Iron - PUB2895

Commodity

Iron is a brittle, steel gray to silvery gray metal used through millennia. Advent of its use was so significant that the Iron Age of human prehistory is named for the dominant use of the metal. Iron has had numerous names throughout human history. Our current name for the metal is likely from Middle English and was itself derived from Proto-Germanic “isarnan” and Celtic “isarnon.” The chemical symbol for iron, Fe, is from “ferrum,” the Latin word for iron metal.

The first iron mining in Missouri was as a pigment by Native Americans. Early mining by Europeans provided a vital resource for development of the region. Its influence can be seen in names such as the town of Ironton, in Iron County. The first iron furnace built west of the Mississippi River was constructed in Iron County in 1815. While Missouri does not have current iron production, it has previously been an important producer of the metal, primarily in the southern and southeastern portions of the state. 

Iron ore deposits in Missouri are divided into three main types: Precambrian-hosted hematite and magnetite, filled-sink hematite and limonite or brown iron ore deposits. Hematite (Fe₂O₃) and magnetite (Fe₃O₄) are the most economically significant ores. Limonite (FeO·OH·nH₂O) was an important ore in the past. Pyrite (FeS₂) and marcasite (also FeS₂) are other iron-bearing minerals previously mined in Missouri; however, they were mined for their sulfur rather than iron content.

Economic Importance

Iron, while not currently mined in Missouri, was economically significant for decades for multiple counties in the state. Surface and near-surface ores were mined throughout the settlement period, and into the mid-1900s. The latter 1900s were dominated by underground mining of large Precambrian-hosted deposits. Timing and economic significance of mining of the different types of iron ores varies greatly.

Mining of limonite for iron experienced a boom after the Civil War, although most producers were small. Limonite ores account for less than 10% of total iron production in Missouri but were highly significant locally. Production of these ores peaked in 1957, declined sharply soon after, and ceased by the mid-1960s. 

Filled-sink hematite deposits reached their greatest levels of production from the 1870s to the early 1900s, accounting for 30% of the iron mined in Missouri during this time frame. Production dropped rapidly after 1930 and ceased by 1960. 

Considerable mining of pyrite for sulfuric acid occurred in the early 1900s, as well as during World War I and early World War II. Most pyrite production in Missouri occurred from 1932 to 1940. Several of the larger deposits produced hundreds of thousands of tons of ore. Since 1940, no pyrite has been mined as a primary ore, but has since been produced as a byproduct of lead, iron and coal mines.

The highest production iron ores in Missouri were the Precambrian-hosted magnetite and hematite deposits. Production of these deposits spans from the early 1800s to 2001, with major production occurring from underground mines from the 1960s onward. Early mining was of smaller, surface exposed deposits. The last operating iron mine in Missouri, Pea Ridge, closed in 2001.

Missouri has potential for future production, with significant reserves of magnetite remaining at Pea Ridge Mine and known reserves in the undeveloped deposits of Kratz Spring, Camels Hump and Bourbon.

Total iron production from Missouri is nearly 62 million long tons (68 million short tons), with the greatest tonnages coming from the Iron Mountain, Pea Ridge and Pilot Knob underground mines. The total production would be valued at more than 5 billion dollars at 2018 prices.

Iron Ore Minerals Found in Missouri

Hematite

• Chemical composition: Fe₂O₃, or iron oxide.
• Color and Luster: Primarily steel-gray, but often red in earthy forms. Luster varies from metallic to earthy depending on the form.
• Hardness: 5.5-6.5.
• Cleavage: none.
• Specific gravity: 5.3.
• Streak: rust red.
• Crystal habit: usually massive; crystals may be tabular, platy, micaceous, fibrous, earthy, or stalactitic.
• Hematite is named from the Greek for "blood stone" due to its red color when powdered.

Magnetite

• Chemical composition: Fe₃O₄, or iron oxide.
• Color and Luster: grayish black to black; metallic to sub-metallic luster.
• Hardness: 5.5-6.5.
• Cleavage: none.
• Specific gravity: 5.2.
• Streak: black.
• Crystal habit: usually massive; single crystals will often form octahedrons.
• Magnetite was originally called lodestone but was formally named in 1845 for a locality in Magnesia, Greece.

