Page:The American Cyclopædia (1879) Volume IX.djvu/423

Rh Spathic Ore, or Siderite. This ore is never found as pure ferrous carbonate, part of the iron being invariably replaced by manganese, lime, or magnesia. The percentage of iron given in the table above is therefore theoretical, and is never perfectly attained. The ore is found crystallized, massive, and concretionary; in the latter form it is called sphærosiderite. It is for many purposes the most valuable ore of iron, owing to its general freedom from injurious ingredients, its easy reducibility, and the presence of manganese (from 1 to 10 per cent. of oxide, exceptionally as high as 25 per cent.), which always enhances its value. It is not very extensively distributed in nature, but a few localities contain it in large deposits. It is almost the only material used in the preparation of spiegeleisen. (See .) Ferrous carbonate also forms the basis of the carboniferous blackband ores, and of most of the clay ironstones, which are very extensively distributed. The ferrous carbonate is in these ores intimately associated with argillaceous, silicious, and often carbonaceous matter. It frequently contains also sulphur as iron pyrites, and phosphorus as calcic phosphate. These ores are therefore much less pure than the spathic ore properly so called, and yield irons of a much inferior character. The carbonated ores, when heated, lose their carbonic acid, and their ferrous oxide is converted into magnetic oxide. They are always calcined before smelting. The carboniferous blackbands contain usually from 15 to 20 per cent. of carbon, and may be roasted without the addition of fuel. On roasting they lose half their weight. Spathic ore becomes brown or brownish black on exposure, owing to a peroxidation of the iron and its passing into limonite; and by a subsequent loss of water it may pass into red hematite. The occurrence of spathic ore is limited principally to the crystalline slates and the older sedimentary rocks, the most extensive and characteristic deposits being in the Devonian formation. The most noted localities are Siegen, Rhenish Prussia; Kamsdorf and Stahlberg in Thuringia; Osnabrück in Westphalia; the Erzberg near Eisenerz in Styria, in the Devonian; and the Erzberg near Hüttenberg in Carinthia. England has also deposits in the Brendon hills in Somersetshire, and at Exmoor, South Moulton, and Walscott in Devonshire; also at Weardale, Durham. A remarkable series of deposits of impure or earthy carbonate is found in the different members of the lias formation in the Cleveland hills, North riding of Yorkshire, England. The main deposit is in the middle lias, showing a workable seam from 8 to 13 ft. thick; it is believed to extend throughout the whole of Cleveland proper. In this region of England the manufacture of iron has reached a higher stage of development than in any other part of the world. The principal deposit of siderite in the United States is at Roxbury, Conn., in a vein of quartz, traversing gneiss.

The clay ironstones are met with in both the carboniferous and tertiary (brown coal) formations. In England, Scotland, Westphalia, and other regions, the blackband ore (carbonaceous carbonate) forms the basis of an extensive industry. This ore, as found in Westphalia, contains an extraordinary amount of phosphoric acid, in some layers as much as 30 to 60 per cent., and in others 20 to 25 per cent. The blackband ores are of subordinate importance in the United States, though they have been found in the coal regions of western Pennsylvania. Earthy carbonates occur extensively in Pennsylvania, West Virginia, Ohio, &c.—4. Magnetite. Magnetic iron ore occurs generally in large masses, but with distinctly crystalline structure. It also occurs in the form of sand, concentrated by fluviatile or tidal action from the debris of rocks containing it. It is readily recognized by its black color and streak, and by its being attracted by the magnet. It derives its name from the Thessalian district of Magnesia, bordering on Macedonia, or, according to Pliny, from Magnes, who first discovered it. There is a magnesian variety in which part of the ferrous oxide is replaced by magnesia, and a titaniferous variety in which a part of the iron is replaced by titanium. This variety bears the same relation to magnetite as iserine to hematite. The amount of titanic acid varies through wide limits. Magnetic ore is often found in a state of almost absolute purity; more generally it is associated with apatite (calcic phosphate), iron pyrites and other sulphides, quartz, and earthy ingredients. It supplies a large amount of the finest iron and steel of commerce. The iron industry of Sweden is based almostly entirely on magnetic ores. “Magnetite is mostly confined to crystalline rocks, and is most abundant in metamorphic rocks, though found also in grains in eruptive rocks. In the azoic system, the beds are of immense extent, and occur under the same conditions as those of hematite. It is an ingredient of most of the massive variety of corundum called emery. By deoxidation through organic matter it is changed to protoxide, which may become a carbonate; by oxidation it becomes hematite.” (Dana.) The principal European occurrences of magnetic ore are at Arendal in Norway; Dalarne, Westmanland, Wermland, Dannemora, Utō, and Smaland in Sweden; in Finland, and in the Ural. In the United States there are are vast beds in the azoic of the Adirondack region, Warren, Essex, and Clinton counties, in northern New York; also in northern New Jersey, in Morris, Sussex, Warren, and Passaic counties, where it is found in beds conformable with the azoic gneiss, and also intimately disseminated in the gneissic strata. In eastern Pennsylvania there are several localities, the most noted being Cornwall in Lebanon co. In Canada it is found at Hull, Grenville, Madoc, &c. In North Carolina, at Greensboro, there is an extensive titaniferous belt of magnetites. Large deposits are known in Sierra co.,