282 



DESCRIPTIONS OF MINERALS. 



of Humite from Vesuvius, and fine chondrodite-like crystals 

 from Brewster's. 



Tourmaline. 



Rhonibohedral. Usual in prisms of 3, 6, 9, or 12 sides, 

 low 3-sided pyramid. The sides of the 

 rounded and striated. Other forms are 



terminating in a 

 prisms are often 

 shown in the figures. 



1. 



The common pyramid is the rhombo- 

 2. 3. 4. 



hedron — J 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, and 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 verv easilv. H. = 7 *0-7 -5. G. 

 = 3-3 3. 



Composition. (R 3 ,B 2 ,Rr,) 5 , Si 2 , in which E includes, in 

 different varieties, Fe, Mg, Is a,, with often traces also of Ca, 

 Mn, K. : , Li..; R includes aluminum, with some boron in the 

 trioxide state replacing Al ; and a little of the oxygen is 

 sometimes replaced by fluorine. 



A black tourmaline from Haddam afforded on analysis, 

 Silica 3T -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 217, lithia 0*41, water 2*57 

 = 100. 



Tourmaline varies much in color owing to its variations in 



