TI:TI;A<;O\AL SYSTKM 



1)9 



Combinations. The pyramids and prisms 



of the first and second orders truncate each 

 other's edtfes >\ nmirt rically ; Fig. 119 repre- 

 sent.-, thr-r tour forms of cassiterite. Several 

 members of a series of a variable form may 

 occur in combination with the fixed forms, 

 fig. 120, Zircon. 



I A.'implo of minerals crystallizing in thedi- 

 tetragoiial equatorial: 



Cassiterite, SnOs ; (110) (100) (310) (111) 

 (101). 



Zircon, ZrSiO 4 ; (110) (100) (111) (331) 

 (311). 



Rutile, TiO 2 ; (110) (100) (310) (111) (101). 



Vesuvianite, Ca(Al, OH) Al 2 (Si0 4 ) 5 ; (100) 

 (110) (310) (210) (001). 



FIG. 120. Combina- 

 tion of .(ill) (Ml) 

 (311), (HO) and (010) 

 in Zircon. 



CLASS, SPHENOIDAL (DIAGONAL-FACED) HEMIHEDRONS 



Symmetry. Crystals of this type possess a ditetragonal alter- 

 ating axis, the c axis; two digonal axes, the a axes; and two 



planes of symmetry intersecting in 

 the c axis and each containing one 

 of the intermediate lateral axes; Fig. 

 121 represents 

 the symmetry of 

 the type. 



This class of 

 hemihedrons in 

 the tetragonal 

 system corre- 

 sponds to the 

 tetrahedral class J 



IG. 121. Type 26: Ditetragonal . 



alternating. in tne isometric 



system and may 



considered as derived from the holohedral FIG. 122. The Minus 

 forms by extending all the faces in alternate 

 ^tants, Fig. 122; the shaded faces when extended will produce 

 form as drawn. 



