188 ALUMINA. 



Occurs also compact massive, and coarse columnar, the col* 

 urans sometimes radiating or divergent from a center. 



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 external- 

 ly, or one color at one extremity and another at the other. 

 Transparent ; usually translucent to nearly opaque. Luster 

 vitreous, inclining to resinous on a surface of fracture. 

 Streak uncolored. Brittle ; the crystals often fractured 

 across and breaking very easily. H=7 # 8. Gr = 3— > 3*1. 

 Electrically polar when heated, (page 62.) 



Varieties and Composition. Tourmalines of different col- 

 ors have been designated by different names, as follows : — 



Rubellite is red tourmaline. 



Indicolite is blue and bluish-black tourmaline. 



Schorl, formerly included the common black tourmaline, 

 but the name is not now used. 



A black variety afforded, on analysis, silica 33*0, alumina 

 38-2, lime 0*8, protoxyd of iron, 23*8, soda 3*2, boracic acid 

 1-9. 



A red variety from Siberia, silica 39*4, alumina 44*0,. pot- 

 ash 1*3, boracic acid 4*2, lithia 2*5, peroxyd of manganese 

 5*0. The presence of boracic acid is the most remarkable 

 point in the constitution of this mineral. It is also observed 

 that lithia is sometimes present ; over 4 per cent, have been 

 obtained from a green tourmaline from Uton, Sweden. 



Before the blowpipe the dark varieties intumesce, and fuse 

 with difficulty ; the red and light-green only become milk- 

 white and a little slaggy on the surface. 



Dif. The black and the dark varieties generally, are 

 readily distinguished by the form and luster and absence of 

 distinct cleavage, together with their difficult fusibility. The 

 black when fractured often appear a little like a black resin. 

 The brown variety resembles zoisite, though very dis- 

 tinct in crystallization. The light brown looks like garnet 

 or idocrase, but is more infusible. The red, green, and yel- 

 low varieties are distinguished from any species they resem- 

 ble, by the crystalline form, the prism of tourmaline always 

 having 3, 6, 9, or 12 prismatic sides, (or some multiple of 



What is the color and hardness of tourmaline ? what has been called 

 schorl? What is rubellite? What are the distinctive characters of 

 tourmaline ? 



