374 MINERALOGY 



products of crystallization, but less often will it contain silicates. 

 It may be found as inclusions in most of the silicates and quartz. 

 Large deposits of magnetite have resulted from this magmatic 

 differentiation, as in the Kirunavaara district of Sweden, where 

 one ridge of magnetite has been estimated to contain 740 millions 

 of tons. The value of magnetite as an iron ore and in the produc- 

 tion of steel depends largely upon the impurities in the form of 

 sulphur and phosphorus which it may contain, even though these 

 are present in only a fraction of a per cent. The sulphur may be 

 oxidized in the Bessemer process and carried off in the slag. The 

 phosphorus is not so easily handled, and very little renders the ore 

 less fit for the production of steel by this process. When titanium 

 is present to the amount of several per cent, the blast furnace is in 

 danger of freezing, owing to the high fusing point of the slag. 



Magnetite deposits in the Eastern states extend from northern 

 New York through the intervening states to Alabama. Those 

 deposits connected with the basic igneous rocks are generally ti- 



FIG. 436. Magnetite twinned after the Spinel Law, from Brewster, New York. 



taniferous, while those connected with, the gneisses and limestones 

 are non-titaniferous and are therefore more valuable to the iron 

 industry. Magnetite in small amounts is very widely distributed 

 as a secondary product derived from the alteration and oxidation of 

 such minerals as pyrite, siderite, garnet, augite, olivine, amphibole, 

 or biotite. Owing to its high specific gravity and proneness to 

 decomposition or solution, it remains behind as a component of 

 the black sands arising from the weathering of metamorphic and 

 igneous rocks. 



Lodestone is a variety of magnetite which is a natural magnet, 

 showing polarity ; good specimens are found at Magnet Cove, 

 Arkansas. It is thought that the unknown people of the Southwest. 



