Iron. This well-known metal fills a very important place in our mineral arrangements, for the substances formed with iron ores occur in great variety of structure, and occasionally in very large masses. They are highly magnetic, and very hard. Were we here treating of iron as a metal, we could give some information respecting its extraction and manifold uses. All we need mention here is the fact that iron occurs in nature in various ores which are essentially composed of iron and oxygen. The iron is extracted in the blast furnace, in which the process is continued for years. The “slag,” or glassy scum, protects the molten iron from the air; its presence is necessary in all blast furnaces. The most important of the iron group of minerals are Magnetic Iron (magnetite), or loadstone. This mineral occurs in Sweden and North America, and is found in primary rocks, and in Scandinavia forms mountains. It crystallizes in the regular (octahedron) system, and often in the form in illustration in the margin. It is highly magnetic, as its name implies.
Fig. 472.—Magnetic iron.
Native iron very rarely occurs, and then only in thin layers. The most extraordinary specimens are those termed meteoric iron, which fall from the atmosphere in great masses; and the meteoric stones, which contain ninety per cent of iron, together with other constituents in small quantities—viz., nickel, cobalt, copper, manganese, carbon, sulphur, arsenic, etc.
Red hematite crystallizes in the hexagonal system. It possesses much the same (chemical) constitution as corundum (q. v.). It is brightly metallic, and shows a red streak. It occurs in various forms, as iron glance or specular iron, which is found in Sweden and Russia; micaceous iron, bloodstone, clay, ironstone, etc.
Brown hematite has not been found in crystals, but brown ironstone (fibrous) is crystalline. The earthy brown, containing clay, gives us yellow ochre and umber. Pea-iron ore and “morass” or “bog” ore also belong to this class. Limonite is the name given to these more recent formations, of which yellow ochre is a pure specimen.
Fig. 473.—Native oxide of iron.
The combinations of iron with sulphur (pyrites) are also important. Iron pyrites and magnetic iron pyrites are two which may be mentioned. The latter first.
Magnetic iron pyrites (or pyrrhotin) crystallizes in six-sided prisms, and is attracted by the magnet. The composition of this mineral has not been exactly ascertained, and no chemical formula has been found for it.