The anomalous high-index faces (hkl) of the mineral calaverite (Au1 - xAgxTe2) measured goniometrically in the year 1931 by Goldschmidt, Palache & Peacock [Neues Jahrb. Mineral (1931), 63, 1-58] are re-interpreted and related to the wave vector q of the displacive incommensurate modulation which was recently found in the crystal structure. All crystal faces (including the high-index ones) can be given four low indices (hklm), using q as a fourth basis vector. From this an almost hundred-year-old anomaly in crystal morphology is in principle solved.

The existence of large-sized satellite faces on crystals of the mineral calaverite (Au1-xAgxTe2) 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 (hklm).

The morphology of Rb2ZnBr4 single crystals as observed in growth sphere experiments is analyzed on the basis of geometrical morphological laws extended to take into account the incommensurate modulated nature of these crystals. Especially in the past, identification of crystals faces (hkl) on the basis of these classical morphological laws has been a powerful tool in the determination of 3-dimensional cell parameters. In analogy, the relative magnitude of the modulation wave vector (the fourth lattice constant in the language of the superspace approach) can be deduced now from the identification of satellite faces described by four low rational indices (hklm).

The macroscopic consequences of displacive modulation on the morphology of incommensurate single crystals are confirmed. Bands of faces in the neighborhood of stable normal crystal faces have been observed on spherically shaped Rb2ZnBr4 crystals. An interpretation is given in terms of classical morphological theory extended to include (four-dimensional) superspace group symmetry. This leads to the view that the formation of these bands involves, at least partly, so-called satellite faces and gives a simple explanation of why the set of bands has a lower point-group symmetry than the set of normal faces.