Note: Magnetite and hematite can appear very similar but are easily distinguished. Hematite has a rust red streak; magnetite has a black streak. In addition, magnetite will attract a magnet, while hematite will not.

Limonite

• Technically a group of yellowish-brown iron-oxide minerals, rather than a single mineral.
• Chemical composition: FeO·OH·nH₂O, or hydrated iron oxide.
• Color and luster: yellow brown; luster is submetallic to dull.
• Hardness: 3-5.5 (range due to variability of minerals)
• Cleavage: none.
• Specific gravity: 3.6-4.3.
• Streak: yellow to yellowish-brown.
• Crystal habit: generally amorphous, earthy, or stalactitic. Often replaces and retains the crystal shape of, or is a pseudomorph of, pyrite or marcasite.
• Limonite is from the Greek for "meadow," as it was often found in bogs.

Pyrite

• Chemical composition: FeS₂, or iron sulfide.
• Color and luster: pale brass yellow; metallic luster.
• Hardness: 6-6.5.
• Cleavage: 2 indistinct cleavages.
• Specific gravity: 5.
• Streak: greenish-black to brownish-black.
• Crystal habit: commonly cubes or dodecahedrons; may be massive, octahedrons, granular, or stalactitic.
• Pyrite is named for the Greek "pyr" for fire, as the mineral sparks when struck by metal.

Marcasite

• Chemical composition: FeS₂, or iron sulfide.
• Color and luster: pale bronze, pale brass yellow, whiter than pyrite; metallic luster.
• Hardness: 6-6.5.
• Cleavage: 2 distinct cleavages.
• Specific gravity: 4.9.
• Streak: dark gray to black.
• Crystal habit: often "cock's comb" or bladed triangles; may be massive.
• The name marcasite is Moorish in origin, and originally included pyrite.

Common Names

Iron ores in Missouri have been known by a variety of names. Hematite is often called specularite when crystals are platy in form, while earthy hematite is often called ochre. Magnetite is called lodestone when the mineral acts as a magnet itself, rather than simply being attracted to a magnet. Limonite was frequently called “bog ore” due to its common occurrence in bogs in parts of the world. Pyrite is often referred to as "Fool's gold" due to its brassy color.

Primary Uses

Iron ores are used for multiple different products. Magnetite and hematite are both used to produce steel but have multiple other uses. Ores from the large Precambrian deposits in Missouri have been mined for steel production, but also for iron pigments, coal desulfurization, concrete densification, and water purification systems. Pyrite, while no longer mined in Missouri, is used today to color amber and brown glass, for sulfuric acid generation, and as beads and in jewelry.

Past Uses

Iron-bearing minerals in Missouri were mined in the past for all of the uses noted above. Magnetite, hematite and limonite were all used for production of iron metal. Limonite was also previously used as an iron pigment, along with magnetite and hematite. Earthy hematite was especially noted as a red pigment. Hematite from filled-sink ores was mined for a cement additive. Most pyrite mined in Missouri was for sulfuric acid generation but was also used to provide sparks for some flintlocks and for starting fires.

Principal Locations of Iron in Missouri

While minor iron mineralization may be found throughout most of Missouri, the majority of mining occurred in several districts in the southern part of the state. The Southeast Missouri Iron Metallogenic Province is considered a world-class iron district. Smaller-scale iron mining also occurred throughout multiple other counties in southern Missouri.

Southeast Missouri Iron Metallogenic Province

• Contains seven major and numerous minor magnetite and hematite deposits.
• Surface-exposed deposits are found in the St. Francois Mountains; known buried deposits are in Washington, Crawford, Franklin, Iron and Dent counties.
• District accounts for the majority of iron production in Missouri.
• Known deposits also contain other metals such as copper, and critical minerals such as rare earth elements and cobalt.

Filled-sink Hematite Deposits

• Primarily located northwest and west of the St. Francois Mountains.
• Most common in Crawford, Phelps and Dent counties.
• Also found in Franklin, Gasconade, Iron, Maries, Miller, Pulaski, Reynolds, Texas and Washington counties.
• Accounted for 30% of iron production in Missouri before 1900; production from these deposits had ceased by 1960.

