THE MINERALS OF IDAHO 385 



altered, contains streaks and veinlets of a greenish-brown fibrous 



mineral with high double refraction. It is believed to be a magnesian 



silicate carried up from the underlying basalt. It is, however, 



neither chlorite nor serpentine for the powder of two rocks in which 



it was abundantly present yielded no magnesia when treated with 



sulphuric acid. It is probably a mineral related to celadonite or 



" green earth." 95 



KAOLINITE (492) 



Hydrous aluminium silicate, Monoclinic. 



2H 2 O.Al 2 03.2Si0 2 . 



Ordinary high grade clays such as are used for porcelain, etc., are 

 usually called kaolin, which name includes kaolinite and the corre- 

 sponding amorphous mineral of the same composition, halloysite. 



Considering their widespread occurrence and commercial import- 

 ance, surprisingly little is known regarding the mineralogy of the 

 clays utilized in the ceramics industries. In addition to kaolinite 

 and halloysite these may include minerals of the sericite-muscovite 

 series and, very recently, it has become increasingly evident that 

 the mineral leverrierite is a clay mineral of widespread occurrence. 



Kaolinite proper is a completely crystalline micaceous mineral 

 and the best and coarsest examples thus far known are not plastic. 

 The mineralogical nature of clays, if they are homogeneous, may be 

 established by optical examination coupled with determinations of 

 the amount of water contained and the temperatures at which it is 

 released. 



Like other similar clayey minerals, kaolinite has its origin in the 

 alteration of feldspars. Formerly it was considered principally a 

 weathering product from the feldspars but a more recent tendency 

 is to attribute a large amount of the kaolinization to hydro thermal 

 processes. The following references to kaolin from Idaho may be 

 quoted from existing literature but no critical discussion of the clays 

 of the State can be offered. 



CUSTER COUNTY 



Kaolinite occurs in the Alder Creek district along fractures and 

 in cavities in the oxidized ores. It seems to here be a normal product 

 of the weathering of the aluminous garnet and of the igneous rocks. 98 



KOOTENAI COUNTY 



Deposits of fine white clay occur near Hutter Station in the Spokane 

 Valley. These clays are very pure but they have not been submitted 

 to a mineralogical examination. 



»« Waldemar Lindgren. U. S. Geol. Survey, 20th. Ann. Rept., pt. 3, p. 179, 1900. 

 '« J. B. Umpleby. U. S. Geol. Survey, Prof. Paper 97, p. 53, 1917. 



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