266 DESCRIPTIONS OF MINERALS. 
The ordinary light-colored micas are mostly Muscovite, and 
the black, mostly Biotite. Lepidolite is a light-colored mica 
containing lithia ; 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 muscovite, 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 1s produced, 
the rays being cleavage lines parallel to the sides of the 
rhombic prism J and the shorter diagonal. 
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 usually less than 1°; crystals appear often 
to be uniaxial. H.=2°5-3. G.=2-7-3°1. | 
Composition. Mostly (K:,Mg,Fe),-Al O,,Si,, 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 easily 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 20°. H.=2:5-3. 
G.=2-78-2°85. In erystals and scales in granular limestone From 
Gouverneur, Edwards, and other places in Northern New York ; Stir- 
ling Mine, and Newton, N. J.; St. Jerome, and Burgess, Canada ; in 
the Vosges. 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. 
