66 Penfield and Ford — Stibiotantalite. 



The general resemblance to columbite is shown by the 

 prominent development on both species of the macropinacoid 

 #, parallel to which stibiotantalite has a highly perfect and 

 colnmbite a distinct cleavage, while both minerals have an 

 indistinct cleavage parallel to the bracbypinacoid h (010). 

 The prism g is always present on stibiotantalite, while on 

 colnmbite it is seldom wanting and often prominent. The 

 macrodome h occurs on both minerals but is not common, 

 while the prominent development of rather flat macrodomes, 

 77 (209) on stibiotantalite. and h (103) and I (106), respectively 

 (206) and (2.0.12), on columbite is a feature common to both 

 species. The pyramid w (4.12.9), although not occurring on 

 columbite, is in the same vertical zone as two of its prominent 

 pyramids, s (263) and y, (133), respectively, (4.12.6) and (4.12. 

 12). If the crystals of stibiotantalite were black and of 

 metallic luster they certainly might be mistaken for columbite, 

 because of similarity in appearance, habit, occurrence and 

 association. Columbite, it should be stated, is not hemimor- 

 phic and does not exhibit pyroelectricity. 



Crystals of stibiotantalite show certain peculiarities as 

 regards the development of the forms and the character of 

 the surfaces, as follows : — 



The macropinacoids a and a', which are generally the most 

 prominent of all the forms, have a bright luster and are 

 usually striated vertically, the strise being rather fine and 

 seldom giving rise to much rounding or irregularity of the 

 surface : on a few crystals they appear almost free from 

 striations. Both (100) and (100) occur without any apparent 

 difference, except when tested for pyroelectricity, and then it 

 generally appears that the same surface develops two kinds of 

 electricity owing to twinning an_d interpenetration. 



The prisms g (130) and g' (130) are present on all of the 

 crystals and are always striated vertically, due to polysynthetic 

 twinning and in part perhaps to oscillatory combination, both 

 causes giving rise to a rounding of the edge between g and g\ 

 or as is frequently the case, to a considerable distortion when 

 one prism face predominates over the other, figure 14. In 

 almost all of the crystals, however, portions of the prismatic 

 faces are quite free from striations, so that good measurements 

 may be had. Any modification of the edge between a and g 

 was rarely observed. On two crystals distinct replacements, 

 indicating the presence of a prism corresponding to m (110) 

 or m' (110) were noted, but the faces were too poorly developed 

 to give good measurements ; compare figure 15. 



The macrodomes rj (209) and rj' (209) are prominent on all 

 of the crystals as shown by the figures. A single face is 

 generally composed of both 77 and n' as explained on page 58. 



