Hexagonal System, Tri-Pyramidal and Hemimorphic Class

The Tri-Pyramidal Class
A division of the Hexagonal System showing lower symmetry than that of the Normal class is known as the Tri-pyramidal Class. It has a vertical axis of hexagonal symmetry and a horizontal plane of symmetry. It is characterized by the form known as the pyramid of the third order. This form consists of twelve faces, which correspond in their position to one-half of the faces of a dihexagonal pyramid. The minerals of the Apatite Group are the only ones of importance in this class, and upon their crystals the pyramid of the third order is rarely to be seen. When it is observed it shows usually only small faces. Fig. 118 represents a complex crystal of apatite with the faces of a third-order pyramid (u) upon it.

Hemimorphic Class
The crystals of certain rare minerals show the forms of the Normal Class but with hemimorphic development. A hemimorphic crystal is one that shows different forms or combinations of forms at the opposite ends of a symmetry axis. Fig. 119 represents a crystal of zincite with a prism terminated by a pyramid above and a basal pinacoid below.