!2 



Penfield and Ford— Stibiotantalite. 



of later origin than the tourmaline and lepidolite, though 

 undoubtedly it was one of the primary minerals of the deposit. 

 The crystals are mostly of a rich dark-brown color, with 

 resinous to adamantine luster. Fragments look exactly like 

 the resinous variety of sphalerite, and a few pieces were found 

 of light brown color, transparent, and so closely resembling 

 the well known sphalerite from Picos de Europa, Spain, that 

 by appearances the two can not be told apart. 



Stibiotantalite crystallizes 

 in the orthorhombic system 

 and is hemimorphic. With 

 few exceptions, the habit of 

 the crystals gives no sugges- 

 tion of hemimorphic develop- 

 ment , which, according to the 

 orientation adopted^ is in the 

 direction of the brachy- or a- 

 axis. All the crystals that 

 have been studied are poly- 

 synthetic twins, thus causing 

 them to imitate the symmetry 

 of the normal group. In 

 habit they resemble colum- 

 bite, and, as will be shown, 

 the two minerals are related 

 both in chemical composition and axial ratio. In order to 

 bring out the crystallographic relationship, however, it has 

 been necessary to assign rather complex symbols to some of 

 the forms of stibiotantalite, but it is believed that it is better 

 to do so than to give simpler symbols to the forms and refer 

 them to other axes. The axial ratio of the two minerals are as 

 follows : 



Stibiotantalite . 

 Columbite 



a : b : e = 0*7995 : 1 : 0*8448 

 a : b : c = 0-8285 : 1 : 0*8897 



The forms observed on stibiotantalite are shown in stereo- 

 graphic projection in figure 1, but, owing to polysynthetic 

 twinning, it is impossible to state as regards some of them 

 whether they occur both in front and behind, or intersect only 

 one end of the brachy-axis, as demanded by hemimorphism. 

 The symbols are as follows : 



a (100) 

 a' (100) 



m (110) or (110)? 

 q (130) 

 q' (130) 



7] (209) 

 rj' (209) 



h (203), probably also ti (203) 

 8 (043) 

 w (4-12-9) probably also w' (4-12-9) 



