22 



MINERALOGY 



FIG. 27. An Open 

 Form. 



FIG. 28. Combination 

 of a Closed and Open 

 Form. 



by a combination with other forms. Combinations of open forms 

 may inclose space. The number of faces occurring on any crys- 

 tal form is very limited, but 

 the number of faces possible 

 on a crystal which is a com- 

 bination of crystal forms is 

 not limited and in some 

 cases may be very large. 

 The forms which may occur 

 in combination on crystals 

 are limited to those possible 

 to be derived from the same 

 point-system, and they will 

 therefore have the same 

 symmetry. From the sym- 

 metry of the type, forms 

 in combination always bear the same relation to each other; 

 Fig. 28 is a combination of the closed form of Fig. 26 and the 

 open form of Fig. 27 ; here equivalent 

 edges are cut by the prism, or the four 

 edges of the pyramid are replaced by the 

 prism faces. When the replacement is 

 symmetrical, as in this case, the angles 

 between the prism and the pyramid faces 

 above and below are equal ; the pyramid 

 edges are said to be truncated by the faces 

 of the prism. In the same way, corners of 

 forms may be truncated by other forms 

 and replaced not only by one face, but by 

 a group of faces, Fig. 29. When the edge of one form is sym- 

 metrically replaced by two faces, it is said to be beveled, Fig. 30. 



Zones. The edge of a form may be 

 replaced by a series of faces, the mutual 

 intersections of which are all parallel to the 

 edge replaced. Such a series of faces is 

 termed a zone. The intersections of all 

 faces possible in any one zone will be rep- 

 resented by possible edges on the crystal, 

 all parallel to each other and parallel to an 



FIG. 30. The Cube bev- . . ,. 



eled by the Tetrahexa- imaginary line drawn through the point of 

 hedron. intersection of the crystal axes, termed the 



FIG. 29. The Cube with 

 the Corners replaced by 

 the Tetragonal Trisocta- 

 hedron. 



