11.7: Crystal Habit and Crystal Faces

Why do halite and garnet, both cubic minerals, have different crystal habits? It is not fully understood why crystals grow in the ways they do. Cubic unit cells may lead to cube-shaped crystals such as typical halite crystals, octahedral crystals such as spinel, dodecahedral crystals such as garnet, and many other shaped crystals. Why the differences? Crystallographers do not have complete answers, but the most important factor is the location of atoms within a unit cell.

Crystals of a particular mineral tend to have the same forms, or only a limited number of forms, no matter how they grow. Haüy and Bravais noted this and used it to infer that atomic structure controls crystal forms. In 1860, Bravais observed what we now call the Law of Bravais:
blank■ Faces on crystals tend to be parallel to planes having a high density of lattice points.

Figure 11.48: Two choices of unit cells and some typical shapes for calcite crystals

This means that, for example, crystals with hexagonal lattices and unit cells often have faces related by hexagonal symmetry. Crystals with orthogonal unit cells (those in the cubic, orthorhombic, or tetragonal systems) tend to have faces at 90° to each other.

The relationship between lattice/unit cell symmetry and crystal habit can be seen in Figure 11.48. The figure shows two choices for calcite’s unit cell (yellow drawings). The rhombohedron is a doubly primitive unit cell (containing two CaCO₃ motifs) and the hexagonal equivalent contains four CaCO₃ motifs. The drawings below the unit cells show some common shapes for natural calcite crystals. There is noticeable resemblance between unit cell shapes, which represent the lattice symmetry, and some of the crystal shapes. Thus, Bravais’s Law works well. The photos below in Figures 11.47 – 11.52 show natural calcite crystals that match quite well the drawings in Figure 11.48.

Figure 11.49: Calcite scalenohedra

Figure 11.50: A calcite rhombohedron

Figure 11.51: Calcite on fluorite

Figure 11.52: Calcite on galena

Figure 11.53: Calcite with orpiment

Figure 11.54: Calcite with siderite

Unfortunately, some minerals, including pyrite (FeS₂) and quartz (SiO₂), appear to violate Bravais’s Law. Bravais’s observations were based on considerations of the 14 Bravais lattices and their symmetries, but in the early twentieth century, P. Niggli, J. D. H. Donnay, and D. Harker realized that space group symmetries needed to be considered as well. By extending Bravais’s ideas to include glide planes and screw axes, Niggli, Donnay, and Harker explained most of the biggest inconsistencies. They concluded that crystal faces form parallel to planes of highest atom density, a slight modification of the Law of Bravais.

Figure 11.55: The rate at which faces grow affects crystal shape

As a crystal grows, different faces grow at different rates. Some may dominate in the early stages of crystallization while others will dominate in the later stages. The relationship, however, is the opposite of what we might expect. Faces that grow fastest are the ones that eventually disappear. Figure 11.55 shows why this occurs. If all faces on a crystal grow at the same rate, the crystal will keep the same shape as it grows (the green crystal in Figure 11.55a). However, this is not true if some faces grow faster than others.

In Figure 11.55b, the diagonal faces (oriented at 45^o to horizontal) grew faster than those oriented vertically and horizontally. Eventually, the diagonal faces disappeared; they “grew themselves out.” The final crystal has a different shape, and fewer faces, than when it started growing. We observe this phenomenon in many minerals; small crystals often have more faces than larger ones.