Limonite or Brown Iron Ores

• Extensive in south-central Missouri, but also found in the central and southwestern portions of the state.
• Most productive mines were in the West Plains and Poplar Bluff regions.

Pyrite and Marcasite Deposits

• Locations similar to those for filled-sink hematite deposits.
• The greatest production was in Phelps, Crawford, Dent and Franklin counties.

Geologic Occurrence

The multiple types of iron deposits in Missouri have widely varying modes of occurrence and are found in different geologic settings. 

Magnetite and hematite ores hosted by Precambrian igneous rocks contain the largest and richest iron ore reserves in Missouri and comprised the largest producing mines. The deposits vary from large stock-like bodies of massive magnetite or hematite to veins, breccia-hosted ores, and bedded deposits. They are generally hosted by rhyolite, trachyte or andesite. Deposits are related to caldera subsidence structures and often have tectonic controls on ore emplacement. Host rocks are extensively altered due to ore emplacement. Individual deposits can contain more than 200 million tons of iron ore.

Filled-sink hematite deposits are hosted by possible sink structures in Cambrian- and Ordovician-age sedimentary rocks, especially the Gasconade and Roubidoux formations. Ores are primarily soft earthy hematite containing boulders of specular hematite, with limonite common near the surface and pyrite and marcasite often found near the base of the deposit. Many deposits are small, being only a few tens of feet in diameter, but range to as large as five acres in area with depths of several hundred feet. Several of these deposits produced significant tonnages of ore over time.

Limonite or brown iron deposits, although often small individually, produced significant iron when looked at collectively. The ores formed by oxidation of pyrite and marcasite; pseudomorphs of limonite after these minerals are common and often contain remnants of the original iron sulfide. The ores are hosted by cherty residuum derived from weathered Ordovician-age dolomite. Deposits varied from surface exposures of tens of feet to 10 acres or more, and depths as great as 150 feet. They were often in demand for blending with other iron ores due to their low phosphorous content.

Pyrite and marcasite – called iron sulfide minerals – are found as filled-sink deposits, with occurrences similar to filled-sink hematite deposits. Sulfide deposits were often capped by hematite or limonite where the iron sulfide minerals were oxidized. Sizes were similar to those of the filled-sink hematite deposits, with the same range of variability. Differences between the two deposit types were often only marked by the degree of oxidation of the iron sulfide minerals.

Mining

Iron ores in Missouri were originally mined at the surface, with later extensive underground mining of the large Precambrian-hosted deposits; mining at Pea Ridge extended to depths of more than 2,700 feet.

Early mining was generally by hand within surface mines or in very shallow shafts and workings. During the mid-1800s, mining and processing of the ores began to vary more widely for the different deposit types.  

Filled-sink deposits were primarily mined by surface methods, with shallow underground workings in some larger deposits. Ores were selectively mined for higher grade hematite and were generally shipped directly to customers without any beneficiation. Original mining was by hand methods, with later shifts to mechanized mining.

Limonite or brown ore mining primarily involved surface methods, beginning with hand mining, and later shifting to power shovels, drag-lines, loaders and bulldozers. Ores were selectively mined for higher grade material. Beneficiation was done with log washers, mechanical jigs, heavy media separation, and in some cases, hand cobbing. The advent of extensive mechanized mining caused a sharp decrease in production of these deposits due to their small size.

Pyrite and marcasite mining was similar to that of the filled-sink ores, as the deposits were very similar. However, pyrite and marcasite ores underwent beneficiation to separate the ores from waste materials before being shipped to customers.

Large-scale underground iron mining in Missouri involved magnetite and hematite ores that had primary crushing underground and hoisting to the surface, followed by more extensive crushing of the ores. Ores were further processed by either flotation methods or separation by magnetic processes for magnetite ores. The ores were then shipped for further processing into steel and other iron oxide-based products.

Production History

Missouri iron production from 1815 to 2000. Production ceased in mid-2001. The increase near 1870 represents production from the Iron Mountain Mine, while the mid-1960s increase represents the start of major production from the Pea Ridge and Pilot Knob underground magnetite mines. Years previous to 1879, with several exceptions, were divided evenly from lump sums. 

Additional Reading

Search the Missouri Geology Bibliography

Visit the department’s Ed Clark Museum of Missouri Geology, where you will find iron samples on display.