72 Penfield and Ford — Stibiotantalite. 



As may be seen from the above, the mineral is characterized 

 by an unusually high index of refraction, the values 7 for 

 yellow, 2*4:568 and 2*4588, being somewhat higher than that of 

 diamond, 2*418, and considerably above that of sphalerite,. 

 2*369, yet so near the latter that, both minerals being of the 

 same color and possessing good cleavage, it is evident that 

 fragments of the two must look exactly alike. An unusually 

 strong dispersion, p < u, is a marked feature of the mineral, 

 and also a high birefringence, 7 — a for yellow being for the 

 crystal of higher specific gravity *0826, and for the other 

 •0611. The character of the birefringence is positive. The 

 plane of the optical axes is the macropinacoid (100), and the 

 vertical axis c is the acute bisectrix. 



The divergence of the optical axes, 2 7, is so great that 

 2E can not be observed ; in fact, the mean index of refraction 

 of the mineral is so far above that of our highest refractive 

 liquids that it was with the greatest difficulty that 2H could 

 be observed : This was due in part to the fact that the sections, 

 prepared from the same crystals from which the prisms were 

 cut, contained some dark inclusions which interfered with the 

 transmission of light. 2H therefore was measured only for 

 sodium light, the illumination from lithium and thallium 

 flames being too weak to yield distinct interference figures. 

 It seems rather anomalous to have 2TI acute as high as 119° 

 and 113°, but this is due to the very high index of refraction 

 of the mineral. In both crystals the agreement between the 

 values of 2 F, as calculated from the three indices of refrac- 

 tion and from 2 Ha, are as satisfactory as could be expected, 

 considering the difficulties encountered in preparing the 

 prisms and sections, and making the measurements. As 

 shown by the tabulated results, the substitution of Nb 2 5 

 for Ta 2 5 causes a slight increase in the values of the 

 indices of refraction, a decrease in birefringence (7 — a) and, 

 except in the case of thallium, a decrease in 2 V. The dis- 

 persion is much more marked in the crystal of lower specific 

 gravity, the differences of 2 Y for green and red being in the 

 one about 4° and in the other about 7°. 



Chemical Composition. — For chemical analysis two crystals 

 were employed, one having a specific gravity of 6*72, and the 

 other 5*98. The method of analysis was as follows : The 

 mineral was dissolved, without heating, in hydrofluoric acid in 

 a platinum dish, and the antimony and bismuth precipitated 

 by hydrogen sulphide. The precipitate was collected on a 

 filter and then digested with a solution of sodium sulphide in 

 order to separate antimony from bismuth. The undissolved 

 bismuth sulphide was collected, dissolved in warm nitric acid 

 and, after removal of the latter, reprecipitated by hydrogen 

 sulphide. The precipitate was then collected on asbestos, 



