144 



MINERALOGY 



FIG. 



291. Penetration 

 Fluorite. 



Twins of 



not an axis of symmetry, as the 

 tesseral central and tesseral polar 

 types. The supplementary twins 

 of pyrite are of this class. 



Twinning in the tetragonal sys- 

 tem. The general law in this 

 system is where the twinning axis 

 is perpendicular to a pyramid face 

 and is a possible mode of twinning 

 in all seven types of the system. 

 Fig. 292 is a drawing of twins of 

 cassiterite, in which the twinning 

 axis is perpendicular to the pyr- 

 amid face 

 of the sec- 

 ond order, 



101, and the composition plane cc is parallel 

 to 101. Fig. 293 is a photograph of a nat- 

 ural twin from Zinnwald, Bohemia. In the 

 ditetragonaJ^V*" 1 ^ type, the normal to the 

 sphenoids 1 1 1 is the twinning axis, with the 

 sphenoidal face the composition face ; when 

 developed as contact twins, they are similar 

 to the spinel twins of the isometric system. 



In the tetragonal polar types a third twinning law is possible, as 

 in these types the lateral axes are not axes of symmetry and are 



therefore possible twinning axes. 

 These are supplementary twins, 

 and when there is no reentrant 

 angle at the equatorial plane 

 there is nothing to indicate the 

 twinned nature of the crystal, 

 and the symmetry is apparently 

 that of an equatorial type. 

 Most crystals of wulfenite are of 

 this character. 



Twinning in the hexagonal 

 system. Twins in the hexago- 

 nal division are rare. A pos- 

 sible law in all the types of the 

 system is where the twinning 



FIG. 292. Twins of Cas- 

 siterite. 



FIG. 293. 



Cassiterite Twins from zimn- 

 wald, Bohemia. 



