GUIDE TO THE MINERALOGIC COLLECTIONS 29 



The remaining forms of the rhombohedral group are geometri- 

 cally the same as the corresponding forms of the normal group 

 and are: prism of the first order; prism of the 

 second order; pyramid of the second order; basal 

 plane. 



Some of the combinations in this group are 

 shown in fig. 111-13. 



Rhombohedral-hernhnorphic group 

 Comparing this group with the preceding one, 

 the main points of difference to be noted are the 

 lack of symmetry to a point, which is character- 

 istic of hemimorphic crystals, the two extremities Fig. no 

 of the vertical axis showing dissimilar modifications, and the 

 fact which results from the above, namely, that the horizontal 



Fig. Ill Fig. 112 Fig. 113 



axes are no longer axes of binary symmetry. A crystal of tour- 

 malin, which is an important species of this type, is shown in 

 fig. 114 and serves to illustrate the main features of the group. 



Trirhombohedral group 

 Trirhombohedral crystals are characterized by the absence of 

 planes of symmetry; they are, however, symmetric to a point. 



Fig. 114 Fig. 11." 



The verticiil ;ixis is an axis of trigonal symmetry. The minerals 

 ilmenite, dolomite, phenacite, dioptase and willemite occur in 

 forms of this group. Fig. 115 shows a crystal of ilmenite. 



