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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	{h0hl}	Six parallelogram faces inclined to c‐axis	Dolomite, calcite, siderite, rhodochrosite, quartz, tourmaline; chabazite
Trigonal Scalenohedron	{hkil} and variations	12 scalene triangle faces inclined to c‐axis	Calcite
Trigonal prism	{hki0} and variations	Three rectangular faces parallel to c‐axis	Tourmaline, calcite, quartz
Trigonal dipyramid	{hkil} and variations	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 crystallographic axis	Stibnite, aragonite, barite, celestite, topaz, enstatite, andalusite, cordierite, epidote, olivine
Pinacoids; front, side and basal	{001} {010} {001}	Two parallel faces perpendicular to a‐, b‐ or c‐axis	Barite, celestite, olivine, andalusite, topaz, hemimorphite