THE MINERALS OF IDAHO 



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could accurately be described as a greenish tinge, the associated 

 green grains being usually augite, titanite, or olivine. A few green 

 and a few perfectly colorless transparent monazite crystals were 

 seen in the columbite-samarskite concentrate from Idaho City 

 described above. The monazite is, for the most part, in sharp and 

 perfect crystals, although many of the larger crystals are broken 

 or abraded. The average diameter of the monazite in the screened 

 sands is less than 1 mm. but in one "oversize" sample from Center- 

 ville rough crystals up to 5 mm. in diameter were observed, and larger 

 masses may have been discarded by screening. In form the monazite 

 from all of the localities represented is very similar. The five figures 

 were all drawn from measurements made upon crystals selected from 

 a sand from Centerville and subsequent examination of numerous 

 other sands did not reveal any additional forms, combinations, or 



131 

 Figs. 131-132.— Monazite crystals. Boise County 



habits. The smaller crystals are often (lawless and transparent, 

 while the larger individuals are more or less opaque from the presence 

 of numerous cracks and rifts. The forms noted on the crystals are 

 few in number and perhaps 90 per cent of those seen had the habit 

 shown in figure 128 and 10 per cent the form shown in Figure 129. 

 Figures 130, 131, and 132 represent quite unusual habits. The simple 

 habit shown in Figure 132 is characteristic of some of the very 

 largest crystals as well as of the colorless and green monazites seen in 

 the samarskite concentrate. The more prominent faces on the 

 crystals gave fairly good reflections and the agreement between the 

 angles measured and the theoretical angles is rather good. The 

 averages of the measurements are given in the following table. 



