W. E. Ford— Mineral Notes. 287 



Art. XXXIV. — Mineral Notes from the Mineralogical Labo- 

 ratory of the Sheffield Scientific School. 

 I. Note on some Analyses of Stibiotantalite ; by W. E. Ford. 



In 1906 the late Prof. S. L. Penfield published with the 

 present writer an article on stibiotantalite.* It was concerned 

 chiefly with the description of crystals from Mesa Grande, San 

 Diego County, California. In it, however, were included two 

 analyses of the mineral. The two samples analyzed were dis- 

 tinguished from each other by marked differences in specific 

 gravity. This variation in the specific gravity made it of 

 interest to determine the relative amounts of the tantalum and 

 niobium oxides in each sample. An attempt was therefore 

 made to estimate the proper proportions of each oxide present. 

 As no very satisfactory method had ever been devised for 

 making a quantitative separation of these oxides, a new method 

 was proposed and used in making the determinations. Briefly 

 this method consisted in taking the specific gravity of the 

 mixed oxides obtained in the analysis of the mineral and com- 

 paring this value with the specific gravities of the pure tantalic 

 and niobic oxides. Time and material failed, however, to ena- 

 ble the proper systematic study of the method to be made. 

 Sufficient work was done, however, to prove that the relation- 

 ship existing between the specific gravities of the pure oxides 

 and any given mixture could be at least approximately expressed 

 diagrammatically as a straight line with the specific gravities of 

 the pure oxides as its terminal points. It was on this assump- 

 tion that the percentages of the two oxides were determined in 

 the analyses given in the article referred to above. The state- 

 ment was made, however, that as the method had not been 

 thoroughly proven, the results could only be accepted .as 

 approximately correct. 



E-ecently Foote and Langley have made a detailed study of 

 this method. f They found that the relationship between the 

 specific gravities of the pure oxides and that of the various 

 mixtures of the two could not be represented by a straight line 

 but rather by a somewhat irregular curve which, however, 

 does not depart anywhere very far from the path of the straight 

 line. The oxides apparently form a solid solution with each 

 other and do not exist together simply as a mechanical mixture. 

 The authors give, however, a table of the specific gravities for 

 various mixtures, and from it, by proper interpolation, the rel- 

 ative proportions of the two oxides in any mixture can be very 

 closely estimated. 



In view of this recent work it seemed of interest to review 

 and correct the analysis made by the writer, and also to add for 

 This Journal, xxii, 61, 1906. f This Journal, xxx, 393, 1910. 



