Growth twins in chalcopyrite

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.