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. [Illustration] 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 5½, and the specific gravity varies with the chemical composition from 4.3 to 5.0.