Iron Ore

Sedimentary, Metamorphic, or Igneous (as primary ore)

Hematite or Goethite (Iron Oxide)

Also known as: Hematite, Goethite, Limonite (a mixture of iron oxyhydroxides, often including goethite)

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Description

Iron ore refers to rocks and minerals from which metallic iron can be economically extracted. The most important iron ore minerals are hematite (Fe2O3) and goethite (FeO(OH)). Hematite is an anhydrous iron oxide, while goethite is an iron oxyhydroxide. These minerals are characterized by their high iron content and are typically found in large, mineable deposits. Iron ores can vary significantly in appearance, from the metallic luster of specular hematite to the earthy, reddish-brown appearance of some goethite-rich ores. The presence of iron oxides often imparts a reddish, brownish, or yellowish color to the surrounding rocks and soils.

How to Identify

Color: Hematite: Steel-gray to black (specular hematite), reddish-brown to red (earthy hematite). Goethite: Yellowish-brown to dark brown, sometimes black.
Luster: Hematite: Metallic to dull/earthy. Goethite: Adamantine to dull/earthy.
Texture: Hematite: Can be massive, botryoidal, reniform, micaceous (specularite), or earthy. Goethite: Often massive, botryoidal, stalactitic, or earthy.
Crystal Form: Hematite: Rhombohedral crystals, tabular, or massive. Goethite: Orthorhombic crystals, acicular, fibrous, or massive.
Cleavage: Hematite: No true cleavage, but parting along rhombohedral planes can occur. Goethite: Perfect cleavage on {010}, but rarely observed in massive forms.
Geological Environment: Banded Iron Formations (BIFs), sedimentary ironstones, lateritic weathering profiles, hydrothermal veins, bog iron deposits, and as accessory minerals in various igneous and metamorphic rocks.

Key Facts

Hardness: Hematite: 5-6.5 on Mohs scale. Goethite: 5-5.5 on Mohs scale.
Specific Gravity: Hematite: 5.26. Goethite: 3.3-4.3.
Crystal System: Hematite: Trigonal. Goethite: Orthorhombic.
Color: Hematite: Steel-gray, black, reddish-brown. Goethite: Yellowish-brown, dark brown.
Luster: Hematite: Metallic to dull/earthy. Goethite: Adamantine to dull/earthy.
Transparency: Opaque.
Fracture: Hematite: Uneven to subconchoidal. Goethite: Uneven.
Cleavage: Hematite: None (parting present). Goethite: Perfect on {010}.
Composition: Hematite: Iron(III) oxide (Fe2O3). Goethite: Iron oxyhydroxide (FeO(OH)).

Quick Check

Color: Hematite: Steel-gray, black, or reddish-brown. Goethite: Yellowish-brown to dark brown.
Luster: Hematite: Metallic to dull/earthy. Goethite: Adamantine to dull/earthy.
Streak: Hematite: Reddish-brown. Goethite: Yellowish-brown.

Physical Characteristics

Crystal Habit: Hematite: Rhombohedral, tabular, massive, botryoidal, reniform, micaceous (specularite), earthy. Goethite: Acicular, fibrous, stalactitic, botryoidal, massive, earthy.
Cleavage Type: Hematite: No true cleavage, but parting on {1011} is common. Goethite: Perfect on {010}, but often not visible in massive forms.
Fracture Type: Hematite: Uneven to subconchoidal. Goethite: Uneven.
Tenacity: Brittle.
Luster Type: Hematite: Metallic, submetallic, dull, earthy. Goethite: Adamantine, silky, dull, earthy.

Formation

Iron ores, particularly hematite and goethite, form through various geological processes. Banded Iron Formations (BIFs), the largest iron ore deposits, formed primarily during the Precambrian Eon (2.5 to 1.8 billion years ago) through the precipitation of iron oxides and silica from ancient oceans. Other significant deposits include oolitic ironstones (sedimentary), lateritic iron ores (weathering products), and hydrothermal deposits. Goethite often forms as a weathering product of other iron-bearing minerals or as a primary precipitate in bog iron ores.

Usage

The primary use of iron ore is in the production of iron and steel. Iron ore is smelted in blast furnaces to produce pig iron, which is then further processed into steel. Steel is an essential material in construction, automotive manufacturing, machinery, infrastructure, and countless other industries. Iron oxides are also used as pigments (e.g., ochre), in magnetic recording media, and as catalysts.

Age Distribution

Predominantly Precambrian (especially Banded Iron Formations), but also found in younger deposits.

Where to Find

Hamersley Basin, Western Australia

One of the world's largest and highest-grade iron ore provinces, primarily hematite from enriched BIFs.

Minas Gerais, Brazil

Significant deposits of high-grade hematite, often associated with itabirite (a metamorphosed BIF).

Lake Superior Region, USA and Canada

Historically and currently a major iron ore producing region, with extensive BIFs and enriched hematite deposits.

Krivoy Rog, Ukraine

Large deposits of high-grade hematite and magnetite within Precambrian BIFs.

Kiruna, Sweden

Known for its large, high-grade magnetite-hematite deposits of igneous origin.

Finding Tips

Look for Reddish Stains

Iron oxides often stain rocks and soil a distinctive reddish-brown or yellow. This can indicate the presence of iron-rich minerals.

Check for Heavy Samples

Iron ore minerals are relatively dense. Samples that feel unusually heavy for their size are good candidates.

Perform a Streak Test

Hematite produces a characteristic reddish-brown streak, while goethite produces a yellowish-brown streak. This is a key diagnostic test.

Examine for Metallic Luster

Some forms of hematite (specular hematite) have a distinct metallic luster, appearing like shiny, dark flakes.

Investigate Geological Maps

Consult geological maps of an area to identify known iron ore deposits or formations associated with iron mineralization, such as Banded Iron Formations.

Similar Rocks

Magnetite

Fe3O4

Also known as: Lodestone

Siderite

FeCO3

Also known as: Iron carbonate

Pyrite

FeS2

Also known as: Fool's Gold

Scientific Classification

Mineral Class: Oxides (Hematite), Hydroxides (Goethite)
Group: Hematite Group (Hematite), Diaspore Group (Goethite)
Crystal System: Trigonal (Hematite), Orthorhombic (Goethite)
Chemical Formula: Fe2O3 (Hematite), FeO(OH) (Goethite)
Composition: Hematite: 69.9% Iron, 30.1% Oxygen. Goethite: 62.9% Iron, 27.0% Oxygen, 10.1% Hydrogen.

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