Magnetites

Magnetite or Lodestone, Fe₃O₄ or Fe₂O₃.FeO, is the richest form of iron ore, containing when pure 72.4 per cent, of metal. Magnetite crystals are usually octahedra and dodecahedra, and are well developed in some chlorite schists. Good crystals have been found at Nornmark in Wermland. They are not scratched by a knife, their hardness on Mohs' scale ranging frem 5.5 to 6.5, and their density from 4.9 to 5.2. Magnetites in the massive form are widely distributed, occurring in large quantities in India, Sweden, South Africa, Russia, Siberia, Canada, and the United States. The Indian deposits are located in the Madras Presidency, and are estimated at milliards of tons, but, owing to the scarcity of fuel, they have not been worked on a large scale. Of even greater magnitude are the American deposits in the neighbourhood of Lake Superior, which form the largest source of iron ore in the world. The metal in these ores ranges from 55 to 65 per cent.

Some of the Swedish magnetites are very pure, and are used in the manufacture of Swedish iron, which is noted for its high quality. As the name implies, magnetite possesses magnetic properties, sometimes exhibiting polarity. Specimens possessing the most powerful magnetism are found in Siberia and the Harz Mountains. It is on account of this property that the ores can be concentrated magnetically on a commercial scale. The colour varies from brownish grey to iron black. When compact, the mineral is opaque, but translucent dendrites have been found in mica, exceedingly thin, and ranging in colour from almost colourless to pale smoky brown. The mineral has a subconchoidal fracture which is bright when fresh. It is fusible only with difficulty; soluble in hydrochloric acid.

According to Nicander, the name magnetite is derived from Magnes, the shepherd, who discovered it in consequence of his shoe-nails being drawn from their soles as he drove his flocks to pasture. More probably the name is derived from Magnesia, a town in Asia Minor, where magnetite was discovered.

A pseudomorph of magnetite has been described under the name of dimagnetite, which crystallises in elongated prisms, black in colour and possessed of feeble lustre. It occurs implanted on crystals of magnetite, is magnetic, and probably has an analogous composition to magnetite. Several varieties of magnetite are known in which the ferrous or ferric, or even both, irons are more or less replaced by other metals. The more important of these are: -

Chromite, chromo-ferrite, sidero-chrome, or chrome iron ore, FeO.Cr₂O₃, in which the ferric oxide of magnetite is replaced by chromium sesqui-oxide. This ore constitutes the main source of chromium compounds, and, on account of its infusibility, is sometimes utilised in furnaces for linings. Chromite is widely distributed, being found in Sweden, Germany, Northern Caucasus, India, Canada, the United States, Rhodesia, New Caledonia, Turkey and Greece. Rhodesia yields more chrome iron ore than any other country, the production in 1912 amounting to 61,857 tons. Chromite occurs in small quantities in most meteorites. The ore resembles magnetite in appearance, having a black colour and a similar crystalline form. It sometimes exhibits magnetic properties; hardness 5.5; density 4.3 to 4.6. It leaves a brown streak.

Acids do not attack chromite, but fusion with alkali hydrogen sulphates effects its decomposition. The Grecian ore, which occurs mainly in the Eastern Provinces and in the island of Skyros, is particularly refractory on account of the impurities it contains. Chrome iron ores containing some Fe₂O₃ may be regarded as mixtures of chromite and chromitite Platiniferous chromites are found in the Urals.

Franklinite, (Mn, Zn)O.Fe₂O₃, is a magnetite in which the ferrous oxide is more or less replaced by zinc oxide and manganous oxide, which latter imparts a red colour to the ore. It is found in New Jersey, U.S.A., in large black octahedra, and in recognition of its zinc content is frequently called zincite. It is slightly magnetic. Hardness 5.5 to 6.5. Density 5.07 to 5.2. It is infusible in the blowpipe; soluble in concentrated hydrochloric acid.

Ilmenite, titanic iron ore or ferrous titanite, FeO.TiO₂, is the ore in which titanium was first discovered. It takes its name from the Ilmen Mountains. In the massive form it occurs in Norway, whilst in Canada, the United States, India, and New Zealand, it is met with as sand. Well-formed crystals are rare, but differ from those of the preceding minerals in that they belong to the hexagonal system, and not to the cubic. They have almost the same angles (viz. 94° 29') as those of haematite (viz. 94° 0') (vide infra).

Ilmenite is faintly but decidedly magnetic; hardness 5.5; density 4.8. It usually contains from 26 to 30 per cent, of titanium, although specimens have been found containing considerably more and considerably less than this amount. The density falls with increase of titanium, and the axial ratio a: с likewise falls, indicating an increase in the vertical axis, с. This is well illustrated by the following data: -

TiO2. Per cent.	Density.	Axial Ratio, a: c.
5.66	5.041	1: 1.368
21.58	4.910	1: 1.3716
47.68	4.852	1: 1.3772
49.68	4.614	1: 1.379
57.23	...	1: 1.3851

The formula for ilmenite may also be written as Fe₂O₃.Ti₂O₃, and, on account of the difficulty of determining the state of oxidation of the titanium, it is not easy to decide, from chemical considerations alone, which formula is correct, although the balance of chemical evidence appears to support the formula FeO.TiO₂. The mineral crystallises, however, in the rhombohedral-tetartohedral division of the hexagonal system, and the axial ratio in no case lies between that of haematite, Fe₂O₃,

a: c = 1: 1.359, and that of artificial titanium sesqui-oxide, Ti₂O₃, namely
a: c = 1: 1.316,

as it might be expected to do if it were an isomorphous mixture of Fe₂O₃.Ti₂O₃. This lends strong support to the view that the mineral is a titanite of ferrous iron, namely FeTiO₃.

Other varieties of this mineral are known as kibdelophane and crichtonite, containing about 30 per cent, of titanium; menaccanite, hystastite, and uddevallite, containing some 25, 15 to 20, and 10 per cent, of titanium respectively.

In Geikielite, from Ceylon, part of the iron is replaced by magnesium, thus (Mg, Fe)O.TiO₂. Density 3.976. Axial ratio,

a: c = 1: 1.370.

It is isomorphous with senaite, (Fe, Mn, Pb)O.TiO₂, which is likewise hexagonal, having axial ratios

a: c = 1: 0.997.

The crystals are black, thin crystals showing a greenish colour by transmitted light.

Bixbyite, FeO.MnO₂, occurs as brilliant black, cubic crystals, possessed of metallic lustre. The composition of bixbyite might also be written as Fe₂O₃.Mn₂O₃. If such were correct, however, the mineral might be expected to crystallise in the hexagonal system, and belong to the haematite group.

Magnesio-ferrite or magno-ferrite, MgO.Fe₂O₃, is a variety of magnetite in which the ferrous oxide has been replaced by magnesia. It crystallises in regular octahedra, and resembles magnetite in colour and general appearance. It is magnetic; hardness 6 to 6.5; density 4.57 to 4.65. It has been found around the fumaroles of Vesuvius. Hydrochloric acid attacks it only with difficulty.

In Ludwigite part of the magnesium oxide is replaced by ferrous oxide, and boric oxide is associated with the ferric oxide. Thus (Mg, Fe)O.Fe₂O₃.B₂O₃, or perhaps 3MgO.B₂O₃ + FeO.Fe₂O₃.