CASSITERITE CRYSTALS. 



69 



Twin groups composed of three individuals are, relatively 

 speaking, larger, the figured group measuring about 1*5 mm. 

 in its greatest dimension (fig. 6). In contrast to the 

 crystals described so far, the larger individual [which 

 supports the two smaller portions twinned to it on the 

 opposite faces of e (101)] is doubly terminated, large dull 

 c faces being present. The composite crystal is inclined 

 to be tabular on the plane containing the twin axes, so 

 that the two opposite faces of a (100) are the largest planes 

 developed. One of these, shown in front in the figure, is 

 smooth and bright, giving a good signal, although it is a 

 composite face formed by coplanar portions of the three 

 twinned segments. The a face at the back of the drawing 

 is rough and irregular and evidently the crystals were 

 attached to the matrix by it. Another feature shown in 

 the figure is the oscillation between «, e and s : this is 

 unusual. 1 



Figure 7 represents a clinogram of a fourling. It is not 

 unlike the triplet described above; the point of attachment 

 to the matrix is again the a face. A third twin segment 

 has been added to the normal large crystal thus eliminat- 

 ing one of the c faces. 



Elements, — Twelve crystals were measured on the two- 

 circle goniometer, five of these gave excellent signals, and 

 the angles obtained from them were utilized to determine 

 the axial ratio. The data and results appear in the follow- 

 ing table. 



Forms 



<t> P 



Limits. 



Ko. of 

 Observations- 



c 



e 

 s 



0° OH' 

 45° 00' 



33° 53' 

 43° 33' 



<f> p 

 0°00'- 0°04' 33°49-33°55' 



44°56'-45°04' 43°3l - 43°36' 



11 



14 



•67155 

 •67217 



1 Cf. Crystals described by Dr. Anderson, supposed to come from 

 Tingha : Rec. Aust. Mus , Vol. xm, No. 1, 1920, p. 23. 



