trigonal

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trigonal (rhombohedral) One of the seven crystal systems (closely related to the hexagonal system) in which the unit cell has the shape of a rhombohedron, consisting of six crystal faces all of which have two pairs of parallel sides. The crystals may be referred to four crystallographic axes: one vertical axis of three-fold crystal symmetry, and three equal, horizontal axes of two-fold symmetry which are separated by an angle of 120°. The minerals calcite and quartz always occur in this system.