70 



MINERALOGY 



I. Tetragonal scalenohedron ; 



na : a : me 



K(hkl)K(hkl). 



FIG. 



This form is bounded by eight similar scalene triangles, Fig. 

 123, four of which are grouped around each extremity of the c 

 axis, with the edges lying in the planes of sym- 

 metry. The lateral axes terminate in the middle 

 of an edge. There are + and forms which 

 become congruent by a 

 revolution of 90 around 

 the c axis. 



II. Sphenoid; a: a: me; 

 K (hhl) K (hhl). 



When the pole is in the 

 plane of symmetry repre- 

 sented by the full lines, 

 Fig. 121, two faces of the 

 scalenohedron will fall in 

 one plane, as e and e' and 



b, b', and the form will be bounded by four FJG 124 . _ Th Plus 

 isosceles triangles, Fig. 124, producing the tet- Tetragonal Sphenoid of 

 ragonal sphenoid of the first order in which the the First Order, K (ill), 

 lateral axes bisect the four equal edges. The 

 c axis bisects the remaining two edges. This 

 form may also be considered as derived from 

 the tetragonal pyramid of the first order by 

 extending alternate 

 faces above and below 

 until they inclose 

 space. 



III. Other forms of 

 the type. All other 

 positions of the pole 

 will in turn produce 

 the holohedral shapes 

 of type 27, here as in 

 the isometric system ; 

 here, also, they must be considered as of 

 lower symmetry, which will be recognized 

 by their combination with the two new 



. ,. , , , ,, . i FIG. 126. Chalcopyrite : 



forms of the type, or by their stnations and Combination of 



FIG. 125. Combination 

 of (122) and (772) of 

 Chalcopyrite. 



etch figures. 



(Ill) 



(110). 



