64 Penfield and Ford — Stibiotantalite. 



show reentrant angles at the top, and the faces of later figures 

 lettered 77 or 77' must in reality be regarded as composite, made 

 up partly of 77 and partly of 77'. 



Figures 3 and 4 both show hemimorphic character by the 

 development of 77' behind, but this face is in reality com- 

 posite, composed in part of 77 in twin position. Figure 3 

 shows prominent reentrant angles at the sides, compare figure 

 6 : none of the other crystals show these so prominently, 



a' 



r 



•£jLo , 



3'°" 



and as a rule reentrant angles are so small as to give the effect 

 of fine striations parallel to the vertical axis, or a rounding of 

 the obtuse edge between g and g\ as if resulting from oscilla- 

 tory combination, figure 4. The crystal represented by figure 

 4 is the only one showing any prominent development of a 

 replacement of the edges between a and g ; the form, how- 

 ever, is so striated and distorted by the development of vicinal 

 faces that no satisfactory measurements could be obtained ; it 

 approximates to (301) respectively (301), but the symbols are 





\Rict 9 e / 



'A 7 9 



6 yfottey / 



' .fff 



r 



Y 



AL 



1 



)* 



a 

 a 



s if 



R 



Y 



A 



H' 



k 



a- 





\fttc/pe 



9\ V 



r 



ff\ 1 



~Off' 



XRidye 

 9\ V 



1$ 



questionable. The pyramid w is developed to so slight an 

 extent and the crystals are so complicated by polysynthetic 

 twinning that it is impossible to state whether it occurs only 

 in front or only behind ; hence it is assumed that both forms, 

 w (4.12.9) and w' (4.12.9), are present. With the exception of 

 the three crystals just described, the general habit is like that 

 of the normal group of the orthorhombic system, as illustrated 

 by figures 10 to 13, in which the forms are represented with 

 ideal symmetry, whereas they all show vertical striations on 

 the pinacoid and prism faces, and some rounding of the edges 

 between g and g' . One crystal only, figure 12, is shorter in 



