148 



MINERALOGY 



twins are possible, as here the lateral axes are not digonal axes of 



symmetry and may be twinning axes with the base as the com- 

 2 position plane. These supplementary 



twins are well illustrated by calamine. 



Twinning in the monoclinic system. 

 The only direction not possible as an 

 axis of twinning is the orthoaxis, and 

 twins are very common in the system. 

 They may be divided into the three types 

 as in the orthorhombic system, with the 

 addition that the two pinacoids may 

 also be twinning planes. In gypsum and 

 augite the twinning plane and composi- 

 tion plane is the orthopinacoid, and the 

 twinning axis is the normal to it. In the 

 Carlsbad twins of orthoclase the twinning 



axis is the vertical axis, while the composition plane is the clino- 



pinacoid, parallel to it, Fig. 300. 



Twinning in the triclinic system. As there is only a center of 



symmetry, any plane is possible as a twinning plane and twins are 



FIG. 299 a. Diagram of the 

 Basal Plane of Aragonite 

 Triplets, showing the Rela- 

 tion of the Individuals. 



FIG. 300. Carlsbad Twins of Orthoclase. Brice, New Mexico. 



numerous and complicated, often repeated polysynthetically as in 

 the plagioclase feldspars. 



