Till- ORTHORHOMBIC SYSTEM 



117 



Olivine, (Mg,Fe) 2 SiO 4 . 

 Knst.-itite, MgSi0 3 . 

 Aragonite, CaCO 3 . 



Topaz, Al[Al(O.F 2 )]Si0 4 . 

 Barite, BaSO 4 . 



Fig. 226 is a combination of the base, brachydomc, macrodome, 

 and macropinacoid in barite. 



CLASS, ORTHORHOMBIC HEMIMORPHIC 

 TYPE 7, DIDIGONAL POLAR 



Symmetry. Crystals of this type possess one didigonal axis, 

 the c axis, and two planes of sym- 

 metry intersecting in the c axis, Fig. 

 227. It is a polar development of 

 the didigonal equatorial, with a loss 

 of all the symmetry lying in the equa- 

 torial plane of that type. 



Forms 



I. Hemipyramids, u/1 



: mc 



(hid), (hkl). FIG. 227. -Type 7, Didigonal 



I here would be upper and lower Polar, 



pyramids of each series, Fig. 228. 



II. Domes. Macrodomes, u/1 ( - -); (hok), (hok), 



Fig. 229. 



/oo y. ' nh ' mr\ 



Brachydomes, u/l(- ^ -J;.(ohk), (ohk), Fig. 230. 



Fio. 228. The Upper Hemipyramid. 



FIG. 229. The Upper Macrodome. 



Both domes would be modified by the symmetry, yielding hemi- 

 domes, while the prisms would suffer no apparent change. 



