Mineralogists use the term form to refer to a group of identically shaped faces on a crystal. The faces of a form are related by crystal symmetry and have identical chemical and physical properties. If a crystal contains only one form, all crystal faces are the same size and shape. Euhedral garnet crystals, for example, generally have one form consisting of 12 identical diamond-shaped faces like the dodecahedron in Figure 3.5, earlier in this chapter. (Some garnet crystals, such as the one depicted at the bottom of Figure 3.9, however, contain a different form called a traphezohedron.)
The drawings in Figure 3.9 show common forms for six different minerals. Different samples of the same mineral may crystallize with different forms, but those shown here are typical. Like most garnet crystals, chabazite crystals generally contain only one form, typically six identical nearly (but not quite) square faces. Five of the drawings show crystals with more than one form. In the ilmenite, corundum, vesuvianite and datolite drawings, the different forms have distinctive different shapes. In the gehlenite drawing, all faces are rectangular but not all are the same size. The gehlenite crystal contains three forms (three pairs of identical rectangular faces) with different sizes.