142 



MINERALOGY 



FIG. 285. Cyclic Twins of Marcasite from 

 Folkstone, England. 



posite direction. When repeated in the same direction a number 

 of times, the complex individual is circular and is termed cyclic 



twins, as in marcasite, Fig. 

 285 ; or the twinning may 

 be repeated first in one di- 

 rection and then in the 

 other, with a zigzag effect, 

 as in rutile, Fig. 286. 



When the twinning is 

 repeated many times at 

 very short intervals, each 

 individual of the complex 

 structure will be confined 

 to a very thin sheet pass- 

 ing through the aggregate, 

 parallel to the composition 

 plane, and indicated on the 

 crystal externally by a re- 

 entrant angle as illustrated in Fig. 287, a twin crystal of albite in 

 which the clinopinacoid is the composition plane and the twinning 

 axis is perpendicular to this face. Fig. 288 is a crystal of albite 

 composed of several individuals 

 twinned after this same law; each 

 individual is indicated by the 

 reentrant angle passing around 

 the crystal parallel to the twin- 

 ning plane. Each individual may 

 be reduced to extreme thinness, 

 when only a striation on the 

 crystal face will remain to mark 

 the plane of contact separating 

 individuals, the whole complex 

 structure building up an appar- 

 ently simple crystal. When often 

 repeated in this manner, the 

 twinning is termed polysynthetic. 

 The striations produced by polysynthetic twinning are quite dif- 

 ferent from those caused by parallel growths; the former pass 

 through the body of the crystal and are caused by the arrange- 

 ment of the molecules and will therefore appear, not only on crystal 

 faces, but also on all cleavage faces intersecting the twinning planes, 



FIG. 286. Zigzag'twins of Rutile. 



