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Common closed crystal forms in the trigonal system include the six‐sided rhombohedron, the 12‐sided scalenohedron, six‐sided hexagonal dipyramids and three‐sided trigonal pyramids. Three‐ and six‐sided prisms, and pinacoids and pedions are also common. Many forms common in the hexagonal division also occur in trigonal crystals, but not vice versa. Common crystal forms and representative minerals in the trigonal crystal system are summarized in Figure 4.27 and Table 4.9.

Figure 4.26 Common crystal forms in the hexagonal crystal system (hexagonal division): (a) hexagonal prism closed by a basal pinacoid, (b) hexagonal dipyramid.

Table 4.8 Common hexagonal crystal forms and associated hexagonal and trigonal minerals.

Crystal forms	Common form indices	Form description	Minerals that commonly exhibit crystal form
Hexagonal dipyramid	and variations	12 triangular faces inclined to c‐axis; top six separated from bottom six by mirror plane	Apatite, zincite
Hexagonal prism	and variations	Six rectangular faces parallel to c‐axis	Apatite, beryl, quartz, nepheline, corundum, tourmaline
Basal pinacoid	{0001}	Pair of faces perpendicular to c‐axis	Apatite, beryl, corundum

Figure 4.27 Common crystal forms in the trigonal system: (a) trigonal dipyramid, (b) rhombohedron, (c) scalenohedron.