ilmenite

[After the Ilmen Mountains, a range of the southern Ural Mountains.]

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A rhombohedral mineral with composition Fe²⁺Ti⁴⁺O₃. The hardness is 5½ on Mohs scale, specific gravity 4.72. Color is black, and there is no cleavage. The mineral usually occurs massive or in thin plates. Two other minerals belonging to the ilmenite structure type are geikielite, MgTiO₃, and pyrophanite, MnTiO₃.

Ilmenite is the most abundant titanium mineral in igneous rocks and the most important ore of titanium. Important occurrences include the Ilmen Mountains, Russia (whence the name); Kragerø, Norway; and Allard Lake, Quebec, Canada. See also Titanium.

A mineral which is commonly used as an aggregate in high-density concrete; also called iron titanate.

Environment

In metamorphic and plutonic rocks and in pegmatites.

Equidimensional to tabular, and down to fine, scaly crystals. Also compact, massive, or granular; and as black sand.

Black to brownish black (geikielite) and deep red (pyrophanite). Luster metallic to submetallic; hardness 5-6; specific gravity 4.1-4.8; streak brown, reddish brown to ocher yellow; fracture conchoidal to subconchoidal; cleavage none in ilmenite (rhombohedral in geikielite and pyrophanite). Brittle; ilmenite is weakly magnetic.

An iron titanium oxide; related to hematite except that half of the iron has been replaced by titanium. Manganese and/or magnesium can also take the place of the remaining iron; then the minerals are known respectively as pyrophanite (MnTiO ₃ ) and geikielite (MgTiO ₃ ). Normal ilmenite has 36.8% Fe, 31.6% Ti, 31.6% O.

Infusible on charcoal, but splinter held in forceps will be slightly rounded on edges in hottest blowpipe flame. Best test is titanium coloration: after fusion in borax and repowdering, the pulverized probe is partially dissolved in hot concentrated hydrochloric acid; the acid is then filtered, leaving a clear yellow solution that, after boiling with real tin, becomes very pale blue or violet. Since this is difficult to observe, it is best to use a very strong solution.

Color of the streak distinguishes it from hematite, the lesser magnetism from magnetite, the great hardness from the black sulfosalts, and the magnetism from brookite or rutile. Nonmagnetic columbite and tantalite are much heavier, but may require a chemical or specific gravity test.

Ilmenite is a common accessory grain in basic igneous rocks, and grains often become concentrated in sands resulting from rock destruction by weathering. Fine sharp crystals have been found in a pegmatite at Kragerø, Norway. Flat plates are common in U.S. pegmatites, as at Bedford, Westchester Co., New York. In the U.S. a formation was mined at Sanford Lake in the Adirondacks, where it forms great masses but no good crystals. Occurrences at Lake Allard, Quebec, are of great economic importance, for titanium has become a valuable commercial metal. The magnesium ilmenite, geikielite, has been found in waterworn pebbles in the gem gravels of Sri Lanka (Ceylon) and in grains in the marble of Riverside, California. The manganese ilmenite, pyrophanite, has been found in small red tabular crystals in Sweden and in a rock in Brazil.

Important as a source of white titanium oxide now, the nonhazardous pigment that has replaced lead in white paint. Technological progress has made fabrication of titanium metal a practical and commercial operation, and there are a number of uses for this lightweight yet strong metal. The titanium content that long made ilmenite worthless as an iron ore has now become the more valuable constituent.

Opaque mineral found in basalt; nearly pure iron-titanium oxide (FeTiO₃.)