It is for any of the mineral that has four axes out of which three are equal in length and lie at an angle of 120° from each other. The fourth one may be long or short but at right angle toward the other corners.
A crystal system characterised by the presence of a hexad (axis of 6-fold rotation). Two of the axes are 120deg apart and are of equal length. The third axis (c) is aligned with the hexad, and is 90deg to the other two axes.
These crystals are usually shaped like six-sided prisms or pyramids. Each crystal has four axes of symmetry. Three lie in the same plane, are the same length, and intersect at 120° angles. The fourth axis is not the same length, and is perpendicular to other three. Example: beryl
(hex-ag´-o-nal) One of the six crystal systems, characterized by one unique axis of threefold or sixfold symmetry that is perpendicular and unequal in length to three identical axes that intersect at angles of 120°. This definition includes the trigonal system.