ISOMETRIC SYSTEM 



61 



the I ype, Imt commonly in (111) (010) ir (hlO) with striations on the 

 cul > face, parallel to its edges, due to alternations of growth be- 

 tween the cube and pyritohedron, Fig. 102. These striations are 

 parallel to the planes of symmetry which bisect the cube edges, 

 and not, as in sphalerite, type 31, parallel to the planes which con- 

 tain the edges and cross the face diagonally. 



Other representatives of the type are smaltite, CoAsj ; cobaltite, 

 CoAsS. 



CLASS, PLAGIOHEDRAL (GYROIDAL) HEMIHEDRONS 

 TYPE 29, TESSERAL HOLOAXIAL 



As the name implies, this type possesses all the axes, 3 tetragonal, 

 4 trigonal, 6 digonal, of the system, but no planes or center of 

 symmetry. 



Forms 



I. Pentagonal icositetrahedron (didodecahedron) r/1 - ; 



T (hkl) T (khl). 



If every other face of the hexoctahedron around the ditetragonal 

 axis is extended, as indicated by the shaded faces of Fig. 39, the 

 solid formed will have the symmetry of this type. It is bounded 

 by 24 pentagonal faces, 4 of which are grouped around the tetrag- 

 onal axes, 3 around the trigonal, and the digonal axes of symme- 

 try bisect the edge between two faces. When the right upper face 

 of the positive octant is extended, then the right pentagonal dido- 

 decahedron is produced, Fig. 103. If the left upper face is ex- 



Fiu. 103. The Right IVn- 

 tagonal Didodecahedron. 



Fi<;. 104. The Left Pen- 

 tagonal Didodecahedron. 



tended, the left pentagonal didodecahedron, Fig. 104, is produced. 

 Figure 105 is a spherical projection of the right form. They differ 

 from + and hemihedra, as there is no way of revolving one into 



