The deposits of iron ore which occur as rocks, interstratified or associated with sedimentary beds, have originated through complex processes, sometimes wholly, sometimes partly, of a purely chemical nature and usually more or less influenced by the agencies of organic life. The most important set of processes are based on the fact that iron exists in the original (the igneous) rocks in the form of silicates, such as biotite, olivine, pyroxene and hornblende, and also as oxides, such as magnetite, hematite and ilmenite, as disseminated grains. It also occurs in the secondary metamorphic rocks as silicates and oxides. It is also pretty generally diffused through the sedimentary rocks, in part as coloring matter and cement, and mostly in the form of ferric oxide, ferric hydroxides and ferrous carbonate. In the igneous rocks it is largely in the ferrous state and to a considerable degree also in the metamorphic ones. Also, to understand the concentration of iron and formation of iron ore rocks, it must be borne in mind that the metal forms only one carbonate, ferrous carbonate or siderite, FeCO3 which, like carbonate of lime, is soluble in water containing carbon dioxide.
When the rocks are decomposed and broken down by the agencies of weathering and erosion, the silicates containing iron are altered; the ferrous oxide in them combines in part with the carbon dioxide in the circulating ground water to form ferrous carbonate which goes into solution, and in part it is oxidized to ferric oxide. The original oxides of iron react in a similar manner. The ferric oxide thus formed or liberated would be insoluble, but in the presence of decaying vegetable matter in the soil and organic acids leached downward into the rocks, deoxidation of the ferric oxide ensues; it is reduced to ferrous oxide and then becomes ferrous carbonate and goes into solution. The reason for this is that decay of organic matter is a process of oxidation, like slow combustion; the organic matter takes oxygen from the air, but in the presence of moisture and ferric oxide it will take oxygen from the latter, reducing it to the ferrous oxide which is then fitted to unite with carbon dioxide and become the carbonate.
The iron of the rocks, which is thus brought into solution, is leached out, and in standing bodies of shallow water, such as swamps, lagoons or estuaries, with small outlets to the sea, it may be concentrated and give rise to extensive deposits. Under some conditions these deposits may be of the carbonate directly, but usually the solution of the carbonate is re-oxidized, carbon dioxide escapes, and the iron is precipitated as ferric hydroxide (limonite). This oxidation is largely, if not wholly, performed by certain bacterial organisms which demand iron in their internal economy, and therefore, secrete the iron from the water, and change it in their cells from the ferrous to the ferric condition, thus rendering it insoluble. Living and dying in unimaginable numbers, though excessively minute, they give rise to large deposits.
The ferric hydroxide which is thus precipitated may accumulate on the bottom as bog iron ore, or limonite, or, as is so often the case in shallow bodies of standing water, like swamps, etc., it may again come in contact with decaying vegetable matter, and be changed back into carbonate. Such beds of iron ore may be quite pure, or they may be more or less mingled with clay and sand, brought in at times of high water, and thus impure limonites, clay ironstones, black-band ore, etc., are formed. This also explains the not infrequent association of stratified iron ore and coal beds in the same series of rocks, and the reason why in this case the iron ore is commonly ferrous carbonate. The moving ground waters containing iron in solution, as described above, may also issue as springs and give rise to deposits of iron ore. Certain masses of iron ore, chiefly limonite, are supposed to be residual products of weathering and solution. This is illustrated in the view that masses of limestone containing ferrous carbonate have been dissolved and carried away, but the iron, oxidized to the ferric condition in the process, has become insoluble and remaining behind has gradually concentrated. The more important iron ore rocks may now be described.
Bog Iron Ore. Limonite. This is sometimes loose and earthy, sometimes firm and porous. It consists mainly of limonite, mixed more or less with humus, phosphates, silicates of iron, clay, sand, etc. Its character has been sufficiently described under limonite among the minerals. It sometimes occurs in concretions. With increasing amounts of clay it passes over into yellow ocher. It is found in all parts of the world. In the United States it is widely distributed, and along the Appalachian belt, from Vermont to Alabama, deposits of limonite, most of which are probably residual in character, have furnished iron ore since the early settlement of the country, and in great quantity.
Clay Ironstone. Siderite. When reasonably pure, siderite, or spathic iron ore, is a coarse to fine crystalline aggregate of siderite grains. It is whitish to yellow, or pale brown in color, but on exposed surfaces much darker brown to black, owing to oxidation of the ferrous carbonate to limonite, or of the manganese carbonate to manganese oxides. It generally contains, more or less, carbonates of lime, magnesia and manganese. Iron pyrites or hematite are commonly associated minerals. For the properties of siderite, reference may be had to its description among the minerals. An impure variety of siderite mixed with clay, sand and limonite in variable proportions, of a compact appearance, and generally of dull brown colors, is known as clay ironstone. It is apt to occur in nodules, often as concretions around some fossil, and lenticular masses which increase until they become interstratified beds of considerable thickness. Another variety which contains so much organic, coaly matter that it is colored black is known as black-band ore. It is especially associated in the strata with coal beds from the Carboniferous upward.
Carbonate ores of iron are of less importance in the United States than the deposits of limonite and hematite. They occur in Pennsylvania, Ohio and Kentucky, of Carboniferous age, and in the Lake Superior region in Michigan and Minnesota, of Algonkian age. They occur in Europe in England, Germany, France and Spain, in deposits of great technical value. Black-band is found in the coal bearing strata of Pennsylvania, England, etc.
Red Iron Ore. Hematite. This occurs in the form of veins, lenticular masses and beds, in various geological formations and especially in those whose strata have been folded. As a rock, it varies from fine grained and compact to earthy or fibrous, is of a red to brown color or, where crystalline, of a dark gray. Its properties as a mineral have been previously given. It occurs pure or nearly so, but with varying mixtures of clay, sand or silica, it passes insensibly into ferrugineous clays, red ochers, or shales, sandstones, cherts, etc. In this connection see jaspilite under flint. While hematite undoubtedly occurs as a normal sedimentary or stratified rock, inter-bedded with other unchanged strata, as in the beds which have such a wide distribution in the eastern United States in the Clinton group of the Niagara period, it is more generally to be considered a metamorphic rock, and as such, might be included among the metamorphic iron rocks described as itabirite and hematite schist. Extensive deposits of hematite are found in various parts of the United States and Canada. The greatest amounts mined as ore come from Tennessee and the Lake Superior Region, the vast production in the latter leading the world in output. Large beds are also found in England and other parts of Europe.
Iron Oolite. The iron rocks described above, and especially red hematite, not infrequently assume a concretionary form in which the rock is composed of rounded, sometimes polygonal, grains which vary in size from that of fine sand to peas. An examination of them shows that they have a concentric shelly structure. The color varies from red to brown. Sometimes the rock is composed of them alone and sometimes they are thickly embedded in a marl or clayey cement. The iron ore appears in many cases to have been deposited around grains of sand, fragments of fossils, etc., as nuclei. The Clinton ores mentioned above frequently assume this oolitic character and it is well known from various European localities. Such ores have sometimes been changed into magnetite while still retaining the oolitic structure.
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