IRON ORES. Although iron is the most abundant of the useful metals, forming 5 per cent of the earth's crust, it is rarely found na tive — one famous native occurrence of it, how ever, being at Ovifak on the west coast of Greenland. The iron ores of chief commercial importance are hematite, magnetite, limonite, siderite and pyrite. Pure hematite contains 70 per cent iron; magnetite 72.4 per cent; limonite 59.89 per cent; siderite 48.27 per cent; pyrite 46.6 per cent. Iron ores, however, are prac tically never mined pure, but as mined in quan tity, averages over 10 per cent less iron than above stated, the principal impurities being silica, alumina and lime.
Hematite, ferric oxide (Fe202), by far the most important iron ore, varies greatly in phys ical characteristics. Specular hematite is black, with a brilliant metallic lustre. Hematite yields 90 per cent of our supply of iron, in the United States. Martite is a variety of hematite. Mag netite, a ferro-ferric oxide (FeO.Fe202), is black, magnetic, and crystallizes in octahedra, but as mined it is usually massive or granular. Ilmenite, an oxide of iron and titanium, is not yet an ore of commercial importance. Frank linite, an oxide of iron, manganese, and zinc, found at Franklin N. J., is used in making spiegeleisen, an alloy of iron and manganese after the zinc has been removed by roasting. Limonite or brown hematie, hydrated ferric oxide (2Fei02.31-1,0), is brown or yellow in color and occurs in massive, earthy or in botryoidal forms. Bog-ore is a variety of limonite. Goethite, differing from limonite in crystalline form and containing less water, is found in large quantities in Minnesota. Siderite, or spathic iron, ferrous carbonate (FeCO,), is white to gray when pure, and crystallizes in rhombohedra. As mined, it varies much in ap pearance, owing to oxidation. Clay iron stone is siderite mingled with clay. Blackband ore is siderite mixed with more or less bituminous matter. Pyrites, ferric disulphide (FeS2), often called "fool's gold,* is used in great quantities to make sulphuric acid. The residue, known as or pyrites clinker, is in some countries smelted in the blast-furnace as an iron ore.
Origin of Iron Many of the large bodies of magnetite in the Adirondacks and in Sweden are believed to be magmatic segre gations (q.v.), having solidified out as great masses of igneous rock cooled. Most of the hematite ores are believed to have been laid down as beds of sedimentary rock. The iron ores of the Clinton (q.v.) age extending in patches from New York to the Birmingham district in Alabama, are believed to have been laid down as beds of hematite in the ocean.
The vast deposits of the Lake Superior region and the still larger deposits of Brazil are thought by many to have been laid down in the ocean as very thick beds of iron carbonate and iron silicate. As the region was uplifted and eroded they weathered to hematite. Limonites in many places seem to have resulted from the weathering of rocks in which there was but little iron. During the weathering process the insoluble iron remained behind, be coming richer by the removal of soluble impuri ties. This also is believed to be the origin of the great Cuban deposits of hematite. In some places limonite is also accumulating in swamps, where it is called iron (q.v.). Similar deposits are now in process of forma tion in the Three Rivers District of Quebec. In smelting an ore the silica, lime and alu mina are removed as slag. Ores high in silica require more limestone in the furnace-charge for fluxing, that is, for combining with the sil ica. Some ores contain silica -..nd lime in such proportions as to be self-fluxing. The higher the iron content of an ore, generally speaking, the greater the yield per ton of material put through the furnace, and the lower the cost per ton of the iron made.
A very hard ore must be broken into small lumps to give best results in the furnace. Hematites often smelt easier than magnetites. A fine granular ore makes trouble in smelting, and a certain proportion may be lost as dust, which clogs furnace-flues. The really injurious impurities most often found in iron ores are sulphur, phosphorus and titanium. Sulphur can he largely removed by roasting the ore be fore smelting; phosphorus cannot, and all the phosphorus in the ore goes into the iron. For making steel by the Bessemer process an ore should contain less than 1-1,000 of its amount of iron ; thus, to be classified as Bessemer, an ore containing 61 per cent iron should contain less than .061 per cent phosphorus. For mak ing steel by the basic process, high phosphorus ores are used The present development of the world's iron mines is the outcome of many factois. Gen erally speaking, it is cheaper to bring the ore to the fuel than the fuel to the ore, hence, coun tries or districts that have great supplies of iron ore may be insignificant producers of pig iron. Low-priced fuel has been the chief factor in determining the location of iron and steel industries, while the chief factors in the development of iron mines are the quality of the ore and the cost at which it can be put down at the furnace.