THE MINERALS OF IDAHO 



407 



to small masses of quartz and muscovite. In the coarse, polycrase- 

 bearing sand from Centerville crystals up to 1 cm. in length occur 

 sparingly, which have the form and appearance of columbite. These 

 are invariably dull with a grayish-black color and somewhat metallic 

 luster. These are more opaque than the small crystals described 

 above. Judging from appearance alone it seems probable that these 

 crystals from Centerville are ordinary columbite while the brilliant 

 black crystals from Idaho City are probably higher in their content 

 of tantalic acid and possibly contain some manganese. The forms 

 and angles measured on crystals from Idaho City are given in the. 

 following; table: 



Measurements of columbite crystals from Idaho City 



Quality, description 



Very good. 



do... 



Poor. 



Very good. 



do 



Poor 



Pair. 



Good TS 



Poor ..._ _ 52 



Good I 90 



Measured 



Calculated 



90 00 



90 00 



00 



90 00 



90 00 



CO 00 



43 48 



61 09 



48 48 



00 39 



In addition to the occurrences of columbite in the heavy sands, the 

 mineral has been found in place in larger masses in this county but 

 the locality is not known. A number of pieces of columbite in the 

 National Museum are labeled as from near Boise. The mineral is 

 typical heavy iron-black columbite, some of the pieces showing crystal 

 faces. The largest fragments in this lot are about 2.5 cm. in diam- 

 eter and are partly coated with a scaly fine micaceous material. 



Another specimen of columbite has recently been received from 

 Boise County. This is a mass of columbite 5 by 7 cm. in size, and 

 Frank L. Hess writes that a much larger mass was received from the 

 same locality by the United States Geological Survey. The speci- 

 men consists of an aggregate of crystals, some of which show a few 

 good faces. The columbite shows attached portions of large mica 

 crystals and is obviously from a pegmatite. 



