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The only crystal forms in the triclinic system, with its extremely low symmetry, are pinacoids and pedions. Common forms and minerals in the triclinic system are illustrated in Figure 4.30 and listed in Table 4.12.

Figure 4.29 Monoclinic crystal forms: (a) front, side, and basal pinacoids, (b) two monoclinic prisms and a side pinacoid.

Table 4.11 Common crystal forms, form indices, and minerals in the monoclinic system.

Crystal form	Form indices	Form description	Minerals that commonly exhibit crystal form
Monoclinic prisms; first, third, and fourth orders	{011} {0kl} {110} {hk0} {hkl}	Four rectangular faces	Gypsum, staurolite, clinopyroxenes, amphiboles, orthoclase, sanidine, sphene (titanite), borax
Pinacoids; front, side, and basal	{001} {010} {001}	Pair of rectangular faces perpendicular to a‐, b‐, or c‐axis	Gypsum, staurolite, sphene (titanite), epidote, micas, clinopyroxenes, amphiboles

Table 4.12 Common crystal forms, form indices, and minerals in the triclinic system.

Crystal forms	Form indices	Form description	Minerals that commonly exhibit crystal form
Pinacoids	{001}{010}{001} {0k1} {hk1} and variations	Two parallel faces	Kyanite, plagioclase, microcline, amblygonite, rhodonite, wollastonite
Pedions	{hk1}	Single face	Similar

Figure 4.30 Triclinic crystal forms: (a) front, side, and basal pinacoids, (b) various pinacoids and a pedion to the lower right.