MICA GROUP. 267 



Muscovite. — Common Mica. 



Monoclinic. In oblique rhombic prisms of about 120°. 

 Crystals commonly have the acute edge replaced, as in the 

 accompanying figure (plane i-i). Usu- 

 ally in plates or scales. Sometimes in 

 radiated groups of aggregated scales or 

 small folia. 



Colors from white through green, 

 yellowish and brownish shades ; rarely 

 rose-red. Lustre more or less pearly. 

 Transparent or translucent. Tough 

 and elastic. H.=2-2*5. G. =2-7-3. 

 Optic-axial angle 44° to 78°. 



Composition. A common variety afforded Silica 46-3, 

 alumina 36*8, potash 9-2, iron sesquioxide 4-5, fluoric acid 

 0*7, water 1*8 = 99 -3. Often contains 3 to 5 per cent, of 

 water, and thus passes to a hydrous mica called Mar gar o- 

 dite. (See page 313). B.B. whitens and fuses on the thin- 

 nest edges, but with great difficulty, to a gray or yellow 

 glass. 



A variety in which the scales are arranged in a plu- 

 mose form is called plumose mica ; another, in which the 

 plates have a transverse cleavage, has been termed prismatic 

 mica. 



Biff. Differs from talc and gypsum in affording thinner 

 and much tougher folia, and in being elastic ; but musco- 

 vite when hydrous loses its elasticity, and becomes more 

 pearly in lustre. 



Obs. Muscovite is a constituent of granite, gneiss and mica 

 schist, and gives to the latter its schistose structure. It also 

 occurs in granular limestone. Plates two and three feet in 

 diameter, and perfectly transparent, have been obtained at 

 Alstead, and Grafton, New Hampshire, and it has been 

 mined at these places, and in Orange and elsewhere. Other 

 good localities are Paris, Me. ; Chesterfield, Barre, Brimfield, 

 and South Royalston, Mass. ; near Greenwood Furnace, War- 

 wick and Edenville, Orange County, and in Jefferson and 

 St. Lawrence counties, N. Y. ; Newton and Franklin, N. J. ; 

 near Germantown, Pa.; Jones's Falls, Maryland. Oblique) 

 prisms from near Greenwood are sometimes six or seven 

 inches in diameter. Western North Carolina affords much 

 mica for commerce. 



