72 



MINERALOGY 



The upper and lower pyramid of the first order ; u/1 



a : a : me 



(hhl) (hhl). 



The upper and lower pyramid of the second order ; u/1 

 (ohl) (ohl). 



GO a : a : me 



FIG. 128. The Upper Di- 

 tetragonal Hemipyramid. 



FIG. 129. The Upper Hemi- 

 pyramid of the First Order. 



The ditetragonal prism ; na : a : oo c ; (hko). 

 The prism of the first order ; a : a : me ; (110). 



The prism of the second order ; oo a : a : oo c ; 

 (010). 



The upper and lower basal pinacoid; 



; (001) (001). 



Examples. There are as yet no minerals 

 known to crystallize in this type, but several 

 organic and artificial salts belong here. Fig. 

 FIG. 130. Silver Fluor- J3Q represents a crystal of silver fluoride, 

 llLtfofoVanun 1 !) AgF,H 2 0; with (111) (111) (113) in com- 

 (113). bination. 



CLASS, PYRAMIDAL (PARALLEL-FACED) HEMIHEDRONS 

 TYPE 24, TETRAGONAL EQUATORIAL 



Symmetry. Crystals of this type have an axis of tetragonal 

 symmetry, the c axis ; one plane, the equator, and a center, Fig. 

 131. 



The forms may be considered as derived from the ditetragonal 

 equatorial type by extending alternate pairs of faces which inter- 

 sect in the equator, as is shown in Fig. 132, where the shaded faces 

 when extended produce the third order pyramid, Fig. 133. 



