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Each of the crystal systems has an associated set of common (and rarer) crystal forms. Crystal forms consist of a three‐dimensional set of one or more crystal faces that possess similar spatial relationships to the crystallographic axes. Some natural crystals possess only one crystal form; others possess multiple or combined crystal forms. Crystal forms can be subdivided into two major groups: closed forms and open forms.

Figure 4.15 A pyritohedron, a closed form in which all faces have the same general relationship to the crystallographic axes.

Closed crystal forms can completely enclose a mineral specimen and therefore can exist alone in perfectly formed (euhedral) crystals which are enclosed completely by the crystal form. Common closed forms include all the forms in the isometric system and many forms in the tetragonal, hexagonal, trigonal, and orthorhombic systems. The pyritohedron (Figure 4.15) is a typical closed form, common in the mineral pyrite. Each closed form possesses a different shape that is related to the number and shape of faces in the form and to their angular relationships to the crystallographic axes. Figure 4.16 illustrates common dipyramid (double pyramid) closed forms in the trigonal, tetragonal, and hexagonal systems.

Open crystal forms (Figure 4.17) cannot completely enclose a mineral specimen and so must occur in combination with other open or closed crystal forms in order to completely enclose a crystal. Common open forms include: (1) pedions, which consist of a single face, (2) pinacoids, a pair of parallel faces, (3) prisms, three or more faces parallel to an axis, (4) pyramids, three or more faces that intersect an axis, (5) domes, a pair of faces symmetrical about a mirror plane, and (6) sphenoids, a pair of faces symmetrical about an axis of rotation. Figure 4.17b illustrates the kinds of prisms that occur in the trigonal, tetragonal, and hexagonal crystal systems.

Figure 4.16 Different types of dipyramid forms in the trigonal, tetragonal, and hexagonal systems.

Source: Klein and Hurlbut (1985). © John Wiley & Sons.

The most common crystal forms in each system are discussed later in this chapter, after we have presented the language used to describe them. More detailed discussions are available in Nesse (2016) and Klein and Dutrow (2007).