Hexagonal System, Rhombohedral Class Normal Division

The forms of this class are to be referred to the hexagonal crystallographic axes, but show a lower symmetry than those of the Normal Class.

Symmetry and Forms. The vertical crystallographic axis is one of trigonal symmetry, and the three horizontal crystallographic axes are axes of binary symmetry. There are three vertical planes of symmetry bisecting the angles between the horizontal axes.

1. Rhombohedron
The rhombohedron is a form consisting of six rhombic-shaped faces, which correspond in their position to the alternate face of a hexagonal pyramid of the first order. There may be two different orientations of the rhombohedron. It is to be noted that when properly orientated the positive rhombohedron has one of its faces, and the negative rhombohedron one of its edges, toward the observer. There are various rhombohedron one of its edges, toward the observer. There re various rhombohedrons, which differ from each other in the inclination of their faces. The symbol of the unit positive rhombohedron is (1011) and of the unit negative rhombohedron (0111). Characteristic combinations of positive and negative rhombohedrons with each other and with other hexagonal. As in the case of the tetrahedron of the Isometric System, to which these forms are analogous, the distinction between the positive and negative rhombohedrons is only one of orientation.

2. Scalenohedron
This form consists of twelve scalene triangular faces. These faces correspond in their position to the alternate parts of faces of a dihexagonal pyramid. the relation of the two forms have the striking characteristics of the scalenohedron are the zigzag middle edges which differentiate it from an ordinary pyramid and the alternating, relatively obtuse and acute angles over the edges that meet at the vertices of the form. There are many different possible scalenohedrons, depending upon the varying slope of their faces. A common scalenohedron having the symbol (2131). Characteristic combinations of scalenohedrons with other forms.