98 W. P. TIeadden — Columbite and Tantalite. 



analysis of the Northfield mineral gives 1 : 1J as the ratio of 

 Cb : Ta, while that of the Branch ville mineral by the same 

 analyst gives 1 : 1 for the ratio. It will be noticed that of the 

 twent}^ specimens from different localities in the Black Hills, 

 seven of them contain the Cb and Ta in the ratio of 3:2, four 

 contain them in the ratio of 1 : 1 and one contains them in the 

 ratio of 1 : 1J. There is no doubt as to the form of these 

 specimens unless it be in regard to the last, but in the case of 

 the Northfield mineral, which gives the same ratio, i. e. 1 : 1J, 

 Professor Dana expresses no doubt. The tantalite from Yan- 

 cey Co., N. C, analyzed by Comstock (Appendix III, p. 118), 

 gives the formula 6BTa 2 6 +4BCb 2 6 , while the columbite from 

 Northh'eld, Mass, and the Yolo Mine, Lawrence Co., S. D., 

 give the formula 5RTa 2 6 -h4BCb 2 6 If the Broddbo and 

 Yancey Co., N. C, specimens are real tantalites and the ^North- 

 h'eld and Yolo minerals real columbites, there is an overlap- 

 ping of specific gravity and chemical composition which 

 destroys their value as guides in determining these minerals 

 when the columbite and tantalite molecules are nearly equal 

 in number. It was my intention to carry my work further 

 and endeavor to show that there is chemically no sharp line 

 between them, but that the tantalate may predominate in a 

 true columbite to even a greater extent than is indicated by 

 any of the analyses. In form the columbite is not always 

 tabular or square prismatic in habit ; the pinacoids to which 

 these habits are due are sometimes very subordinate and the 

 columbite becomes as pronouncedly prismatic as tantalite. 

 I have so far been unable to determine the superior limit of 

 tantalic acid compatible with the columbite form. 



The Turkey Creek (Colorado) mineral, deserves mention as 

 being an almost typical columbite, but is, like the greater number 

 of the Dakota columbites, rich in manganese. It is also rather 

 remarkable that it is the only one which contains tungstic acid. 

 A pure ferriferous columbite has not yet been found in the 

 Black Hills, the only specimen approaching it is that from the 

 Yolo Mine which contains the Fe and Mn in the ratio of 5 : 1 ; 

 by far the greater number of all the others are rather mangan- 

 iferous than ferriferous columbites, this is in marked contrast 

 to the tantalites, analyses of which are given later. 



2. Tantalite. 



Professor 'Schaeffer published in the Transactions of the 

 American Institute of Mining Engineers, vol. viii, page 233, 

 the identification of a mineral from the Etta Mine as tantalite 

 and gives the following analysis : Ta 2 5 79*01, Sn0 2 0*39, FeO 

 8-33, MnO 1213 = 99'86, sp. gr. 772. I have good reasons for 

 believing that Professor Schaeffer's material was from the 



