THE MINERALS OF IDAHO 405 



task only to be undertaken by a chemist experienced in this particular 

 work and requires much time and a large amount of material. 



The rare-earth minerals are associated in the heavy sands with 

 abundant monazite and their source has not been proven by the find- 

 ing of any of them in place in matrix, although the fact that they 

 sometimes contain embedded mica and other minerals suggests that 

 they came from the granitic rock. The source is quite probably in 

 pegmatitic phases of the granite and a careful examination of peg- 

 matitic streaks and segregations in the bedrock would doubtless 

 reveal them in place. 



FERGUSONITE (523) 



Columbate and tantalate of yttrium, Tetragonal, 



erbium, cerium, etc., approximates the 

 formula (Y, Er, Ce) 2 3 .(Cb,Ta) 2 5 . 



BOISE COUNTY 



Rare brown resinous grains observed under the microscope in a 

 concentrate from a placer sand from the Idaho City dredge, Idaho 

 City may be fergusonite. In one case a crystal was seen which 

 appeared to have the tetragonal form and pyramidal hemihedrism 

 of fergusonite. When crushed and examined under the microscope 

 the material of these grains is transparent, light brown in color and 

 perceptibly isotropic with very high refractive index. 



COLUMBITE (525) 



Iron columbate, FeO.Cb 2 5 . Orthorhombic. 



Columbite has been obtained only in Boise County where it occurs 

 as small crystals in heavy sands and also as larger masses probably 

 in pegmatites. 



BOISE COUNTY 



A separation product from a concentrate from placer gravels 

 worked by the Idaho City dredge at Idaho City, consisting largely 

 of samarskite, contains important amounts of a black mineral in 

 crystals, which proved, upon measurement, to have the angles of 

 columbite. Aside from the difference in form, which is not always 

 manifest, this mineral greatly resembles ilmenite, which occurs com- 

 monly in the sands. The columbite makes up about 10 per cent of 

 the samarskite concentrate and is more abundant than samarskite 

 in another concentrate from the same locality which consists largely 

 of garnet. 



The columbite crystals vary considerably in habit, ranging from 

 tabular parallel to the pinacoid 5(010) to square prismatic. The 

 common forms and habits are illustrated in the drawings, Figures 

 122 to 125, inclusive. The color is black and the luster is more vitre- 

 ous than metallic. The prismatic planes are usually very brilliant, 

 but the terminal faces are frequently more or less dull or pitted. 



