8 



PROCEEDINGS OF THE NATIOISTAL MUSEUM. 



VOL. 60 



Bear Creek, iii Camas County. In part the mineral occurs in irregu- 

 lar grains, and it can not then be distinguished from similar grains 

 of numerous other opaque black minerals. The majorit}' of the 

 ilmenite is in more or less distinct tabular crystals which are hex- 

 agonal-trigonal in form. In the monazite-bearing sands of the Boise 

 Basin and the Clearwater region ilmenite is present in amount 

 greatly in excess of magnetite. Here this mineral occurs in fine 

 to coarse grains, which are, for the most part, distinctly tabular in 

 form with three or six sided bright basal pinacoids present although 

 the edges are dull or etched and rarely show good crystal faces. 

 Often the basal pinacoid shows triangular markings which are very 

 characteristic. A typical crystal is shown in the orthogonal and 

 perspective drawings of figure 27. The angles measured on a crystal 

 similar to this are given below : 



Angles of ilmenite crystal from Rhodes Creek, Pierce City district. 



Although the general aspect does not differ greatly, the forms 

 present vary somewhat from crystal to crystal. A second crystal 

 measured eave the following forms and angles : 



Angles of ilmenite crystal from Idaho City. 



The ilmenite of the monazite sands, which is in all probability de- 

 rived, as are the associated monazite and zircon, from the granitic 

 rock of the Central Idaho batholith, is not attracted to a magnet. In 

 the fine sands of the Snake River about one-half of the total amount 

 of black iron ore is extracted by a magnet, both the nonmagnetic 

 and the magnetic portions being composed of irregular black grains 

 and disseminated brilliant thin tabular crystals of ilmenite. Only 



