RELATION OF INDIVIDUAL CRYSTALS 



141 



FIG. 282. Supplementary Twins of Pyrite. 



syinmetrv nr ;i -ymmetry 



of a higher type, as tin- 

 upper and lower t'orms of 



a polar type when joined 



alon; the plane of the base 



will possess the symmetry 



of an equatorial type. 

 The plus and minus 



forms may penetrate each 



other and be distinguished 



as complex individuals by 



the reentrant angles. Fig. 



282 is a drawing of the 



supplementary twins of 



pyrite, formed by the plus 



and minus pyritohedrons ; while Fig. 283 is a photograph of these 

 twins from Prussia. In interpenetrating 

 twins there is no marked plane of contact 

 between individuals, but a very irregular 

 and ill-defined area separates the two indi- 

 viduals internally. 



In the growth of crystals the twinning 

 position may have been assumed at the 

 very outset, in the nucleus of crystalliza- 

 tion, and the complex nature existed at the 

 beginning of crystallization; or again the 



individuals may have developed as simple crystals, when through 



a changed condition mole- 

 cules have separated in a 



twinned position and the 



axis of the simple crystal is 



abruptly changed. Where 



there is but one angle in the 



axis of an elongated crystal 



they are often termed genic- 



ulate twins, as the, geniculate 



twins of rutile, Fig. 284. 

 This bending or angle in 



the axis of the crystal may 



b.- repeated either in the ,,, , sl . __ Geniculate Twins of Rutii, from 



Same direction Or in the Op- Lancaster Count;. , Pennsylvania. 



Fi<;. 'Js:i. Supplemen- 

 tary Twins of Pyrite. 



