26 



CRYSTALLOGRAPHY. 



and hence it affords an example of hemihedrism — a kind that, ia 

 presented by many crystals of pyrite. Fig. 48 is the hemihe- 



dral for 

 to the 



m resulting when these twelve planes i-2 are extended 

 obliteration of the cubic faces ; and fig. 49 is another, 

 made of the other twelve of these planes. Again, 

 in fig. 50, a cube is represented having only 

 three out of the six planes of fig. 22, and this 

 is another example of hemihedrism. These kinds 

 differ from the inclined hemihedrons in having 

 opposite parallel faces, and hence they are called 

 parallel hemihedrons. 



4. Internal Structure of Isometric Crystals, or Cleavage. — 



The crystals of many isometric minerals have cleavage, or 

 ^ greater or less capability of division in directions situated 

 symmetrically with reference to the axes. The cleavage direc- 

 tions are parallel either to the faces of the cube, the octahe- 

 dron, or the dodecahedron. In galenite (p. 145) there is easy 

 cleavage in three directions parallel to the faces of the cube ; 

 in fluorite (p. 208), in four directions parallel to the faces of the 

 octahedron ; in sphalerite (p. 154). in six directions parallel to 

 the faces of the dodecahedron. These cleavages are an impor- 

 tant means of distinguishing the species. 



The three cubic cleavages are precisely alike in the ease with 

 which cleavage takes place, and in the kinds of surface obtained ; 

 and so is it with the four in the octahedral directions, and the six 

 in the dodecahedral. Occasionally cleavages of two of these sys- 

 tems occur in the same mineral ; that is, for example, parallel to 

 both the faces of the cube and the octahedron ; but when so, 

 those of one system are much more distinct than those of the 

 other, and cleavage surfaces in the two directions are quite un- 

 like as to smoothness and lustre. 



5. Irregularities of Isometric Crystals. — A cube has its faces 

 precisely equal, and so it is with each of the form* ^presented 



