414 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



Angles measured on rnonazite from Centerville 



In the Idaho Basin, where the occurrence is typical, the monazite 

 is a heavy yellow sand associated with the gold in placer washings. 

 It is associated with much ilmenite and considerable zircon and lesser 

 amounts of rare earth minerals. It is more abundant in some 

 places than in others, but is universal in its distribution. A sample 

 from the sandy lake beds forming the false bedrock in a gravel 

 bench at the junction of Moore and Granite Creeks, 3 miles east of 

 Idaho City, gave 0.025 per cent of heavy residue containing sharp 

 crystals of ilmenite, no magnetite, sharp zircons, and abundant 

 greenish to yellowish grains of monazite which rarely show crystal 

 faces. After extraction of the ilmenite the remaining sand was 70 

 per cent monazite and contained 48 per cent of oxides of the cerium 

 metals and 1.20 per cent of thoria. 20 



Inclusions in the monazite are not abundant and it is rarely attached 

 to any other mineral. In one case a hexagonal tablet of biotite was 

 seen embedded in a crystal and another crystal was penetrated by a 

 tabular crystal of ilmenite. 



CLEARWATER COUNTY 



Monazite was found in a number of concentrates from the Pierce 

 district in the usual yellow crystals entirely like those from Boise 

 County. It is associated with titanite, from which it is difficult 

 to distinguish, in a sand consisting largely of ilmenite and rose pink 

 garnet. 



NEZ PERCE COUNTY 



The occurrence of monazite in Nez Perce County has been carefully 

 described by Schrader 21 who writes that the mineral is common 

 along the Musselshell Creek and probably occurs also in the adjoining 

 valleys. It is associated with the placer gold and is more concen- 

 trated in the lower part and on the bedrock than in the upper part 



»o Waldemar Lindgren. U. S. Geol. Survey, 18th Ann. Rept., pt. 3, p. 677, 1898. 



31 Frank .C Schrader. An occurrence of monazite in northern Idaho, U. S. Geol. Purvey, Bull. 430, 

 pp. 184-190, 1910. 



