218 ECONOMIC GEOLOGY 



one-third of the world's yield of each of these commodities. The 

 value of pig iron produced in the United States for the banner 

 year of 1907 was $529,958,000. 



Aluminum : Its Properties, Occurrence and Uses 



Properties. Aluminum, symbol Al, is a tin-white, lustrous 

 metal. It is ductile, sectile and malleable. It is a good conduc- 

 tor of both heat and electricity. It is permanent in either dry 

 or moist atmosphere. Aluminum is scarcely attacked by nitric 

 acid, but is readily soluble in hydrochloric acid. It is extremely 

 sonorous, for when struck it emits a clear and sustained note. 

 It has a tensile strength of about 35,000 Ib. per square inch. 

 It is the lightest of all the useful metals. Its specific gravity is 

 2.58, melting point, 657.3 C., and its atomic weight is 27.1. 



Ores of Aluminum. Aluminum is the most widely distributed 

 of all the metals, but it does not occur free and uncombined in 

 nature on account of its remarkable affinity for oxygen. All of 

 the aluminum minerals save the double fluoride, cryolite, are 

 oxygenated compounds. It does not occur in nature alloyed with 

 other metals. It is unlike all the other metals considered in this 

 volume save gold in that its sulphide does not exist in nature. 

 The most important source of many of the useful metals is the 

 sulphide, but aluminum finds its most important ores like iron 

 in the oxides and hydrous oxides. 



Corundum, A1 2 O 3 , 52.9 per cent. Al. Distinguished from all 

 other minerals, save the diamond, by its superior hardness. 



Ruby, A1 2 O 3 , 59.2 per cent. Al. The red variety of corundum 

 used as a gem. 



Sapphire, Al 2 Os, 59.2 per cent. Al. The blue variety of corun- 

 dum used as a gem. 



Emery, xA! 2 O 3 , yFe 2 O 3 . The oxide of aluminum sometimes 

 contains as high as 35 per cent, Fe. Magnetite may replace the 

 hematite. 



Diaspore, A1 2 O 3 , H 2 O, 85 per cent. A1 2 O 3 . 



Bauxite, A1 2 O 3 , 2H 2 O, 73.9 per cent. A1 2 O 3 . 



Gibbsite, A1 2 O 3 , 3H 2 O, 65.4 per cent. A1 2 O 3 . 



Cryolite, A1F 3 , 3NaF, 12.8 per cent. Al. 



Turquoise, A1 2 O 3 , P 2 O 6 , 5H 2 O. The mineral is used as a gem. 



Alunite, K 2 O, 3A1 2 3 , 4SO 3 , 6H 2 0. 



Alunogen, A1 2 (SO4) 3 , 18H 2 O. 



