Carbonates

Carbonates consist essentially of ferrous carbonate, FeCO₃, the purest form of which is spathic iron ore, which occurs both in the crystalline and the massive form. It is mined in Russia, Poland, the Balkans, Styria (Austria), Westphalia, and other parts of Germany. Its phosphorus content is low, but a considerable quantity of manganese is present. It is usually of a light brown colour and is possessed of a pearly lustre.

Stvrian ore is obtained by quarrying, there being three kinds of veins, each several yards in thickness, the richest of which contains some 45 per cent, of metallic iron. In 1913 the production of ore amounted to 1,950,000 tons. It is estimated that the reserve of rich ore exceeds 200 million tons. It is practically free from sulphur, contains 0.01 per cent, of phosphorus and 2.23 per cent, of manganese.

When crystalline, spathic ore is known as siderite or chalybite. The crystals belong to the hexagonal system

a = 0.81715,

and Cornwall has yielded many fine specimens.

The theoretical percentage of metallic iron is 48.3. Hardness 3.5 to 4.5; density 3.7 to 3.9. It is brown or grey in colour and leaves a white streak. It readily oxidises when wet, being converted into limonite. Siderite from East Pool Mine, Cornwall, has been found containing cobalt, nickel, and even indium and rubidium in small quantity.

The spathic ores of the Brendon Hills, West Somerset, were at one time worked largely for the manufacture of Spiegeleisen, as they contained some 12 per cent, of manganese, probably as carbonate, since ferrous and manganese carbonates are isomorphous. The ore was worked in early times, perhaps by the Romans.

Staffordshire, West Yorkshire, and South Wales, yield an argillaceous iron ore, also known as clay iron stone, which contains some 10 per cent, of clay and from 30 to 40 per cent, of metallic iron. The Staffordshire ore has many local names; when found in concretionary and globular masses it is called sphaero-siderite. It contains about 0.25 per cent, of phosphorus. Cleveland iron stone is one of the lowest grades of ore worked for iron in this country, and contains about 33 per cent, of metallic iron. It occurs in bands in the Middle Lias, the most important band being nearly 20 feet thick. Its phosphorus content is high, averaging 0.75 per cent. The bluish-green colour of the ore is due to ferrous silicate. Traces of zinc, gallium, nickel, and cobalt have been detected in the ore.

Under the microscope the ore has an appearance suggestive of oolitic limestone, from which it has very probably been formed by molecular replacement of the calcium carbonate by ferrous carbonate, through the infiltration of waters containing the latter in solution.

Beneath the brown haematite ores of Northamptonshire, an impure unaltered ferrous carbonate deposit occurs which is bluish or greenish- grey in appearance. The depth at which it lies represents the depth to which weathering or oxidation of the upper layers has occurred.

A clay iron stone containing some 35 per cent, of iron has been worked from Roman times, if not earlier, in the Weald of Sussex and Kent, charcoal being used as the fuel.

In Linlithgow and Lanark a clay iron stone occurs, impregnated with some 15 per cent, of carbonaceous matter. It is also found in North Staffordshire and in South Wales, and is known as blackband iron stone. The carbonaceous material present is often sufficient to allow the ore to be calcined without the further addition of fuel. A product containing 50 to 70 per cent, of iron is yielded.

An ore, containing magnesium carbonate in the proportion represented by the formula 2FeCO₃.MgCO₃, is known as sidero-plesite, and has been found at Salzburg and in Nova Scotia. Pistomesite, FeCO₃.MgCO₃, is found at Salzburg and Piedmont; and mesitite, FeCO₃.2MgCO₃, at Piedmont. Ankerite is 2CaCO₃.MgCO₃.FeCO₃.