THE MINERALS OF IDAHO 367 



SHOSHONE COUNTY 



A few unimportant scattered pegmatities in the southern part of 

 the Avery quadrangle contain mica in the usual coarse plates. Much 

 of this area is occupied by metamorphosed sedimentary rocks which 

 contain abundant muscovite in fine flakes. In places granular 

 pseudomorphs of muscovite after large cyanite crystals are found. 



WASHINGTON COUNTY 



Mica deposits have been prospected 12 miles southeast of Council 

 near the Middle Fork of Weiser River. The specimens consist of 

 clear mica with smooth flat cleavage and also of twinned plates 

 having the so-called A structure. In thick pieces the mica has a 

 dark green color. Some of it is of good qualitj- and could be trimmed 

 into fair sized sheets. 77 



MARGARODITE 



VARIETY OF MUSCOVITE 



Margarodite or pearly mica includes varieties of muscovite which 

 are usually somewhat less elastic than ordinary muscovite and have 

 a more pearly luster. The variety grades into ordinary muscovite 

 and differs little from it in optical properties. Chemically it differs 

 in having relatively less potash and more water. The place of 

 occurrence of the margarodite is usually in quartz of veins inter- 

 mediate between ordinary quartz veins and pegmatites. It is fre- 

 quently associated with molybdenum and tungsten ores in such 

 veins. Specimens of the margarodite variety of mica have been 

 seen from two localities in Idaho as follows: 



BONNER COUNTY 



Pale grayish rosettes and scaly masses of the margarodite variety 

 of muscovite occur in quartz of molybdenum ore from a molybdenite 

 prospect 21 miles by trail southwest of Porthill, Idaho. The associ- 

 ated molybdenite has the same foliated and rosetted habit as the 

 margarodite. 



SHOSHONE COUNTY 



Specimens from the dump of the Monitor mine in the Avery 

 quadrangle contain scaly masses of typical margarodite associated 

 with quartz, chalcopyrite, and carbonates. 



SERICITE 



VARIETY OF MUSCOVITE 



While of little value as cabinet material and of unpreposessing 

 appearance, sericite is a variety of muscovite of distinct importance 

 to the mining geologist, prospector, and penologist. It is commonly 



" U. S. Qeol. Survey, Bull. 740, p. 93, 1923. 



