THE MINERALS OF IDAHO 295 



to brownish red garnets in schists and slates are principally alnian- 

 dite. While these are usually very largely almandite it is rare that 

 one of them approaches the maximum refractive index of pure 

 almandite, 1.830, since they usually contain small amounts of pyrope 

 and spessartite which lower the index to below 1.820. The brown- 

 red garnets grade into the last group by increase in manganese and 

 they can not be separated from them either on the basis of color or 

 refractive index. Some distinctly brown-red examples react strongly 

 for manganese; (3) the brown to greenish or sometimes nearly color- 

 less garnets occurring as the products of contact metamorphism at 

 the contacts of limestone with granitic intrusives are variable mix- 

 tures of grossularite and andradite. In this group the composition 

 may be judged with some certainty by the refractive index. 



ALMANDITE-SPESSARITE SERIES 



All of the distinctly red garnets of the slaty and schistose rocks 

 and the purplish to rose-red garnets from the pegmatites may be 

 classified as almandite, since they doubtless contain a preponderance 

 of the almandite molecule. The pegmatite garnets having the rose- 

 red color are probably largely free from pyrope and consist of more or 

 less pure isomorphous mixtures of the almandite and spessartite 

 molecules. Thus their composition can be estimated from their 

 refractive index as follows: 



Almandite Rrefractive index between 1.820 and 1.830 



Almandite-spessartite Refractive index between 1.810 and 1.820 



Spessartite Rrefractive index between 1.800 and 1.810 



This relationship may be completely confused by the entry of a 

 little of the low index pyrope molecule into the mineral. The classi 

 fication by color is fairly reliable, most but not all of the notably 

 manganiferous garnets having the rose tint. The color is, however, 

 the only basis for classification of the loose grains and pebbles found 

 in placers, in the absence of a chemical analysis. It has, of course, 

 been impossible to note all of the occurrences of red garnet known in 

 the State, especially in the heavy residues from placer sands, and 

 only a few of the more important can be mentioned here. 



ADAMS COUNTY 



Almandite is mentioned from the Peacock claim by Palache., 18 but 

 it is believed that the mineral was identified by color and specimens 

 conforming to the description given, although red brown in color and 

 superficially resembling almandite were found to be almost free from 

 ferrous iron and to be andradite. Palache's description is accord- 

 ingly given under andradite. 



i« C. Palache. Amor. Journ. Sci., vol. 8, p. 300, 1899 



