 |
ILMENITE, a mineral known also as titanic iron, formerly regarded as an iron and titanium sesquioxide (Fe, Ti)2O3 isomorphous with haematite (Fe2O3), but now generally considered to be an iron titanate FeTiO3 isomorphous with pyrophanite (MnTiO3) and geikielite (MgTiO3). It crystallizes in the parallel-faced hemihedral class of the rhombohedral system, thus having the same degree of symmetry as phenacite and pyrophanite, but differing from that of haematite. The angles between the faces are very nearly the same as between the corresponding faces of haematite; but it is to be noted that the rhombohedral angle (94° 29′) of ilmenite is not intermediate between that of haematite (94° 0′) and of the artificially prepared crystals of titanium sesquioxide (92° 40′), which should be the case if the three substances were isomorphous. Analyses show wide variations in chemical composition, and there is a gradation from normal ilmenite FeTiO3 (with titanium dioxide 52.7, and ferrous oxide 47.3%) to titaniferous haematite and titaniferous magnetite. Frequently also, magnesia and manganous oxide are present in small amounts, the former reaching 16%. The formula (Fe, Mg)TiO3 is then analogous to those of geikielite and pyrophanite. Many analyses show the presence of TiO2 and (Fe, Mg)O in this ratio of 1:1, yet there is often an excess of ferric oxide to be accounted for; this may perhaps be explained by the regular intergrowth on a minute scale of ilmenite with haematite, like the intergrowth of such substances as calcite and sodium nitrate, which are similar crystallographically but not chemically.
In many of its external characters ilmenite is very similar to haematite; the crystals often have the same tabular or lamellar habit; the twin-laws are the same, giving rise to twin-lamellae and planes of parting parallel to the basal plane and the primitive rhombohedron; the colour is iron-black with a submetallic lustre; finally, the conchoidal fracture is the same in both minerals. Ilmenite has a black streak; it is opaque, but in very thin scales sometimes transparent with a clove-brown colour. It is slightly magnetic, but without polarity. The hardness is 512, and the specific gravity varies with the chemical composition from 4.3 to 5.0.
Owing to the wide variations in composition, which even yet are not properly understood, several varieties of the mineral have been distinguished by special names. Crichtonite occurs as small and brilliant crystals of acute rhombohedral habit on quartz at Le Bourg d’Oisans in Dauphiné; it agrees closely in composition with the formula FeTiO3 and has a specific gravity of 4.7. Manaccanite (or Menaccanite) is a black sandy material, first found in 1791 in a stream at Manaccan near Helston in Cornwall. Iserite, from Iserwiese in the Iser Mountains, Bohemia, is a similar sand, but containing some octahedral crystals, possibly of titaniferous magnetite. Washingtonite is found as large tabular crystals at Washington, Connecticut. Uddevallite is from Uddevalla in Sweden. Picrotitanite or picroilmenite (Gr. πικρός, “bitter”) is the name given to varieties containing a considerable amount of magnesia. Other varieties are kibdelophane, hystatite, &c. The name ilmenite, proposed by A. T. Kupffer in 1827, is after the Ilmen Mountains in the southern Urals, whence come the best crystals of the mineral. The largest crystals, sometimes as much as 16 ℔ in weight, are from Kragerö and Arendal in Norway.
Ilmenite occurs, often in association with magnetite, in gneisses and schists, sometimes forming beds of considerable extent, but of little or no economic value. It is a common accessory constituent of igneous rocks of all kinds, more especially basic rocks such as gabbro, diabase and basalt. In these rocks it occurs as platy crystals, and is frequently represented by a white, opaque alteration product known as leucoxene. (L. J. S.)