460 



H. W. Foote — Occurrence of Pollucite, 



Mangano- Columbite. 



This mineral is found in imbedded crystals which are sel- 

 dom over 10 mm long. The color is a dark reddish brown, 

 very closely resembling that of rntile. The available material 

 was not sufficient for a quantitative analysis, but a qualitative 

 examination showed the the presence of manganese, tantalum 

 and niobium. A specific gravity determination on 0'3242 gr 

 of material taken very carefully on a chemical balance gave 

 6*44, which would indicate a chemical composition about mid- 

 way between a niobate and tantalate and near that of the man- 

 gano-columbite from Branchville, analyzed by Comstock.* 



/ 



133 

 121 



n, 163 



The crystals show a considerable variation in habit, being 

 usually prismatic in the direction of the c axis, fig. 1, some- 

 times lengthened parallel to a, fig. 2, while occasionally they 

 are about equally* developed in all directions. They show a 

 very good cleavage parallel to b, 010. The forms which were 

 observed are similar to those found on columbite and are as 

 follows : 



a, 100 m, 110 /, 102 



b, 010 g, 130 e, 021 

 d, 730 k, 103 o, 111 



They gave a few excellent reflections which served to es- 

 tablish the lengths of the cry stall ographic axes. The values 

 are given below, together with the axes for columbite as de- 

 termined by E. S. Dana.f 



Mangano-Columbite. 

 m^m, llO/OlO = 79° 47' 

 /X/, 102^102 = 55 40 



d:b: c — 0-8359 : 1:0-8817 0-8285 



The variation in the ratios is undoubtedly due to a change 

 of both acid and base, the measurement by Dana having been 



*This Journal, xix p. 131, 1880. - fZeitschr. Kryst. xii, p. 266, 1886. 



Columbite. 

 79° 17' 

 56 28 



1: 0-8S98 



