298 



BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



Analysis and ratios of garnet from Avon 

 (E. V. Shannon, analyst) 



Constituent 



Silica (SiO s )— 



Alumina (AI2O3)-- 



Ferric iron (Fe2Ch) 



Lime (CaO) 



Ferrous iron (FeO) 



Manganous oxide (MnO) 

 Magnesia (MgO) 



Total 



Per cent 



35.84 



21.20 



.28 



Trace. 



27.84 



14.33 



1.04 



100.53 



Ratios 



0. 594 0. 594 

 .207) 

 .0021 



1.00X3 

 209 1.06X1 



.202 

 .026 



.615 1.04X3 



These ratios yield the garnet formula, 3RO.R 2 3 .3Si0 2 . The 

 small amount of ferric iron can not be present as the andradite mole- 

 cule (Ca 3 Fe2(Si0 4 )3) which is the only recognized garnet molecule 

 containing iron in the trivalent State, as the amount of lime re- 

 quired (0.29 per cent) does not occur in the mineral. Grouping the 

 ferric iron with the alumina in the almandite molecule the garnet 

 has the following molecular and percentage compositions: 



The measured index of refraction of the analyzed powder is 1.818 

 while the calculated value for a garnet of the above composition, based 

 upon Ford's data for the end members of the group, is 1.816, the two 

 agreeing within the limit of error. 



LEMHI COUNTY 



Garnet occurs at several places in Lemhi County in schists. The 

 crystals sometimes reach 2 cm. in diameter. The garnet is appar- 

 ently almandite. 



MINIDOKA COUNTY 



Small grains of both brownish-red and purplish-red garnet are 

 frequently observed from placer concentrates from Minidoka and 

 elsewhere in Minidoka County along Snake River. 18 



NEZ PERCE COUNTY 



Numerous grains of garnet both of rose-red and brown-red color 

 have been noted in a placer concentrate from the Early Bird placer 

 on Clearwater River near Lewiston. Fine purple almandine gar- 

 nets of gem quality have been reported to occur in quantity in 



"Earl V.Shannon, 

 art. 3, 1921. 



Mineralogy of some black sands from Idaho, Proc. U. S. Nat. Museum, vol. 60. 



