SII.ICATKS, TITANATE8, ETC, i'.H 



ference figure is symmetrical in the field showing well-defined dark 

 shadows, hut in thin sections no color hands. I'leochroi-m i- not 

 marked, luit absorption takes place in tho.-e -ret ions containing 

 deavagr crack.- and is (lie strongest in sections parallel to the verti- 

 cal axis and parallel to the cleavage cracks, which is the rev \ 

 that found in tourmaline, whore absorption is t he strongr.-t parallel 

 to the vertical axis. 



Muscovite is a common primary mineral of granites, syenites, and 

 pegmatites, and usually separates from the magma directly after 

 /ircon, apatite, and magnetite and before the feldspars. It is 

 particularly characteristic of pegmatite-, in which the -hrrt- may 

 be of enormous size, as in the Black Hills, where sheet.- a yard across 

 have been found. 



It is also widely distributed in the schists and gneisses, where t ho 

 scales are parallel in position and, in the variety sericite, very small 

 and silky in appearance. As a secondary mineral it is a product 

 derived from the alteration of numerous aluminium silicates, as the 

 feldspars, andalusite, cyanite, scapolite, nepheline, and also from 

 corundum. Muscovite itself is very stable under the conditions 

 of weathering, persisting in an exceptionally fresh condition in the 

 residual products of such rocks as granite and syenite, long after 

 other minerals have succumbed to oxidation or kaolinization. Un- 

 der the influence of hot aqueous solutions and pressure, the reac- 

 tions above may be reversed, and muscovite will be transformed to 

 leucite, nepheline, or feldspars. The complex nature of micas is 

 demonstrated on fusion, for then they break down on cooling to 

 various products, among which, in case of muscovite, are leucite, 

 sillimanite, glass, and water. 



Commercially muscovite is mined in the pegmatites of the Black 

 Hills, South Dakota; along the Blue Ridge in North and South 

 Carolina, Virginia, and Georgia; at Cafion City, Colorado, while 

 several other states are producers of small quantities. The product 

 of sheet mica in the United States is about one million pounds an- 

 nually. 



The micas, especially muscovite, are used for many purp* 

 when in large sheets, as for stove doors and lamp chimneys and 

 shades. The value varies greatly with the size of the sheets. Frag- 

 inenlal mica is used as an insulator in various electric apparatus, 

 and when finely ground it is used as a paint for frosting, and, as in 

 the case of graphite, as a lubricant. 



The synthetic production of mica is uncertain. Biotite lias been 



