74 Penfield and Ford — Stibiotantalite. 



Goyder, in his analysis of the original stibiotantalite from 

 Australia, made no separation of the tantalum and niobium 

 oxides, but assuming that the ratio of (Ta,Nb) 2 5 : (Sb,Bi) 2 3 

 = 1:1, the proportions of Ta 2 5 and Nb 2 5 were determined 

 indirectly. Thus, considering the first of our analyses, where 

 the per cent of (Ta,Nb) 2 5 is 55*33 ; the per cent of Ta 2 5 may 

 be taken as x, when that of Nb^ will be 55*33 — x: now 

 having the molecular weights, Ta 2 5 = 446, Nb 2 B = 268, 

 Sb 2 3 = 288*4 and Bi 2 3 = 465, the following equation results : 



x 55-33— x 44-26 0*33 „ . . _ 



i46 + ~26^ = 288T + lA6> fr0m Wh ' Ch * = 35 ' 15 - 



Treating all three analyses in like manner, the results, together 

 with the theoretical composition of the end products, are as 

 follows : 





Theory for 











Theory for 





(SbO) a Ta a 6 









(Sb'.: 



Sp. 



, gr. 7-90 



7-37 





6-72 



5-98 



573 



Ta„O s . 



60*73 



51*13 (52 



•16)* 



35-15 



10*60 



o-oo 



Nb,0, 



000 



7*56 ( 6 



•53) 



20*18 



39 70 



48*17 



sb.o. 



39-27 



40*23 





44-26 



49*28 



5T83 



Bi .0. - 







0-82 





•33 



•53 





NiO._ 







0*08 

















H.O.. 





0*08 



















100-00 



99-90 





99*92 



100*11 



100-00 



The i 



'esults of the 



indirect 



determinati 



on of Ta 2 5 and 



Nb 2 5 in the last two analyses show a very satisfactory agree- 

 ment Avith the values derived from the specific gravities of the 

 oxides, and it may be pointed out that a slight variation in 

 the per cent of Sb 2 3 causes about four times as great a varia- 

 tion in the percentages of Ta 2 5 and Nb 2 5 , while, also, varia- 

 tions up to 4 per cent in Ta 2 6 or Nb 2 5 have little effect upon 

 the ratio owing to the large molecular weights of the acid oxides. 



The ratios in the two new analyses, being almost exactly 1 : 1, 

 indicate that in chemical composition stibiotantalite is an iso- 

 morphous mixture of Sb 2 3 .Ta 2 5 and Sb 2 3 .Nb 2 B , with a 

 little bismuth replacing the antimony. It is a curious and 

 interesting fact that, in materials from such widely separated 

 localities, the amounts of bismuth should be so nearly alike. 



The chemical relation between stibiotantalite and columbite 

 may perhaps be represented best by developed formulas, as 

 follows : 



= Sb-0-Nb=0 ^0-Nb = 



0< > O Fe<" 0< > O 



= Sb-0-Nb^O \0-Nb = 



* The percentages given in parentheses result from the molecular weights 

 made use of in. this article. The figures given by Goyder vary somewhat 

 because he used slightly different molecular weights. 



