THE MINERALS OF IDAHO 3G5 



Kootenai County, agrees in indices with faroelite but again the 

 elongation is negative. 



UNCLASSIFIED ZEOLITES 



In addition to the several occurrences of zeolites noted above the 

 following occurrences have been mentioned without more specific 

 data than just that they are zeolites. 



BLAINE COUNTY 



An unidentified pearly zeolite was observed by D. F. Hewett at the 

 Golden Bell mine at the head of Minnie Gulch near Bellevue in the 

 Wood River district. 



CUSTER COUNTY 



Good specimens of zeolites have been reported from the vicinity of 



Salmon. 



MUSCOVITE (458) 



WHITE MICA, POTASH MICA 



Hydrous silicate of potash and alumina, Monoclinic. 



K 2 0.3Al 2 3 .6Si0 2 .2H 2 



Muscovite or white mica is a mineral of very common occurrence, 

 especially in metamorphic rocks such as schists, etc., where it occurs 

 in the usual small flakes and scales. In this form the mineral occurs 

 at many places in Idaho as mica schists and micaceous quartzites as, 

 for example, in the Archaen area about Lake Coeur d'Alene in 

 Kootenai County, in the metamorphosed Belt rocks of southern 

 Shoshone County, in the crystalline rocks of Mount Hyndman, and 

 the adjacent range in the Hailey quadrangle in Blaine and Custer 

 Counties, and in many other places in the State. 



Muscovite is rare as an original constituent of unmetamorphosed 

 igneous rocks either intrusives or lavas, and is almost invariably 

 secondary when present in such rocks. It does occur, however, as 

 a primary constituent of pegmatites, usually as a prominent mineral 

 and in large crystals. These are of value when they are large enough 

 and free from twinning and flaws, so that they may be split into thin 

 clear sheets which are used for windows in stoves, for electrical insula- 

 tion, etc. Such mica is mined to a considerable extent in Latah 

 County as set forth below. A few of the principal occurrences of this 

 mineral are mentioned below. Two varieties of muscovite, margaro- 

 dite, and sericite, are discussed separately following the discussion of 

 ordinary muscovite. 



BLAINE COUNTY 



In Blaine County muscovite occurs in scales and grains in the 

 crystalline rocks which form the cores of the peaks about Mount 

 Hyndman in the Hailey quadrangle and also in larger crystals in 

 occasional pegmatites which cut some of the metamorphic rocks. 



