ALUMINA. 367 



indicates the negative or resinous. It is said that the finest transparent crystals, especially 

 when cut by the lapidary, are constantly electric, without artificial heat. 



Its property of becoming electric by heat, and its vitreous conchoidal fracture, will distin- 

 guish tourmaline from several varieties of hornblende, some varieties of which it often resem- 

 bles. Allanite, which it also resembles, forms a jelly with muriatic acid. 



Varieties. Schorl. This term is often applied to the black opaque specimens with an 

 uneven fracture. 



Indicolite. This has the crystalline forms of the other varieties, but has an indigo-blue 

 colour. 



Rubellite. This includes those crystals which are of various shades of red ; and sometimes 

 they are violet. The colour is owing to the presence of a minute portion of oxide of man- 

 ganese. 



Composition. The analysis of this mineral is exceedingly diificult. 



Black Tourmaline — Silica 34.48, alumina 34.75, soda 1.75, potash 0.48, oxide of iron 

 17.44, oxide of manganese 1.89, boracic acid 4.02, magnesia 4.68 {Gmelin). 



Blue Tourmaline — Silica 40 . 30, alumina 40 . 50, oxide of iron 4 . 85, oxide of manganese 

 1.50, boracic acid 1.10, lithia4.39 (Arfwedson). 



Green Tourmaline — Silica 39.16, alumina 40.00, oxide of iron 5.96, oxide of manganese 

 2.14, boracic acid 4.59, lithia3.59 (Gmelin). 



Red Tourmaline — Silica 39.37, alumina 44.00, potash 1.29, oxide of manganese 5 . 02, 

 boracic acid 4.18, lithia 2.62 {Gmelin). 



Geological Situation. Tourmaline is almost always found in primary rocks, as coarse 

 granite, gneiss, mica slate, chlorite slate, dolomitic and common white limestone. 



localities. 

 Dutchess County. Tourmaline is said to have been found at Rhinebeck in this county.* 

 Fig. 357. Essex County. The brown variety is found in white limestone, in the imme- 



diate vicinity of Port Henry. It is usually in six-sided prisms, with the alter- 

 nate planes very narrow. Fig. 357. Z on Z 120°. The black variety occurs 

 imperfectly crystallized in quartz, in Ticonderoga. The same variety is asso- 

 ciated with fibrous phosphate of lime, near Hammondsport in Crown-Point. 



Jefferson County. On the banks of Vrooman lake, near Oxbow, tourma- 

 line, in imperfect crystals, is found associated with mica and pyroxene. It is 

 not abundant. 

 On High island, in the St. Lawrence, opposite to Alexandria, there occur 

 long and large prisms with many sides, irregularly grouped, but the crystals are seldom suf- 

 ficiently pcrfecft for measurement. 



* Schseffer. CUavetancC$ Mineralogy. 



