ISOMI:TI;K SYSTEM 





of the cuhe, which mark it as a hemihedral cube belonging 

 to the tetrahedral type of >ymmet ry, even if (he tetrahedron did 

 imt truncate the corners. 



1 Aamples of minerals crystallizing in the type. 



Diamond. C, (ill) (100) (321) (210) (320). 



Sphalerite, ZnS, (100) (111) ( 110) (311) (331) (210). Fig. 96. 



Tetrahedrite, l('u,S, SbzSs, (100) (110) (111) (221). Fig. 97. 



Boracite, (100) (110) (111) (410) (531) (221). Fig. 98. 



CLASS, PYRITOHEDRAL (PARALLEL-FACED) HEMIHEDRONS 

 TYPE 30, TESSERAL CENTRAL 



All forms of this type possess 

 four trigonal axes ending in the 

 center of the octants; three di- 

 digonal axes corresponding to the 

 crystallographical axes; three 

 planes, the diametral planes and a 

 center of symmetry. All forms of 

 the type are therefore bounded by 

 pairs of parallel faces. 



Symmetry. 4 trigonal axes, 

 3 didigonal axes, 3 planes and a 

 center, Fig. 99. 



Forms 



FIG. 99. 



I. Diploid; 



na : a : ma 



ir (hkl) IT (hkl.) 



If planes be grouped on the axes to conform to the symmetry 

 of the type, it will be found that if half of the faces of the hexocta- 



Fio. 100. The Minus Dip- 

 loid, ir (hkl). 



FIG. 100 a. 



