CHAPTER VII 



RELATION OF INDIVIDUAL CRYSTALS 



CRYSTALS as found in nature are rarely simple, or composed of one 

 individual. During the process of formation they must necessarily 

 come in contact with each other ; this contact modifies them, not 

 only producing distortions and irregularities in external form, but 

 reentrant angles are formed. The angles of all simple crystals 

 must be less than 180, and whenever an angle greater than 

 180 or a reentrant angle appears, it is proof that the crystal is of a 

 compound nature or consists of more than one individual. At the 

 time of separation, one crystal may have an influence upon the 

 position or direction of the axes of its neighbor, and this influence 

 may show itself in various ways. Minerals totally different in 

 composition and crystalline structure are sometimes so placed that 

 certain axes and edges are parallel in the two species, as in case 



of staurolite and 

 cyanite from St. 

 Gothard, Fig. 268 ; 

 while belonging to 

 different crystal 

 systems, these two 

 species are often 

 so placed that 

 their c axes are 

 parallel. Such 

 parallel growths, 

 however, occur the 

 more often be- 

 tween individuals 

 of the same species, or between species belonging to the same 

 isomorphic group; in such cases large aggregates will have all 

 their crystalline directions parallel as in Fig. 269, sulphur, or as in 

 Fig. 270, microcline from Pike's Peak. In such parallel growths 

 equivalent faces will reflect light or appear bright at the same 

 time. 



134 



FIG. 268. Cyanite and Staurolite in Parallel Position 

 from St. Gothard, Switzerland. 



