300 BULLETIN 131, UNITED STATES NATIONAL MUSEUM 



A specimen of garnetiferous mica schist from "head of gulch east 

 of Prospect Knob" contains brownish red garnets, somewhat iron 

 stained, up to 1 cm. in diameter. 



One of the largest of the garnet crystals seen from this area is 

 dodecahedral in form and is 7 cm. in diameter. It is greatly fractured 

 and the fractures are partly filled with manganese oxide. After 

 the manganese oxide is dissolved the garnet is purplish pink in color. 

 It reacts strongly for manganese. The refractive index is 1.805. 

 The exact locality or mode of occurrence of this specimen (ll-J-84) 

 was not learned. A specimen containing numerous garnets up to 8 

 cm. in diameter (10-L-64B) shows a central feldspathic streak, prob- 

 ably pegmatite, bordered by the coarse garnets associated with 

 black biotite. The garnet may owe its origin to the pegmatitic 

 material. The only locality given for the latter specimen is " Clear- 

 water Basin." 



ANDRADITE-GROSSULARITE SERIES 



Brown to green garnets occurring as contact metamorphic minerals 

 in limestone silicate zones adjacent to igneous intrusions range in 

 composition from grossularite, the lime-alumina garnet, to andradite r 

 the lime-ferric iron garnet. Admixture of other garnet molecules is 

 so rare in this situation as to be negligible. Every gradation exists 

 between these two end members and, owing to the large range in 

 refractive index covered by the isomorphous series it is possible to 

 estimate accurately the composition of a given garnet from its 

 refractive index. They may be roughly classified by the following 

 scheme. 



Andradite refractive index from 1.842 to 1.895 



Andradite-grossularite refractive index from 1.788 to 1.842 



Grossularite refractive index from 1.735 to 1.788 



Garnets of this series are known to occur in a number of places 

 in Idaho, some of which furnish fine mineral specimens. The lime 

 garnets have an economic importance because of their frequent 

 association with valuable ores. 



ADAMS COUNTY 



Lime silicate garnets are abundant in many places in the Seven 

 Devils and adjacent districts making up silicate contact zones in 

 limestone which are mined for copper. The garnet is present as 

 well-formed crystals and the locality is capable of furnishing cabinet 

 specimens of garnet comparable to the best that have been found 

 either in the United States or abroad. The garnet from this district 

 was first described by Melville, 22 who mentions dark brown garnet 

 as occurring throughout the Peacock mine in crystals exhibiting a 



" W. H. Melville. Amer. Jour. Sci. f vol. 41, p. 138, 1891. 



