11.12.1: Zones and Zone Axes

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11.68.png — Figure 11.68: Zones and zone axes

As discussed in Chapter 9, a set of faces parallel to a common direction defines a zone. We designate zones by the common direction, which we call the zone axis. Figure 11.68 shows examples of tetragonal, hexagonal, and orthorhombic crystals. The green faces in the tetragonal crystal (Figure 11.68a) form a zone. The faces are parallel to the a-axis. Three other faces (not visible) are also part of the zone. The zone axis is [100], parallel to a. Zones may contain faces from more than one form, as in this example.

Figure 11.68b shows a common zone in hexagonal minerals: 6 (blue) faces (three not seen) are parallel to the c-axis. The zone axis is [001]. And in Figure 11.68c, the zone (purple faces) comprises three forms and the zone axis is [010]. In all three drawings, additional zones are present besides those just mentioned. In these three examples, the zones axes have simple indices because they are parallel to crystal axes. Zones need not be parallel to axes, and consequently the indices may contain values other than 0 and 1.