THE MINERALS OF IDAHO 



197 



ILMENITE (233) 



MENACCANITE, TITANIC IRON 



Ferric titanate, Fe 2 03.Ti20 3 , variable. 



Rhombohedral, tetartohedral . 



Ilmenite has been noted in Idaho chiefly as a constituent of the 

 black sand residues from gravel washings, especially in the Snake 

 River sands and in the Idaho Basin. It is abundant in the heavy 

 sands, probably being present in excess of magnetite in all of those 

 examined except in one from Bear Creek, in Camas County. In 

 part the mineral occurs in irregular grains, and can not easily be 

 distinguished from similar grains of numer- 

 ous other opaque black minerals. The 

 majority of the ilmenite is in more or less 

 distinct tabular crystals which are hexag- 

 onal-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, dis- 

 tinctly 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 pina- 

 coid shows triangular markings which are 

 very characteristic. A typical crystal is 

 shown in the orthogonal and perspective 

 drawings of Figure 35. The angles measured 

 on crystals similar to this are given in the 

 present author's paper on the black sands 

 of Idaho. 5 



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

 derived, 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 

 rarely does a magnetic grain show the typical octahedral form of 

 magnetite, and it may be that the black mineral is practically all 

 ilmenite. The ilmenite of Snake River sands thus is in considerable 

 part magnetic as contrasted with the nonmagnetic character of the 

 same mineral in the sands of the granite regions. 



Fig. 35.— Ilmenite. Typical crys- 

 tal FROM BLACK SAND FROM 



Boise County 



* E. V. Shannon, Mineralogy of some black sands from Idaho, with a riA^riptinn nf the methods used 

 for their study. Proc. U. S. Nat. Mus., vol. 60, art. 3. do '-'« °'- M 



