dodecahedron
has twelve diamond shaped or rhombic faces. Other forms are the
tetrahedron with only four triangular faces. These forms may occur
together in a crystal, which is then known as a combination of forms.
(See figs. 1-5).
2. Tetragonal system.—Tetragonal
prisms have four or eight faces and are parallel to the vertical axis.
Tetragonal pyramids have eight or sixteen faces. Sasal pinacoids are
the top and bottom faces. Combinations of the above forms are seen in
zircon. (See figs. 6-9).
3. Hexagonal system.—The
forms in this system have faces in numbers which are in multiples of
six. The hexagonal prism has six or twelve faces arranged parallel to
the vertical axis. Other forms must be present in combination with a
prism. So a prism is known as an open form. Hexagonal bipyramids
represent a form with two pyramids placed base to base. The forms may
have twelve or twenty-four faces. These are closed forms. The Basal pinacoid has two parallel faces at right angles to the vertical axis. The top and bottom faces of prisms are pinacoids. The Rhombohedron has six rhombic or diamond shaped faces. The Scalenohedron has
twelve faces which are scalene triangles and hence the name.
Combinations of prisms, bipyramids and basal pinacoids are common as in
beryl; combinations of prism and rhombohedron are common in quartz.
(See figs. 10-18).
Orthorhombic, monoclinic and triclinic systems.—These three systems have their forms so much alike and are considered together. Prisms: Forms parallel to the vertical axis, but intersect the lateral axis; domes similar to prisms, but are parallel to a horizontal, axis. Bipyramids are similar to those already dealt with. Pinacoids: There
may be three kinds of pinacoids, each consisting of two parallel faces.
Each face cuts one of the axes and is parallel to the other two. (See
figs. 19-23).
In the triclinic system forms can have only two similar or parallel faces. The different faces in crystals are
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