114 



MINERALOGY 

 Forms 



I. Orthorhombic pyramids, na : b : me; (hkl). 

 The poles in Fig. 217 represent the orthorhombic pyramid; it is 

 bounded by 8 similar scalene triangular faces, which inclose space, 



FIG. 217. Type 8, Didigonal 

 Equatorial. 



FIG. 218. The Orthorhombic Axes and Planes 

 of Symmetry. 



Fig. 219. The crystallographical axes terminate in the tetrahedral 

 angles. There are three series of pyramids : 



a. The unit series, a : b : me, (hhl), where the variable lies on the 

 c axis ; Fig. 220 represents the unit series of pyramids. 



FIG. 219. The Unit Pyramid of 

 Barite. 



FIG. 220. The Unit Series of Pyra- 

 mids. 



6. Macro series of pyramids, a : nb : c, (hlh), when the intercept 

 on b is greater than unity. 



c. Brachy series of pyramids, na : b : c, (Ihh), when the intercept 

 on the & axis is greater than unity. 



