THE MINERALS OF IDAHO 



375 



scent marcasite from the Garfield tunnel. This was analyzed with 

 the following results: 



Analysis and ratios of clay from Garfield tunnel 

 (E. V. Shannon, analyst) 



Per cent 



Ratios 



Silica (Si0 2 ) 



Alumina (AI2O3) 



Ferric iron (Fe203> -. 



Lime (CaO) 



Magnesia (MgO) 



Potash (K 2 0).. 



Soda (Na 2 0) 



Water (H2O) above 110° C 

 Water (H 2 0) below 110° C 



Total 



1.05X3 



1.00X1 



The formula derived from this analysis is — 



3(H,K) 2 0.2Al 2 3 .5Si0 2 .H 2 



which is indeed like the composition which might be obtained from 

 a mixture of muscovite and kaolinite. It is also similar to the 

 composition obtained for the sericite from Boise County except in 

 that the alumina is low and there is one molecule of water which 

 is lost at 110° C. Optically the material is nearer leverrierite with 

 the indices of refraction approximately a = 1.547, 7= 1.556, bire- 

 fringence 0.009. It is so exceedingly fine grained that it is not 

 possible to say definitely that there is no amorphous material mixed 

 with the minute crystalline flakes but the material all appears of the 

 same refractive index and birefringence which is lower than for either 

 muscovite or kaolinite. The evidence seems to be in favor of the 

 conclusion that these clays are not so simple as just mixtures of 

 kaolinite and sericite. Many of them may be pure sericite, even 

 when low in potash, while others may be more properly classifiable 

 as leverrierite, a mineral of similar properties whose exact relation 

 to the muscovite and kaolinite groups is not yet known. These 

 clays are further discussed under the headings leverrierite and 

 kaolinite below. 



SHOSHONE COUNTY 



In Shoshone County extensive formations of quartzitic or slaty 

 rocks are characterized by the more or less abundant occurrence of 

 sericite throughout. The Burke formation is characteristically a 

 slaty sericitic quartzite and the Wallace formation is, in places, 

 practically a sericite slate or schist. The occurrence of this sericite 

 is regional and its formation long antedates the deposition of the 

 ores, and is in no way connected with them. A small amount of 

 sericitization occurred during the period of vein-forming activity, 

 but this consisted mainlv of the conversion to sericite of crushed 



