Penfield and Ford — Stibiotantalite. 



67 



The domes are generally free from striations and dull in com- 

 parison with the other faces, appearing as if corroded, but 

 on a few of the crystals they are bright and somewhat striated, 

 not sufficiently so, however, to interfere with measurements. 

 The domes h (203) and perhaps h f (203) occur on only a few of 

 the crystals, two of which are shown as nearly as possible in true 

 proportions in figures 2 and 15. On both crystals the surfaces 

 were so dull, seemingly etched, that on 

 the goniometer they gave no reflections 

 of the signal, but by placing bits of mi- 

 croscopic cover glass against the faces 

 satisfactory measurements were ob- 

 tained which leave no doubt as to 

 the correctness of the symbol. Owing 

 to the interposition of a twin lamella, 

 the dome h, in front, shown in figure 

 15, extends only about one-third way 

 across the crystal. 



The pyramid w (4.12.9) appears on 

 only a few of the crystals, is never 

 very prominent and its development 

 is so complicated by polysynthetic 

 twinning, that it is difficult to state 

 whether jt occurs in front (4.12.9), or 

 behind (1.12.9), or in both positions. 

 The pyramid faces are often striated 

 parallel to the edge 4.12.9 ,\ 4.12.9, as 

 if in oscillatory combination, and fre- 

 quently distinct reentrant angles occur, 

 figures 3, 14, and 15, which indicate 

 rather that the striatums result, in 

 part at least, from polysynthetic twin- 

 ning. The pyramid faces generally have bright surfaces which 

 yield good reflections even when the accompanying dome faces 

 are dull. 



A basal plane c and brachypinacoid h have not been ob- 

 served, but occasionally owing to oscillatory combinations of 

 g*g\ or as a result of polysynthetic twinning, a striated sur- 

 face results, approximating in position to h. 



The surfaces as a whole are not the best for reflecting light, 

 and striations gave rise to further difficulties, but it is believed 

 that the measurements and axial ratios derived therefrom 

 must be very nearly correct. 



The crystal shown in figure 16, considerable portions of 

 which are missing, is unique. It measures about 5 cm in length, 

 4 in height and 3 in thickness, respectively in the directions of 

 the a, c and b axes, and weighs 150 grams (over 5 ounces), or 



