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 7-7). Usu- 
ally in plates or scales. Sometimes in 
radiated groups of aggregated scales or 
small folia. . 
Colors from white through green, 
seeyellowish and- brownish shades ; rarely 
rose-red. Lustre more or less pearly. 
Transparent or translucent. ‘Tough 
and elastic. H.=2-2°. 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, fluorie 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 Margaro- 
dite. (See page 313). B.B. whitens and fuses on the thin- 
nest edges, but with great difficulty, to a gray or yellow 
lass. 
7; A variety in which the scales are arranged in a plu- 
mose form is called plwmose mica; another, in which the 
plates have a transverse cleavage, has been termed prismatic 
mica. 
Diff. Differs from tale 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 Kdenville, 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. 

