The existence of large-sized satellite faces on crystals of the mineral calaverite (${\mathrm{Au}}_{1-x}{\mathrm{Ag}}_x{\mathrm{Te}}_2$) is reported. These anomalous high-index faces can be related directly to the wave vector q of the displacive incommensurate modulation recently found in the crystal structure. If we extend the classical morphological law of rational indices to include q as a fourth basic vector, the high-index faces can be described by four low integers ($\mathrm{hklm}$).

• Received 19 July 1985

DOI:https://doi.org/10.1103/PhysRevLett.55.2301