62 COLORADO FERBERITE AND THE WOLFRAMITE SERIES. 



TWINNING. 



Twin crystals were often observed. Only one twinning law was 

 noticed, namely, with {023} as the twinning plane. The many 

 groupings parallel to a{100} were carefully examined for evidences 

 of twinning on the a(100) face, but none were seen. As these parallel 

 groupings generally have the 2(102) face well developed, twinning on 

 the a(100) face would cause a twinned face of (102) to assume the 

 position of (T02), and there would then be an apparent rear orthodome 

 as well as a front one on such a twinned crystal. The absence of such 

 a rear orthodome would indicate that these parallel groupings were 

 not twinned. 



Though only one twinning law was observed, namely {023}, there 

 were two kinds of twins according to this law, namely, contact and 

 penetration twins. Both were observed many times, though the 

 contact twins are more abundant. 



The contact twins are illustrated in figures 14 and 15, which show 

 the twinned crystal facing in two directions, or in other words show 

 the two positions a twinned group will take according to which face 

 of {023} acts as twinning plane. Both positions of the twinned group 

 have the same crystallographic relationship. The position shown in 

 figure 14 shows the clmopinacoid &{010} of the twinned crystal adjoin- 

 ing the same form of the untwinned crystal. In figure 15 the basal 

 pinacoid c{001} of the twinned crystal is shown. In the two illustra- 

 tions the relationships of the contact twins are more clearly indicated 

 than can be done by a written description. . 



The line of contact of the two crystals affords some points of interest, 

 but for the present only a description of it is given. Theoretically 

 it should be a straight line, as drawn in figures 14 and 15, but actually 

 it is very uneven, the two parts of the twinned crystal being very 

 irregularly dovetailed, as illustrated diagrammaticaUy in figure 16, 

 which was drawn from a twinned crystal (notebook No. A13), as seen 

 on the goniometer. 



In the penetration twins tne containing unit is always much larger 

 than the one which is partly embedded and projects out of the middle 

 of the larger crystal, as shown in figure 17. The two crystals, though 

 of different appearance, have the same combination of crystal forms 

 as those shown in figures 14 and 15. Similarly figure 18 shows the 

 same kind of penetration twin with the smaller twinned crystal in the 

 opposite direction to that in figure 17. The large untwinned crystal 

 in figure 18 is shown with one end cleaved off, due to the clino- 

 pinacoidal cleavage 6(010). (See PL VIII, A, p. 19.) 



The angle e(001): (010) (fig. 17) is 29 58', and the angle c(001): 

 c(001) (fig. 18) is 60 02'. These angular values are here given as 

 reference will be made to them later and the position of the faces 

 forming the angles can here well be seen. 





