282 DESCRIPTIONS OF MINERALS. 
of Humite from a and fine chondrodite-like crystals 
from Brewster's 
Tourmaline. 
Rhombohedral. Usual in prisms of 3, 6, 9, or 12 sides, 
terminating in a low 3-sided pyramid. The sides of the 
prisms are often rounded and striated. Other forms are 
shown in the figures. The common pyramid is the rhombo- 

hedron —4 in the figures, the angle of which is 133° 8’. The 
crystals often have unlike secondary planes at the two ex- 
tremities, as shown in figure 3. Occurs also compact mas- 
sive, and coarse columnar, the columns sometimes radiating 
or divergent from a centre. 
Color black, blue-black, and dark brown, common ; also 
bright and pale red, grass-green, cinnamon-brown, yellow, 
gray, aud white. Sometimes red within and green externally, 
or one color at one extremity and another at the other. 
Transparent ; usually translucent to nearly opaque. Di- 
chroic. Lustre vitreous, inclining to resinous on a surface 
of fracture. Streak uncolored. Brittle; the crystals often 
fractured across and breaking very easily. H.=7:0-7:5. G. 
=3-3 3. 
Composition. (R,,B,,R,)O;, Si, 11 which R ineludes, in 
different varieties, Fe, Mg, Na, with often traces also of Ca, 
Mn, K,, Li,; 8 includes aluminum, with some boron in the 
trioxide state replacing Al; anda little of the oxygen is 
sometimes replaced by fluorine. 
A black tourmaline from Haddam afforded on analysis, 
Silica 37°50, boron trioxide (by loss) 9:02, alumina 30°87, 
iron protoxide 8°54, magnesia 8°60, lime 1°33, soda 1:60, 
potash 0°73, water 1 81=100. A red ‘tourmaline from Paris, 
Maine, afforded Fluorine 1:19, silica 41°16, boron trioxide 
(by loss) 8-93, alumina 41 83, manganese "protoxide 0°95, 
magnesia 0°61, soda 1°37, potash 2°17, lithia 0-41, water 2 5Y 
—100. 
Tourmaline varies much in color owing to its variations in 
Soe nee St he tes tee 
