Taconite

Taconite (ˈtækənaɪt) is a variety of banded iron formation, an iron-bearing (over 15% iron) sedimentary rock, in which the iron minerals are interlayered with quartz, chert, or carbonate. The name "taconyte" was coined by Horace Vaughn Winchell (1865–1923) – son of Newton Horace Winchell, the Minnesota State Geologist – during their pioneering investigations of the Precambrian Biwabik Iron Formation of northeastern Minnesota. He believed the sedimentary rock sequence hosting the iron-formation was correlative with the Taconic orogeny of New England, and referred to the unfamiliar and as-yet-unnamed iron-bearing rock as the 'taconic rock' or taconyte. Following development of high grade direct shipping iron ore deposits on the Mesabi Range, containing up to 65% iron and as little as 1.25% silica, miners termed the unaltered iron-formation wall rock taconite. The iron content of taconite is generally 30% to 35%, and the silica content generally around 45%. Iron in 'taconite' is commonly present as magnetite, iron silicates, and iron-bearing carbonates, and locally martite (hematite) formed by oxidation of magnetite. Horizons containing magnetite as the dominant mineral have been extensively mined since 1955 to produce iron ore pellets; the term 'taconite' has consequently been colloquially adapted to describe the magnetite iron-formation ores (taconite iron ore), the mining, milling, magnetic separation, and agglomerating process (taconite process), and the product iron ore pellets (taconite pellets). In the late 19th and early 20th centuries, the United States was mining such an abundance of iron ore of high quality that taconite was considered an uneconomic waste product. By the end of World War II, however, much of the high-grade iron ore in the United States had been exhausted. Taconite became valued as a new source of the metal. To process taconite, the ore is ground into a fine powder, the magnetite is separated from the gangue by strong magnets, and the powdered iron concentrate is combined with a binder such as bentonite clay and limestone as a flux.