

266 DESCRIPTIONS OF MINERALS. 



The ordinary light-colored micas are mostly Muscovite, and 

 the black, mostly Biotite. Lepidolite is a light-colored mica 

 containing litliia ; and Lepidomelane a black mica containing 

 more iron than biotite. Muscovite and biotite are so closely 

 related that crystals of the latter often occur that are fin- 

 ished out uninterruptedly by nrascovite, the axial lines of the 

 one continuous with those of the other; and such crystals 

 are sometimes several inches across. There is here a com- 

 pound structure chemically, but no twinning in the crystal- 

 lization. When a thin plate of mica is struck with a pointed 

 awl or other tool a symmetrical star of six rays is produced, 

 the rays being cleavage lines parallel to the sides of the 

 rhombic prism i" and the shorter diagonal. 



P^- Biotite. 



Monoclinic. Crystals usually short erect rhombic or hex- 

 agonal prisms. Common in disseminated scales; also in 

 masses made up of an aggregation of scales. 



Color dark green to black, rarely white. Transparent to 

 opaque. Lustre more or less pearly on a cleavage surface. 

 Optic-axial angle usuallv less than 1' ; crvstals appear often 

 to be uniaxial. H. =2*5-3. G. =2*7-3 *i. 



Composition. Mostly (K 2 ,Mg,Fe) 3 Al la Si 6 , a variety af- 

 forded Silica 40*91, alumina 17*79, iron oxides 10*00, mag- 

 nesia 19*04, potash 9*96. B.B. whitens and fuses on thin 

 edges ; sometimes the flame is red owing to the presence of 

 a little lithium, 



Lepidomelane. Like biotite, but containing" more iron oxides and 

 less of magnesia than biotite, and folia brittle. Fuses easi]y to a 

 black magnetic globule. Annite (from Cape Ann, Mass.) is near Lepi- 

 domelane. 



Phlogopite. Contains much magnesia and little or no iron. Color 

 yellowish brown to brownish red, somewhat copper-like in its reflec- 

 tions ; also white or colorless. Optic-axial angle 3° to 30'. H =2 5-8. 

 Gr.=2-?8-2-So In crystals and scales in granular limestone From 

 Gouverneur, Edwards, and other places in Northern New York ; Stir- 

 ling Mine, and Xewton, X. J. ; St, Jerome, and Burgess, Canada ; in 

 the Vosge.s Aspidolite. from the Tyrol, is a related mica. 

 ^ Astrophyllite. A bronze-yellow mica affording nearly 8 per cent, of 

 titanium dioxide. From Brevig, Norway, and El Paso County, Colo- 

 rado. 





