Trigonal system (hexagonal system, trigonal division) forms
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 |
|
12 triangular faces inclined to c‐axis; top six separated from bottom six by mirror plane | Apatite, zincite |
| Hexagonal prism |
|
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.
Orthorhombic crystal system
Common crystal forms in the orthorhombic system include four‐sided rhombic prisms and pyramids, and eight‐sided dipyramids. Pinacoids are the dominant form and pedions are also common. Common crystal forms and associated minerals in the orthorhombic crystal system are indicated in Figure 4.28 and Table 4.10.
Monoclinic crystal system
Because of its lower symmetry, the only crystal forms in the monoclinic crystal system are four‐sided prisms, two‐sided domes, sphenoids and pinacoids, and pedions. More complex forms cannot exist in systems with low symmetry in which some crystallographic axes do not all intersect at right angles. Common crystal forms and associated minerals in the monoclinic crystal system are depicted in Figure 4.29 and Table 4.11.
Table 4.9 Common trigonal crystal forms and associated minerals.
| Crystal forms | Form indices | Form description | Minerals that commonly exhibit crystal form |
|---|---|---|---|
| Rhombohedron |
|
Six parallelogram faces inclined to c‐axis | Dolomite, calcite, siderite, rhodochrosite, quartz, tourmaline; chabazite |
| Trigonal Scalenohedron |
{hki |
12 scalene triangle faces inclined to c‐axis | Calcite |
| Trigonal prism |
{hki |
Three rectangular faces parallel to c‐axis | Tourmaline, calcite, quartz |
| Trigonal dipyramid |
{hki |
Six triangular faces; top three separated from bottom three by a mirror plane | Tourmaline |
Figure 4.28 Common crystal forms in the orthorhombic crystal system: (a) rhombic dipyramid, (b) front, side, and basal pinacoids, (c) rhombic prism with a pinacoid.
Table 4.10 Common crystal forms, form indices, and minerals in the orthorhombic system.
| Crystal forms | Form indices | Form description | Minerals that commonly exhibit crystal form |
|---|---|---|---|
| Rhombic dipyramids | {111} {hkl} and variations | Eight triangular faces; top four separated from bottom four by a mirror plane | Topaz, aragonite, witherite, olivine |
| Rhombic prisms; first, second, and third orders | {011} {0kl} {101} {h0l} {011} {0kl} |
Four rectangular faces parallel to a single
|