CRYSTALLOGRAPHY 



21 



Fio. 24. 



the molecules are the nearest in the planes. They will also possess 

 small intercept- and a simpler relation of their parameters. 



Crystal forms. When space is divided by 

 the three axial planes into eight octants it is 

 evident that one set of parameters may rep- 

 resent more than one face ; in fact there will be 

 eight planes, or one in each octant. The solid 

 hounded by these eight faces is known as a 

 crystal form; the symmetry of the type may 

 not, however, require all the eight faces to be 

 present. A crystal form may be defined as the 

 solid bounded by the combination of all those 

 faces possible to be represented by one set of 

 I mn i meters irrespective of sign; and required by 

 the symmetry of the type. The combination of planes may inclose 

 space or may not. In Fig. 24, eight faces are shown, all of 



which are represented by the set of param- 

 eters, a : b : c, and the symmetry of the 

 type requires all eight faces to be pres- 

 ent ; Fig. 25 represents four of the same 

 faces, but producing an entirely differ- 

 ent form, as the symmetry of the type 

 requires only four of the eight faces to 

 be present. When one face only is in- 

 tended to be represented by a set of 

 parameters, or indices, they are written 

 without pa- 

 renthesis 111 ; 

 when the entire 



form is represented, they are written (111). 

 The number of faces possible on any 

 crystal form may vary from 48, in the 

 type which possesses the highest sym- 

 metry, to one face in the type which con- 

 tains no symmetry. As at least four 

 faces are required to inclose space, there 

 are two classes of crystal forms, those 

 that inclose space and are termed closed FlG - 26. Unit Pyramid of 



r TI- oo j j.u u' u j x Barite ; a Closed Form. 



forms, Fig. 26, and those which do not in- 

 close space, and are termed open forms, Fig. 27 ; theoretically the 

 open forms extend to infinity on the open sides, unless terminated 



FIQ. 25. 



