118 



MINERALOGY 



III. Of the three pinacoids, the base would yield hemi forms, the 



/ oo S * oo b * c \ 

 upper and lower base, u/l( - -), (001), (001). 



Possible forms to combine in the type would be : 



Pyramids upper and lower, (hkl) , (hkl) . 



Prisms, two series, (hko). 



Domes ; upper and lower macrodomes, (hok), hok). 



Domes ; upper and lower brachydomes, (ohk), (ohk). 



Macropinacoid, (100). 



Brachypinacoid, (010). 



Upper and lower base (001,) (001). 



Examples. Calamine, ZnSiO 3 , 2H 2 O. Fig. 231 represents a 

 combination of two upper hemimacro- and brachydomes, the 



FIG. 230. The Upper Brachydome. 



FIG. 231. Calamine. 



unit prism, the lower unit hemipyramid, the macro- and brachy- 

 pinacoids, and the upper base. 

 Struvite, NH 4 MgPO 4 , 6 H 2 O, also crystallizes in this type. 



CLASS, SPHENOIDAL HEMIHEDRONS 

 TYPE 6, DIGONAL HOLOAXIAL 



Symmetry. Crystals of this type possess three axes of digonal 

 symmetry corresponding to the crystallographical axes, but no 

 planes of symmetry. Fig. 232 illustrates the symmetry of the type. 



I. Sphenoids, 



l b:mc 



Forms 



K (hkl), K (hkl). 



The poles of Fig. 232 represent the general form of the type. 



