ILMENITE  , a

mineral known also as titanic iron, formerly regarded as an iron and titanium sesquioxide (Fe,Ti)203 isomoX-phous with haematite (Fe203), but now generally considered to be an iron titanate FeTiO3 isomorphous with pyrophanite (MnTiOa) and geikielite (MgTiO3) . It crystallizes in the.parallell- ILOILO 327 faced helnihedral 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° o') 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 51, and the specific gravity varies with the chemical composition from 4.3 to 5.o . 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 Dauphine; 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. snobs, " 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 lb in weight, are from Kragero 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 . Iu these rocks it occurs as platy crystals, and is frequently re--presented by a white, opaque alteration product known as leucoxene . (L . J .