BAUXITE 269 



occurs in the mineral either free or in com))ination in clay. 

 Minor impurities are compounds of phosphoric acid, sulplnir, 

 carbon dioxide, lime, and magnesia. In composition the 

 French mineral appears to correspond to the monoh^'drate 

 whihi tliat of the United States approximates the trihydratc. 

 The composition of bauxite can be determined only by chemi- 

 cal analysis since it is not indicated by the physical properties 

 of the mineral. According to Francis Laur, the average com- 

 position is from 06 to 09 per cent of AloOa) 27 percent of 11^ 

 O, SiOo, and F2O3; with from three to four per cent of TiOa 

 and other impurities. In general the value of the ore is 

 in direct proportion to the richness and purity of the 

 alumina content. In ore of good quality the percentage of 

 alumina is high, while that of iron oxide and silica is low. 

 Bauxite has a strong affinity for water, which makes it neces- 

 sary^ that the ore be dried before shipment. For the manu- 

 facture of aluminum the presence of iron oxide and titanic 

 oxide in the ore is not objectionable, but for making alum 

 these insoluble materials should not exceed 7 per cent, and 

 the iron oxide should be less than 2.75 per cent. The trihy- 

 dratc, on account of its greater solubility, is best suited for 

 the manufacture of alum, and for this purpose the bauxite 

 of the United States is preferable to that of France. 



Bauxite occurs in the earth's crust in the form of veins, 

 beds, or amorphous masses, not crv'stallized, and without 

 any constant organoleptic characteristics. The hardness, 

 color, texture, and density often change in the same deposit. 

 Usually the ore occurs in concretionary or pisolitic masses, 

 although it is sometimes a hard, compact, homogeneous fine- 

 grained rock. In some cases the structure is oolitic, and in 

 others it is earthy, resembling clay. It may be hard or soft 

 and compact or porous. The color varies from pure white 

 to a deep red or black, passing through shades of cream, gray, 

 yellow, and pink. The mineral is sometimes speckled or 

 mottled, and is more or less stained b}^ iron oxide, manganese 

 minerals, or organic matter. The colors shade into one 

 another gradually or abruptly, and seldom, if ever, does a 

 deposit possess a uniform color. The hardness of average 



