Penfield and Ford — Stibiotantalite. 71 



Orthorhombic symmetry was shown by the measurements of 

 g ^w in four upper octants of one crystal, which, though vary- 

 ing from 39° 15' to 39° 25', are as nearly alike as the charac- 

 ter of the reflections warrant. The crystal on which these 

 measurements were made is the one shown in figure 15, where 

 the distribution of the w faces is like that of the normal 

 group of the orthorhombic system, owing perhaps to poly- 

 synthetic twinning. 



Optical Properties. — Thin sections cut parallel to the three 

 pinacoids, when examined in polarized light, all showed 

 extinction parallel to directions corresponding to the crystal- 

 lographic axes. Some of the sections were not uniformly 

 colored throughout, some portions being darker than others, 

 but whether this phenomenon is due to any essential variation 

 in chemical composition or not has not been determined. It 

 is believed that if the difference in color indicates any varia- 

 tion in chemical composition, it is probably slight. 



The indices of refraction vary somewhat for crystals of dif- 

 ferent specific gravity. Determinations have been made on 

 two crystals ; one having a specific gravity of 6'818, and the 

 other of 6*299. In both cases the indices of refraction were 

 determined from two prisms ; one with a face parallel to the 

 base and its edge parallel to the &-axis, which gave with 90° 

 incidence the values for rays vibrating parallel to the a and b 

 axes, respectively, (3 and a ; the other prism with its edge 

 parallel to the c-axis gave the value 7. The optical orienta- 

 tion is therefore a=h, b=a and c — c. With basal sections, also, 

 2H acute was measured in potassium mercuric iodide solu- 

 tion, ft,y= 1*7156. The values for the two crystals are as 

 follows : — 



Specific gravity 6*818, corresponding to about 39 per cent 

 Ta 2 5 and^l7*5 per cent Nb 2 5 . 



a (3 y 2V, Calculated y—a 



For Li 2*3470 2*3750 2*4275.*. 73° 40' *0805 



" Na ... 2*3742 2-4039 2*4568.'. 75 5 '0826 



" Tl 2-4014 2-4342 2*4876.'. 77 38 -0862 



" Na 2Ha = 119° 10' .-. 2 V = 75° 58' 



Specific gravity 6*299, corresponding to about 22*5 per cent 

 Ta 2 5 and 30 per cent Nb 3 5 . 





a 







y 



2 V, Calculated 7 — a 



For Li ... . 



2-3686 



2-3876 



2-4280 . 



*. 70° 0' *0594 



" Na.... 



2-3977 



2-4190 



2-4588 . 



'. 73 25 -0611 



" Tl .... 



2*4261 



2-4508 



2-4903 . 



77 50 -0642 



Na 2Ha = 113° 11' .*. 2 V = 72° 37' 



