Triferric tetroxide, Fe₃O₄

Ferroso-ferric oxide, Triferric tetroxide, Magnetite, or Magnetic oxide of iron, Fe₃O₄, occurs in nature as the mineral magnetite and may be regarded as a compound of ferrous oxide and ferric oxide, namely FeO.Fe₂O₃, or as the ferrous salt of hypothetical meta-ferrous acid, namely FeFe₂O₄. It has a black, metallic appearance, and crystallises in octahedra and dodecahedra, of hardness 5.5 to 6.5, and density approximately 4.9 to 5.2. It was first recognised as a definite oxide of iron by Gay-Lussac, who obtained it by the action of steam on iron. As its name would appear to imply, magnetite possesses magnetic properties, being attracted by a magnet. It is quite possible, however, that the name of the ore does not really refer to the magnet, but to Magnesia, a town in Lydia, Asia Minor, where the ore was first found. In the laboratory the oxide may be obtained in a variety of ways. Thus, when iron is heated in steam to upwards of 820° C., it becomes covered with a skin of magnetic oxide, the layers underneath consisting of various amounts of ferrous oxide associated with magnetic oxide in solid solution. If the iron is in the form of very thin plates, these may, by prolonged heating in the steam, be converted completely into the higher oxide.

Magnetic oxide also occurs as a superficial layer when iron is heated to dull redness in air - that is, at a temperature of 625° to 650° C. The oxide beneath the outermost skin has the composition represented by the formula Fe₃O₄.xFeO.

When ferric oxide is maintained at 1500° C. in nitrogen or at very high temperatures in air, such as those obtaining in the electric furnace, it is reduced to magnetite, for which reason it is possible, though scarcely profitable, to apply magnetic concentration to haematites at high temperatures. When iron burns in oxygen, the magnetic oxide is produced, and also when ferric oxide is heated to 400° C. in a current of hydrogen saturated with water vapour at 30° to 50° C. At higher temperatures products increasingly rich in ferrous oxide are obtained. Thus: -

700° С	85 per cent. FeO
800° С	92 per cent. FeO

On heating reduced iron in carbon dioxide at 440° C., and by reducing ferric oxide by hydrogen or carbon monoxide at 500° C., Moissan has been able to obtain the magnetic oxide.

Crystals of magnetic oxide have been obtained in a variety of ways, such as by calcination of sodium carbonate and ferrous chloride; by fusion of potassium sulphate and iron phosphate; by the action of hydrogen chloride upon heated ferrous oxide; and by ignition of ferrous fluoride with boric anhydride.

When strongly heated with excess of sodium chloride for several hours ferric oxide is converted into black crystals of magnetic oxide. Pure iron wire, heated to 1200° C. in a current of carbon dioxide, yields crystalline magnetic oxide, the crystals frequently exhibiting magnetic polarity. The presence of moisture facilitates the formation of larger crystals.

When iron wire is subjected to prolonged fusion with sodium sulphate, it is converted into magnetite, the sodium sulphate acting catalytically being first reduced to sulphite and re-oxidised to sulphate by the oxygen of the atmosphere. Ferrous sulphide may be oxidised to magnetite in a similar manner. As obtained in this manner, the crystalline magnetic oxide closely resembles the natural product. Its crystals are opaque, magnetic octahedra, possessed of a metallic lustre. Hardness 6 to 6.5; density, 5.21 to 5.25. The crystals are not affected by steam or carbon dioxide at bright red heat, are not attacked by diluted mineral acids, and are but slowly dissolved by the concentrated acids or aqua regia.

Ordinary magnetic oxide of iron melts at 1527° C. Its specific heat is 0.1655. When heated with platinum to 1600° C. in contact with air, magnetic oxide is reduced to the metal, oxygen being evolved, the iron passing into solid solution in the platinum. Under low oxygen pressures reduction in the above manner can take place at 1400° C. When heated in air for a prolonged period at 1300° C., the oxide is almost completely converted into ferric oxide. According to the reversible reaction 4Fe₃O₄ + O₂ ⇔ 6Fe₂O₃,

magnetic oxide has no perceptible dissociation pressure at 1350° C.

It does not react appreciably with sulphur dioxide at dull redness. Raised to white heat in a current of hydrogen sulphide, it yields ferrous sulphide, accompanied by the evolution of hydrogen, sulphur dioxide, and a little sulphur trioxide.

When placed in freshly-fused potassium hydrogen sulphate, a crystal of magnetite is only slightly attacked; but at a higher temperature there is an energetic action.

Hydrogen reduces magnetic oxide to the metal, the reaction being perceptible at 305° C.

Magnetic oxide dissolves in hydrochloric acid. If the latter is not present in sufficient quantity to yield a complete solution of ferrous and ferric chlorides, ferric oxide and ferrous chloride are produced.

The heats of formation of magnetite are as follow: -

3[Fe₂O₃](calcined) = 2[Fe₃O₄] + (O) - 45,180 calories.
3[Fe] + 4(O) = [Fe₃O₄] + 265,200 calories.
3[Fe] + 4(O) = [Fe₃O₄] + 265,700 calories.
[FeO] + [Fe₂O₃] = [Fe₃O₄] + 9200 calories.
3[FeO] + (O) = [Fe₃O₄] + 85,800 calories.
3[FeO] + (O) = [Fe₃O₄] + 75,600 calories.
3[FeCO₃] + (О) = [Fe₃O₄] + 3CO₂ + 0 calories.
and at 490° C. under constant pressure,
3[Fe] + 4(O) = [Fe₃O₄] + 267,380 calories.

According to Moissan, magnetic oxide of iron exists in two polymorphic forms, according to its method of preparation. The one form, obtained by high temperature methods, such as the combustion of iron in oxygen, the action of steam on iron at red heat, and the calcination of ferric oxide at bright red heat, is characterised by its insolubility in concentrated boiling nitric acid, by its high density (5 to 5.09), and by its resistance to further oxidation when heated in air.

The second variety resembles the former in its black appearance and magnetic properties, but differs from it in density (4.86), in its solubility in nitric acid, and in its tendency to oxidise to ferric oxide when calcined in air. It is converted into the other variety when raised to white heat in nitrogen. As explained in the case of ferrous oxide, however, these differences may simply be due to variations in the states of aggregation of the oxide, according to its method of preparation.

Several substances, such as 4FeO.Fe₂O₃, 3FeO.Fe₂O₃, etc., have been described. It is highly probable, however, that these are not definite chemical entities.

When ferroso-ferric ammonium carbonate is decomposed by a hot concentrated solution of potassium hydroxide in the absence of air, hydrated ferroso-ferric oxide separates out, which, when dried at 100° C., corresponds in composition to the formula Fe₂O₃.4FeO.5H₂O. It is readily acted on with air, yielding hydrated ferric oxide, Fe₂O₃.H₂O.

A bluish black oxide, corresponding in composition to 2FeO.3Fe₂O₃, is described as resulting when potassium nitrate is added to a boiling solution of ferrous sulphate rendered alkaline with ammonia.

Magnetic oxide dissolves in hydrochloric acid, and the solution so obtained yields, on pouring into an excess of sodium hydroxide solution, a black precipitate which, on drying, is attracted by a magnet. It is the monohydrate, Fe₃O₄.H₂O.

The sesqui-hydrate, 2Fe₃O₄.3H₂O, is obtained by precipitation in an analogous manner from mixed solutions of ferrous and ferric salts in the proportions theoretically required. The precipitate is dark green to black in colour, and strongly magnetic. It may be washed in the presence of air without oxidising, and loses water only, on heating.