146 



MINERALOGY 



FIG. 295. Twins of Calcite in which the Twinning Axisis c and the 

 Composition Plane is the Base. Guanajuato, Mexico. 



Twinning in the orthorhombic system. There are three classes 

 of twins possible in all three types of the system : 



I. Where the twinning axis is normal 

 to a. pyramid face. This is in fact a pos- 

 sible law in all 32 types of crystals, consid- 

 ering the octahedron and its resulting 

 hemihedral forms as pyramids. In Fig. 

 297 a, twins of staurolite, in which the 

 twinning axis is normal to the pyramid 

 (232). 



II. Where the twinning axis is normal 

 to a prism face. Fig. 298 is a diagram 

 of aragonite, in which the twinning axis is 

 normal to the prism 110, often repeated, 

 and as the prism angle is nearly 120, the 



F IG . 296. -Brazilian Twins twinned crystals have a pseudohexagonal 

 of Quartz. symmetry, Fig. 299. 



