THE MINERALS OF IDAHO 177 



QUARTZ (210) 



Silicon dioxide, Si0 2 Hexagonal RhombohedraL 



The mineral quartz, under which are included many varieties, 

 including chalcedony, is an exceedingly common mineral and occurs 

 very widely distributed in Idaho as elsewhere. The several varieties 

 are discussed separately below under the several varietal names, in 

 accordance with the following outline: 



A. Phenocrystalline varieties. 



a. Common quartz, massive. 



b. Rock crystal. 



c. Amethyst. 



d. Pseudomorphous. 



B. Cryptocrystalline varieties. 



e. Chalcedony and agate. 

 /. Jasper. 



Numerous other varieties of this mineral are recognized but they are 

 not known to occur in good specimens in Idaho. 



A. PHENOCRYSTALLINE VARIETIES 



a. COMMON QUARTZ 



MILKY QUARTZ, BULL QUARTZ, VEIN QUARTZ, GREASY QUARTZ. 



Common massive quartz occurs throughout the State in veins and 

 masses in rocks of all kinds, both as a gangue in metal bearing veins 

 and as large deposits which are not known to contain any valuable 

 metal. It varies in color from clean milky white, as for example in 

 the gold veins of many districts in Idaho County, to bluish, or gray 

 varieties, which pass under the miners name of "bull quartz." En- 

 tirely similar massive quartz occurs in considerable masses in pegma- 

 tites and in grains in granites and other igneous rocks. When these 

 rocks disintegrate the quartz, being chemically the most stable con- 

 stituent, remains behind in the form of waterworn grains and most 

 sand is made up of such quartz grains. Consolidation of the sand 

 gives rise to extensive formations of sandstone, also consisting essen- 

 tially of more or less pure quartz, and by further metamorphism the 

 sandstone is converted to quartzite, much of which is comparable in 

 purity to the massive quartz of veins. 



b. ROCK CRYSTAL 



Under this head are included all the forms of colorless quartz which 

 occur in distinct crystals. Small crystals of quartz occur in very 

 wide distribution. The following occurrences have been noted. 



Boise County. — Small crystals occasionally occur in open spaces 

 in the gangue of the gold veins. A specimen of sericitized and pyri- 

 tized porphyry from the Gold Hill mine dump, Quartzburg district, 



