54 



MINERALOGY 



combinations of a few simple forms. The general appearance or 

 habit is fixed by the simple form predominating in the combination. 

 Figure 84 is a combination of a hexahedron and octahedron, the 

 former predominating ; Fig. 85 is the same combination with the 

 latter predominating. 



Examples crystallizing in the Type 

 Copper, forms (100) (110) (111) (410) (211) (531). Fig. 86. 



FIG. 86. Combination of a (001) 

 and d (110) on copper. 



FIG. 87. Magnetite, a (001) 

 and d (110). 



Lead, forms (100) (110) (111) (410) (550). 

 Silver, forms (110) (111) (310) (751). 



Galena, PbS, forms (100) (110) (111) 



(221). Fig. 84. 

 Magnetite, Fe 3 O 4 , forms (100) (110) 



(111) (210) (221) (432). Fig. 87. 

 Fluorite, CaF 2 , forms (100) (111) (421) 



(110) (211). 

 Analcite, NaAl 3 (SiO 3 )2, H 2 O, forms 



(100) (211) (575). 

 Garnet, forms (110) (111) (310) (221). 



Fig. 88. 



FIG. 88. Garnet; Combi- 

 nation of n (221) andd(HO). 



CLASS, TETRAHEDBAL (DIAGONAL-FACED) HEMIHEDRONS 

 TYPE 31, DITESSERAL POLAR 



All forms of this type possess four ditrigonal polar axes, terminat- 

 ing in the center of the octants. The conditions surrounding one 

 extremity of the axis are different from those at the other extrem- 



