THE MINERALS OF IDAHO 369 



colorless serpentine, or kaolinite, from which it may, however, be 

 distinguished by its refractive indices. 



Chemically sericite has long been called hydro-mica. It is chemi- 

 cally and optically closely related to muscovite, but is distinguished 

 by having less potash in many instances, and more combined water. 

 The term also includes, petrographically, micas of the same micro- 

 scopic appearance, which may contain soda and thus be properly 

 classifiable as paragonites. Just to what extent muscovite may 

 grade toward kaolinite is not known. Neither is it clear just what 

 relationship exists between sericite, which contains no water which is 

 released at a temperature of 110° C. and the "leverrierites," clayey 

 minerals of widespread occurrence, which are similar in composition 

 and properties but contain a varying and usually large number of 

 molecules of water released at a low temperature. These points 

 are further noted in the following discussion and also under the 

 heading leverrierite below. The occurrence of sericite in several 

 Idaho mining districts is described here. 



BOISE COUNTY" 



Throughout the Boise Ridge and Idaho Basin the primary gold 

 veins present a certain similarity. They are all inclosed in granitic 

 rocks or associated dikes and are all either fissure veins or impregna- 

 tions connected with fissures. A marked change appears in the 

 rock in the vicinity of the veins. The dark constituents are bleached 

 or disappear and the feldspar is altered to a soft white opaque ma- 

 terial, only the quartz remaining unaltered. Besides abundant 

 pyrite, arsenopyrite also appears in small scattered crystals. This 

 change has previously been described as kaolinization. The width 

 of the altered zone may be from a foot up to 50 or 60 feet or more. 

 The soft white substance which often has a greasy feel is also referred 

 to by the miners as "talc." This change in appearance and com- 

 position is without the slightest doubt directly due to the chemical 

 action of the solutions from which the mineral content of the vein 

 was deposited. The process consists in a sericitization or replace- 

 ment of the ferromagnesian silicates, feldspars, and part of the 

 quartz by sericite, a fine fibrous or felted variety of muscovite, in 

 composition a hydrous silicate of aluminium and potassium. In 

 many places a carbonatization or replacement by carbonate of lime 

 and magnesia goes on at the same time and sulphides, chiefly pyrite 

 and arsenopyrite are formed in the rock as minute and perfect crys- 

 tals. It is certain that this metasomatic process is a common one 

 in fissure veins and its chemical character is very different from 



78 Reprinted with little change from Waldemar Lindgren. U. S. Oeol. Survey, 18th Annual Report, 

 pt. 3, pp. 63S-C40, 1897. 



