43. Ores of Iron.—Even though the earth seems to be a huge iron meteor with but a thin covering of rocks, the exasperating proneness of iron to oxidize explains readily why this metal is only rarely found native, except in the form of meteorites. They are four important iron ores, magnetite, haematite, limonite and siderite, and one of less but still considerable importance, pyrite or pyrites.
44. Magnetite, Fe3O4, contains 72.41% of iron. It crystallizes in the cubical system, often in beautiful octahedra and rhombic dodecahedra. It is black with a black streak. Its specific gravity is 5.2, and its hardness 5.5 to 6.5. It is very magnetic, and sometimes polar.
45. Haematite, or red haematite, Fe2O3, contains 70% of iron. It crystallizes in the rhombohedral system. Its colour varies from brilliant bluish-grey to deep red. Its streak is always red. Its specific gravity is 5.3 and its hardness 5.5 to 6.5.
46. Limonite, 2Fe2O3, 3H2O, contains 59.9% of iron. Its colour varies from light brown to black. Its streak is yellowish-black, its specific gravity 3.6 to 4.0, and its hardness 5 to 5.5. Limonite and the related minerals, turgite, 2Fe2O3 + H2O, and göthite, Fe2O3 + H2O, are grouped together under the term “brown haematite.”
47. Siderite, or spathic iron ore, FeCO3, crystallizes in the rhombohedral system and contains 48.28% of iron. Its colour varies from yellowish-brown to grey. Its specific gravity is 3.7 to 3.9, and its hardness 3.5 to 4.5. The clayey siderite of the British coal measures is called “clay band,” and that containing bituminous matter is called “black band.”
48. Pyrite, FeS2, contains 46.7% of iron. It crystallizes in the cubic system, usually in cubes, pentagonal dodecahedra or octahedra, often of great beauty and perfection. It is golden-yellow, with a greenish or brownish-black streak. Its specific gravity is 4.83 to 5.2, its hardness 6 to 6.5. Though it contains far too much sulphur to be used in iron manufacture without first being desulphurized, yet great quantities of slightly cupriferous pyrite, after yielding nearly all their sulphur in the manufacture of sulphuric acid, and most of the remainder in the wet extraction of their copper, are then used under the name of “blue billy” or “purple ore,” as an ore of iron, a use which is likely to increase greatly in importance with the gradual exhaustion of the richest deposits of the oxidized ores.
49. The Ores actually Impure.—As these five minerals actually exist in the earth’s crust they are usually more or less impure chemically, and they are almost always mechanically mixed with barren mineral matter, such as quartz, limestone and clay, collectively called “the gangue.” In some cases the iron-bearing mineral, such as magnetite or haematite, can be separated from the gangue after crashing, either mechanically or magnetically, so that the part thus enriched or “concentrated” alone need be smelted.
50. Geological Age.—The Archaean crystalline rocks abound in deposits of magnetite and red haematite, many of them very large and rich. These of course are the oldest of our ores, and from deposits of like age, especially those of the more readily decomposed silicates, has come the iron which now exists in the siderites and red and brown haematites of the later geological formations.
51. The World’s Supply of Iron Ore.—The iron ores of the earth’s crust will probably suffice to supply our needs for a very long period, perhaps indeed for many thousand years. It is true that an official statement, which is here reproduced, given in 1905 by Professor Tornebohm to the Swedish parliament, credited the world with only 10,000,000,000 tons of ore, and that, if the consumption of iron should continue to increase hereafter as it did between 1893 and 1906, this quantity would last only until 1946. How then can it be that there is a supply for thousands of years? The two assertions are not to be reconciled by pointing out that Professor Tornebohm underestimated, for instance crediting the United States with only 1.1 billion tons, whereas the United States Geological Survey’s expert credits that country with from ten to twenty times this quantity; nor by pointing out that only certain parts of Europe and a relatively small part of North America have thus far been carefully explored for iron ore, and that the rest of these two continents and South America, Asia and Africa may reasonably be expected to yield very great stores of iron, and that pyrite, one of the richest and most abundant of ores, has not been included. Important as these considerations are, they are much less important than the fact that a very large proportion of the rocks of the earth’s crust contain more or less iron, and therefore are potential iron ores.
Table II.—Professor Tornebohm’s Estimate of the World’s Ore Supply.