

SILICATI S. TITAN. \Ti;S, ETC. 475 



well illustrated by the double terminated crv-t.il- from Pierrepont, 

 New York, when- they occur in a lime-tone. They an- short, stout 

 prisms, combinations of a and m, and less commonly tin- di trigonal 

 prism h (4150) in the prism zone, terminated .-it one end by the 

 pyramids r and e, which are flat. At the other end they are termi- 

 nated by the much steeper pyramid o and a small basal plane; 

 the two ends are quite different in appearance. 



Owing to the great variation in composition possible, tourmaline 

 differs widely in color. The transparent pale pink, blue, green, 

 and colorless specimens are rich in alkalies. Their color is often 

 unevenly distributed, as different parts of the same specimen will 

 differ in color; this distribution of color may be from end to end, 

 one end being colorless, green, or pink, while the other may be 

 blue; or the distribution may be concentric around the vertical 

 axis, as in the Brazilian specimens, in many of which the central 

 axis is pink, then a colorless area, while the outside is green. Beau- 

 tifully colored and transparent tourmalines are obtained at 

 Haddam, Connecticut ; Paris, Maine ; Mesa Grande, California ; 

 Madagascar; and Brazil. When transparent and flawless, they 

 are cut and polished as gems. The pink varieties are known as 

 rubellite, the green as Brazilian emeralds, the yellow as Ceylon 

 peridote, the blue as indicolite, and the white as achroite. 



The black varieties are rich in iron ; that from Pierrepont con- 

 tains 9.08 per cent, of FeO. The brown varieties are high in mag- 

 nesium ; that of Gouverneur, New York, contains 14.9 per cent, of 

 MgO. 



Tourmaline is very strongly pleochroic ; even the gems cut from 

 the transparent specimens are often of different color, according to 

 the direction in which the light passes through the crystal. This 

 absorption is more strongly marked in the darker varieties and is 

 developed to such an extent in the dark brown specimens that in a 

 section parallel to the vertical axis the ordinary ray is entirely 

 absorbed. Such sections are used, as in the tourmaline tongs, to 

 replace the nicols in viewing interference figures of mineral sections 

 placed between them. 



Tourmaline is also the best example of the pyroelectric property 

 in minerals. When a crystal of tourmaline is cooling after being 

 heated, one end, the analogous end (the sharp end), is usually nega- 

 tively charged, while the antilogous end (the blunt end) is posit ively 

 charged. The reverse of this is true when the temperature is 

 rising. If a crystal of tourmaline after being heated is dusted 



