THE MINERALS OF IDAHO 411 



pinacoids and on two pyramid faces indicating roughly the form 

 s(lll) of polvcrase. The remaining faces were coated. The form 

 and appearance of this crystal, which is tabular, are shown in the 

 drawing (fig. 127). There is, as shown in the figure, a smaller 

 crystal in parallel position projecting from the face of the larger 

 individual. 



The so-called Poncia tract, from which the most of this mineral 

 has come, is worked by Carlos Poncia in Swede Gulch near the center 

 of section 9, township 7 north, range 5 east, 1% miles southwest of 

 Pioneerville. The polycrase is concentrated in the heavy sands with 

 brilliant crystals of yellow monazite and colorless zircon. 



MONAZITE (537) 



Phosphate of the cerium metals Monoclinic. 



(Ce, La, Di) 2 03.P 2 5 . 



Monazite is abundant in a number of places in Idaho as a con- 

 stituent of concentrates from the gold-placer deposits. The presence 

 of this mineral in the State was first recognized by Lindgren 17 in the 

 gold placers of the Boise Basin, where he found it as a resinous 

 brown mineral in subangular grains in part exhibiting crystal faces. 

 Roughly quantitative analyses by Hillebrand made upon the purified 

 sand showed the principal constituents to be phosphoric acid and 

 cerium earths, with a small amount of thorium. The absence of 

 yttrium earths showed that xenotime probably was absent. Later, 

 Day, 18 in his work on the black sands of the Pacific slope, reported 

 the mineral from 37 localities in 10 counties in Idaho. Some of these 

 are in error, since several of the sands listed are from Snake River 

 localities, and a reexamination of the same samples failed to detect 

 any monazite. Schrader has recently 19 described the occurrence of 

 monazite in Nez Perce County. 



BOISE COUNTY 



The monazite occurs most abundantly in the gold-placer region 

 about Centerville, in Boise County, and preparations were made 

 some years ago by the Centerville Mining & Milling Co. to recover 

 and clean the sand for market. The plant which was built was 

 burned before any important production was made and the commer- 

 cial outlook was not sufficiently bright to encourage its rebuilding. 

 At present the only value which attaches to monazite is dependent 

 on its thorium content. The Idaho monazite is seemingly lower in 

 its thorium content than similar sands from Brazil and North CarO- 

 if Waldemar Lindgren. U. S. Qeol. Survey, 18th Ann. Rept., pt. 3, pp. 677-679, 1898. 

 is David T. Day and R. H. Richards. U. S. Qeol. Survey, Mineral Resources of United States for 1905 

 pp. 1195-1201, 1906. 



" Frank C. Schrader. An occurrence of monazite in northern Idaho, U. S. Qeol. Survey, Bull. 430, pp., 

 184-190, 1910. 



