Hemimorphic, Tri-Rhombohedral and Trapezohedral Division
Tourmaline crystals show the forms of the Rhombohedral Class but with hemimorphic development. They are also commonly characterized by the presence of three faces of a triangular prism. Characteristic hemimorphic tourmaline crystals.
Rhombohedral Class. Tri-Rhombohedral Division
This is a subdivision of the Rhombohedral Class, which contains only a few and rare minerals. It is characterized by the forms known as the rhobohedrons of the second and third orders. The faces of a scond-order rhombohedron correspond in position to one-half the faces of the second-order hexagonal pyramid, and those of the third order to one-quarter of the faces of the dihexagonal pyramid.
Rhombohedral Class. Trapezohedral Division
The only important mineral of this class that is commonly found in crystals is quartz, and its crystals as a rule do not show forms other than those of the Rhombohedral Class, Normal Division. at times, however, small faces may occur of a form known as a trapezohedron, which shows a lower symmetry. This form has sis faces, which correspond in their position to one-quarter of the faces of a dihexagonal pyramid. the quartz crystals are said to be right-or left-handed, depending upon whether these faces are to be observed truncating the edges between prism and rhombohedron faces at the right or at the left.
Characteristic of Hexagonal Crystals
Hexagonal crystals are most readily recognized by the following facts: The vertical crystallographic axis is one of either hexagonal or trigonal symmetry. the crystals are commonly prismatic in habit. When viewed in the direction of the vertical axis, they usually show a hexagonal cross section.
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