440 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



Analysis of niter from near Dubois, Idaho 



(E. V. Shannon, analyst) 



Per cent 



Insoluble in water 0. 24 



Alumina and ferric iron Trace. 



Water (below fusion) . 12 



Lime None. 



Potassium nitrate (KN0 3 ) 98. 74 



Sodium nitrate (NaN0 3 ) 1. 74 



Total 100.84 



The results indicate an unusually pure salt. 



ELMORE COUNTY 



Niter has been found in cavities and cellular material in volcanic 

 tuff at a locality 30 miles north of King Hill near Camas Prairie. 42 



OWYHEE COUNTY 



Niter has been found in several places southwest of Homedale in 



Oregon and probably occurs in similar deposits in shelly rhyolite in 



Owyhee County. 



LUDWIGITE (694) 



Iron-magnesium borate, 3MgO.B 2 3 , Orthorhombic. 



(Mg, Fe)O.Fe 2 3 . 



The rather rare iron and magnesium borate, which occurs in the 

 copper ore deposit of the Bruce Estate in the Texas district in Lemhi 

 County south of Dry Gulch, has already been described. 43 The 

 essential features of the description are repeated here. 



LEMHI COUNTY 



The ludwigite was identified as the most abundant constituent of 

 a specimen from the Bruce Estate copper prospect. It forms fairly 

 pure masses up to some 10 cm. in diameter, which contain dissemi- 

 nated grains of diopside and irregular patches of bornite, and also 

 occurs in disseminated form intergrown with bornite and chalcopy- 

 rite as a replacement of calcite of a calcite-diopside rock. The 

 prospect from which the mineral came occurs in limestone at the 

 side of a large dike of quartz-diorite and the ludwigite is a meta- 

 morphic mineral resulting from the action of the emanations accom- 

 panying the diorite intrusion on the adjacent limestone. 



The mineral is black in the hand specimen and has a very finely 

 felted structure in which the fibrous texture is too fine to be con- 

 spicuous to the unaided eye, but is manifested by a faint silky luster 

 which might easily be overlooked and then the mineral might be 

 mistaken for magnetite. The hardness is above 5 as the ludwigite 

 scratches apatite but is itself scratched by orthoclase. It is not 

 attracted by a common magnet. In thin fragments it is transparent 

 and very pleochroic in tones of brown parallel to the elongation of 



41 Robert N. Bell. Ann. Rept. State Inspector of Mines, Mining Industry of Idaho for 1913, p. 211. 

 <» Earl V. Shannon. Ludwigites from Idaho and Korea. Proc. U. S. National Museum, vol. 59, pp 

 667-669. 



