62 



MINERALOGY 





O; 



FIG. 105. The Right Pentagonal 

 Icositetrahedron. 



a congruent position with the other ; they are not superimpos- 

 able; one is a mirror image of the other. 

 Such pairs of forms are enantiomorphous. 



.....^ Other forms of the type do not 



differ geometrically from those of 

 type 32, the forms which may be 

 found in combination with the pen- 

 tagonal didodecahedron, the only new 

 form of the type will be the tetra- 

 hexahedron (hlo). 



Tetragonal trisoctahedron (hhl). 

 Trigonal trisoctahedron, (hhi). 

 Hexahedron, (010). 

 Rhombic dodecahedron, (110). 

 Octahedron, (111). 

 As six forms of the type are in shape holohedrons, minerals which 

 crystallize in the type are distinguished by a study of the symmetry 

 of the etch figures; this is especially 

 necessary as the occurrence of the most 

 general form is always rare. Sylvite, 

 KC1, occurs in combinations of the cube 

 and octahedron, Fig. 106. Etch figures 

 appear on the cube faces as shallow pits 

 with square outline. The position of 

 the square pits depends upon the sym- 

 metry of the point-system, as the type 

 contains no planes of symmetry; they 

 are so oriented on the cube that none of the possible planes which 

 are traced on the cube face will cut them symmetrically. 

 Examples. 



Cuprite, Cu 2 O; (111) (100) (110) (211) rarely in T (hid), from 

 Cornwall, Eng., 



Ammonium Chloride NH 4 C1; (111) (110) (100) r(875). 



TETARTOHEDRAL CLASS 

 TYPE 28, TESSERAL POLAR 



Symmetry : crystals of this type must conform to 4 trigonal 

 axes ending in the center of the octants; these four trigonal axes 

 maintain their position in all five types of the isometric system 

 and to three digonal axes corresponding to the crystallographic 

 axes. They have no planes or center of symmetry. 



FIG. 106. Diagram of Etch 

 Figures on Sylvite. 



