194 BULLETIN 12, UNITED STATES NATIONAL MUSEUM. 



Bauxite is au abundant source of aluminium. Three characteristic 

 samples from a deposit in Floyd County, Georgia, are shown : 



(1) Oolitic and porous, slightly stained by iron. (66576.) 



(2) Oolitic, stained by iron. (6G577.) 



(3) Oolitic, hard and compact, very red from iron. (66578.) 



An analysis by Prof. J. W. Langley shows it to contain : 



Per cent. 



Silica (SiOi) 16.01 



Titanic acid (TiOa) 6.33 



Water (HjO) 25.27 



Sesqnioxide of iron (FejOs) 0. 70 



Alumina (AliO;i) "51.69 



100. 00 

 Aluminium (Al) 27.49 



CRYOLITE. 



Composition : Al2lSra6F,2. Fluoride of aluminium and sodium, contain- 

 ing aluminium 13 percent., sodium 32.8 per cent., and fluorine 54.2 

 per cent. 



It crystallizes in the triclinic system in tabular crystals, generally 

 occurs massive, showing cleavage. Its hardness is 2.5, and specific 

 gravity varies from 2.9 to 3.077. It is generally white, although it may 

 be tinged or black, and translucent. 



Cryolite is an abundant source of ahimiuiuni since it used for the man- 

 ufacture of carbonate of soda, and alumina is obtained as a by-product. 

 It is found in large amounts only in Greenland, and is illustrated by a 

 single characteristic specimen. 



(1) Massive, containing siderite, galena, and chalcopyrite. (17571.) 



CLAY. 



Under this head are included several hydrated silicates of aluminium 

 of very varying composition, which will be more fully described fur- 

 ther on. 



Aluminium is a constituent of many other minerals, but they are not 

 used for the manufacture of the metal. 



Extraction of Aluminium. 



Although aluminium is a constituent of so many common minerals, 

 its extraction is very difficult and expensive, owing to the tenacity with 

 which it holds on to oxygen. The first step is the j)reparation of the 

 anhydrous sesqnioxide, alumina (AI2O3), in as pure a condition as pos- 

 sible. For this purpose the aluminium minerals are treated chemically 

 by long and complicated i>rocesses. Having a pure alumina, it is first 

 converted into anhydrous chloride, and the metal is then reduced from 

 this by means of metallic sodium. Formerly this was the only method 



* By diflerence. 



