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 ssss1 and listed in ssss1.

ssss1 Monoclinic crystal forms: (a) front, side, and basal pinacoids, (b) two monoclinic prisms and a side pinacoid.

ssss1 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

ssss1 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

ssss1 Triclinic crystal forms: (a) front, side, and basal pinacoids, (b) various pinacoids and a pedion to the lower right.