distorted by 204 twinning.