The Isometric Pyritohedral Class
They Pyritohedral Class is one of the subordinate divisions of the Isometric System. It differs from the Normal class, since its crystals commonly show forms that do not possess as high symmetry as those of that class. The name of that class is derived from that of its chief member, pyrite.
Symmetry and Forms
The symmetry of the Pyritohedral Class is as follows: they three crystal axes are axes of binary symmetry; the four diagonal axes, each of which emerges in the middle of an octant, are axes of trigonal symmetry; the three axial planes are planes of symmetry.
The characteristic forms of the Pyritohedral Class are as follows:
1. Pyritohedron or Pentagonal Dodecahedron
This form consists of twelve pentagonal-shaped faces, each of which intersect one crystallographic axis at unity, the second axis at some multiple, and is parallel to the third. There re a number of pyritohedrons which from each other in respect to the inclination of their faces. Perhaps the most common in occurrence has the parameter relations 1a, 2b, c, the symbol of which would be. It is to be noted that the parameter relations of the pyritohedron are the same as those of the tetrahexahedron. A piritohedron may be considered as derived from a corresponding tetrahexahedron by the omission of alternate faces and the extension of those remaining. Fig. 56 shows the relations of the two forms, the shaded faces of the tetrahexahedron being those which when extended would form the faces of the pyritohedron.
2. Diploid
The diploid is a rare form. It is composed of twenty-four faces which correspond to one-half the faces of a hexoctahedron. Fig. 57 represents a diploid.
In addition to the two forms described above, minerals of this class show also the cube, octahedron, dodecahedron, trapezohedron and triscotahedron. Sometimes these forms may appear alone and so perfectly developed that they cannot be told from the forms of the Normal Class. This is often true of octahedrons of pyrite. Usually, however, they will show by the presence of striationlines or etching figures that they do not possess the high symmetry of the Normal class buy conform rather to the symmetry of the Pyritohedral Class. This is shown in Fig. 58, which represents a cube of pyrite with characteristic striations, which represents a cube of pyrite with characteristic striations, which are so disposed that the crystal shows the lower symmetry.
These figures should be studied in order to impress upon one’s mind the characteristic symmetry of the class.
The chief mineral of the Pyritohedral Class is pyrite; other much rare members of the Pyritohedral Class is pyrite; other much rare members are smaltite, chloanthite, cobaltite, gersdorffite and sperrylite.
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