TETRAC JON A L SYSTEM 



71 



The possible forms to be found in combination in this type are : 



The positive and negative tetragonal scalenohedrons, K (hkl), 

 icihkl 



Tin- positive and negative sphenoids of the first order, K(hhl), 

 K(hhl). 



The pyramid of the second order, (ohl). 



The ditetragonal prism, (hko). 



Tlie prism of the first order, (hho). 



The prism of the second order, (oho). 



The basal pinacoid, (ooi). 



I Examples of minerals crystallizing in the type. 



( 'halcopyrite, FeCu^; (111) (111) (122) (772). Figs. 125 and 

 126. 



CLASS, HOLOHEDRAL HEMIMORPHIC 

 TYPE 25, DITETRAGONAL POLAR 



Hemimorphic forms may be considered as derived from an equa- 

 >rial form by the suppression of all the 

 ;es around one extremity of the c 

 cis and the development of those 

 round the opposite extremity, as an 

 idependent form, or if the equatorial 

 form is cut along the equatorial plane, 

 will yield an upper hemiform and 

 lower hemiform. 



Symmetry. All the elements of 

 letry in the equatorial plane are 

 namely the four axes, the equa- 

 >ri;il plane, and the center. The type 

 symmetrical in regard to a ditetrag- 

 >nal axis, the c axis and four planes of symmetry intersecting in 

 the c axis, Fig. 127. 



Forms 



Each pyramid of the ditetragonal equatorial type yields an upper 

 nd lower hemipyramid, each of which is independent. The basal 

 inacoid is also divided into upper and lower forms. 



The possible forms to combination in this type are therefore : 



The upper and lower ditetragonal pyramid; u/1 - ; 



(hki) (hki). 



FIG. 127. Type 25: Ditetrag- 

 onal Polar. 



