58 



MINERALOGY 



considered as produced by the extension of alternate faces of the 

 octahedron. 



Combinations 



Of the seven forms which may combine in the tetrahedral class, 

 the tetrahexahedron, hexahedron, and rhombic dodecahedron are 

 apparently holohedrons in shape. They can be distinguished from 

 the latter when not in combination with tetrahedral forms only by 

 special markings, striations, etch figures, or other physical proper- 

 ties, which indicate a symmetry of the lower type. 



Crystal faces often contain striations, parallel to the edge of 

 common combinations found represented on the crystals. They 



FIG. 95. 



FIG. 96. The Plus and Minus 

 Tetrahedrons of Sphalerite. 



are attributed to alternations of growth, in which the face is reduced 

 to the minimum, and appear as a striation parallel to the common 

 edge. Fig. 95 represents a combination of the hexahedron and 

 tetrahedron in sphalerite. The striations on the hexahedral faces 



FIG. 97. Combination of 

 ic (111), ic (211), (110) on 

 Tetrahedrite. 



FIG. 98. Combination of 

 (100), K(lll), K(211) on 

 Boracite. 



are traces of tetrahedral faces. It is to be noted that the striations 

 on the cube faces are symmetrical to planes containing opposite 



