Читать книгу The Rare Earths: Their Occurrence, Chemistry, and Technology - Stanley Isaac Levy - Страница 175

—This is a titanate of iron, usually written FeTiO₃. Its constitution has given rise to very considerable discussion[57]; not only do the relative proportions of iron and titanium vary greatly, but the iron is undoubtedly present in both the ferrous and the ferric states, and in the former state is partly replaced in some specimens by manganese and magnesium. In 1829 Mosander put forward the view that the mineral consisted of FeTiO₃, ferrous titanate, with varying proportions of ferric oxide, the forms and angles of ilmenite being very similar to those of hæmatite, Fe₂O₃. This view was disputed by H. Rose, who concluded that the mineral must have been originally an isomorphous mixture of ferric oxide, Fe₂O₃, and titanic oxide, Ti₂O₃, which on exposure to high temperature in the earth’s crust would change according to the equation

Fe₂O₃ + Ti₂O₃ = 2TiO₂ + 2FeO

so that the proportion of ferrous iron increases with the proportion of titanium dioxide, as is actually found to be the case. This condition, however, is also satisfied by Mosander’s view. The latter view was also supported by Rammelsberg, who pointed out that the presence of magnesium indicated the existence of ferrous iron as a primary constituent. Additional support is lent to this view by the discovery of Pyrophanite, MnTiO₃ (see list), which is found to be isomorphous with ilmenite, so that there can be little doubt that MgTiO₃, which can be only a titanate, would, if it existed in the crystalline form (see Geikielite in list), also be isomorphous with ilmenite. Friedel and Guérin (1876) prepared artificial titanium sesquioxide, Ti₂O₃, and found it to be isomorphous with hæmatite, Fe₂O₃; they concluded that FeFeO₃, FeTiO₃ and TiTiO₃ formed an isomorphous series, and that ilmenite was a mixture of the second with the other two. In 1890 Hamberg pointed out that there was no reason to suppose that hæmatite, Fe₂O₃, contains ferrous iron, i.e. has the constitution Fe´´Fe^ivO₃, analogous to Fe´´Ti^ivO₃, since in corundum, the analogous compound of aluminium, Al₂O₃, divalent aluminium can hardly exist; nevertheless, strict analogy of constitution is not necessary for isomorphism, as shown by the case of potassium nitrate, KNO₃, and aragonite, CaCO₃, so that hæmatite, Fe₂O₃, and ferrous titanate, FeTiO₃, might form solid solutions in varying proportions without the strictly analogous formula FeFeO₃ being true for the former. The balance of opinion inclines to the constitution (mFeTiO₃ + nFe₂O₃ in isomorphous mixture) originally proposed by Mosander. The evidence in support of this view has been greatly strengthened by the recent work of Manchot,[58] which has proved the absence of titanium sesquioxide, Ti₂O₃; the mineral is therefore to be regarded as a titanate.

[57] For a full account of the earlier work on the constitution of ilmenite vide Hintze, i. 1858 et seq.

[58] Zeitsch. anorg. Chem. 1912, 74, 79.

Crystal system—rhombohedral; in forms and angles very close to hæmatite, but the two differ in symmetry (hæmatite has t, 3δ, c, 3π; ilmenite has only t, c).

c = 1·38458; (111) ∧ (100) = 57° 58¹⁄₂´; habit, tabular, thick; or in thin laminæ. Usually in embedded grains or rolled crystals in sand.

Hardness 5 to 6; sp. gr. 4·5 to 5·0, increasing with percentage of ferric oxide. Iron black, opaque; streak black to brownish-red. Lustre sub-metallic. Slightly magnetic.

The mineral is infusible; when powdered, it dissolves slowly in boiling hydrochloric acid, the filtered yellow solution giving the characteristic blue colouration of titanium salts on addition of tinfoil. In fused potassium hydrogen sulphate it dissolves readily. The variation in composition can be judged from the following limits:

TiO₂	Fe₂O₃	FeO
3·5	93·6	3·3	per cent.
52·8	1·2	46·5	„

Ilmenite is a widely distributed mineral. In crystals it occurs chiefly at Kragerö and Arendal in Norway, at Miask in the Ilmen mountains, in Dauphiné, the St. Gothard, etc.; in the massive form at Bay St. Paul, Quebec, and other localities in America; and in sands at Menaccan in Cornwall, Iserwiese in Bohemia, Puy de Dôme, dép. Haute Loire, France, and in Brazil, Australia, and New Zealand.

The mineral was discovered at Menaccan in Cornwall by McGregor, about 1790. He described it as containing iron and a new oxide; the unknown oxide was obtained in 1795 from rutile by Klaproth, who gave the name Titanium to the new metal it contained.

Short descriptions of the following titanates are also given (see list):

Davidite and Knopite; these are complex titanates containing elements of the cerium and yttrium groups.

Arizonite and Pseudobrookite—ferric titanates.

Perovskite, calcium titanate, and its variety Hydrotitanite.

Pyrophanite, a manganese titanate isomorphous with ilmenite, and Senaite, a species intermediate in composition between these two.

Geikielite, the magnesium analogue of ilmenite, with the variety Picroilmenite, which is rich in iron.

Uhligite, a titanate of zirconium, calcium and aluminium.

Derbylite, Lewisite and Mauzeliite, an interesting series of titano-antimonates.

Warwickite, a boro-titanate.