316 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



divide at the head of Big Lost River, contain hair-brown prisms 

 of vesuvianite associated with masses of greenish-brown garnet. 

 These, like those previously described, are vertically elongated im- 

 perfectly developed crystals. One which was measured is illus- 

 trated in Figure 81. This shows one form which appears to have 

 the indices (6-12-5), but it is doubtful. The measurements are as 



follows : 



Measurements of vesuvianite, Figure 81, Basin Group 



ZIRCON (394) 



Zirconium silicate, ZrO.Si0 2 . Tetragonal. 



Zircon is known from Idaho only as a constituent of heavy sands 

 concentrated fron gold-placer washings. It is widely distributed in 

 such sands, being especially common in association with monazite. 

 Like the monazite much of the zircon is probably derived from the 

 granite by disintegration of that rock or of small pegmatitic segrega- 

 tions in it. Ordinarily the mineral is clear and colorless and it is 

 invariably in beautifully sharp and clear crystals, most of which are 

 very transparent and brilliant but it varies in color through smoky 

 and gray to brown and orange, pink or flesh red. The smoky crys- 

 tals are much like smoky quartz in coloring and the pigment is often 

 unevenly distributed. Inclusions are frequent, the most highly 

 transparent and brilliant forms containing minute spherical bubble- 

 like cavities and also minute microlite-like prismatic crystals of a 

 transparent colorless mineral having a lower refractive index than the 

 zircon. Many of the translucent smoky gray crystals appear to 

 owe their color to minute inclusions of iron oxide. The several 

 crystals illustrated in Figures 83 to 91, inclusive, from various 

 localities in the State have been measured and the measurements 

 have been previously published so that they need not be repeated 

 here. 40 



ADA COUNTY 



A few very minute crystals of zircon occur in a concentrate from 

 a locality on Snake River in Ada County near Boise. These are 

 pyramidal in habit with no prism faces, as shown in Figure 90, and 

 have an orange red color. 



<• Earl V. Shannon. Mineralogy of some black sands from Idaho, Proc. U. S. Nat. Mus., vol. 60, art. 

 3, pp. 11-15, 1921. 



