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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 a hydrated iron oxide. These minerals often occur together, especially in weathered deposits. Iron ores are typically dense and can range in color from reddish-brown to black. They are characterized by their high iron content and often exhibit a characteristic reddish-brown streak.
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
- Can be massive, botryoidal, reniform, oolitic, or earthy. Specular hematite has a micaceous or platy texture. Goethite often forms stalactitic or radiating fibrous aggregates.
- Crystal Form
- Hematite: Rhombohedral, tabular, or massive. Goethite: Orthorhombic, typically 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), lateritic weathering profiles, bog iron deposits, hydrothermal veins, sedimentary beds, and as alteration products in various rock types.
Key Facts
- Hardness: Hematite: 5-6.5 on Mohs scale. Goethite: 5-5.5 on Mohs scale.
- Specific Gravity: Hematite: 4.9-5.3. 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, submetallic, dull, earthy. Goethite: Adamantine, silky, dull, earthy.
- Transparency: Opaque
- Fracture: Hematite: Uneven to subconchoidal. Goethite: Uneven.
- Cleavage: Hematite: None (parting present). Goethite: Perfect on {010}, but rarely seen.
- Composition: Hematite: Iron(III) oxide (Fe2O3). Goethite: Hydrated iron(III) oxide (FeO(OH)).
Quick Check
- Color: Reddish-brown to black (hematite), yellowish-brown to dark brown (goethite)
- Luster: Metallic to dull/earthy
- Streak: Reddish-brown (hematite), yellowish-brown (goethite)
Physical Characteristics
- Crystal Habit: Hematite: Rhombohedral, tabular, massive, botryoidal, reniform, micaceous (specularite), earthy. Goethite: Acicular, fibrous, radiating, stalactitic, massive, botryoidal, earthy.
- Cleavage Type: Hematite: None (parting on {1011}). Goethite: Perfect on {010}.
- Fracture Type: Uneven to subconchoidal
- Tenacity: Brittle
- Luster Type: Metallic, submetallic, adamantine, silky, dull, earthy
Formation
Iron ores, particularly those dominated by hematite and goethite, form through various geological processes. Banded Iron Formations (BIFs), the largest source of iron ore, are sedimentary rocks formed in ancient oceans during the Precambrian. They consist of alternating layers of iron-rich minerals (hematite, magnetite) and chert. The precipitation of iron was likely driven by oxygenic photosynthesis from early microorganisms. Weathering of iron-rich rocks can also lead to the formation of secondary iron oxides and hydroxides like goethite and limonite, often concentrating iron in lateritic soils or bog iron deposits. Hydrothermal processes can also contribute to iron ore formation.
Usage
The primary use of iron ore is in the production of iron and steel. Iron is a fundamental component of infrastructure, machinery, vehicles, and countless other manufactured goods. Hematite is also used as a pigment (e.g., in paints, cosmetics), a polishing agent (rouge), and in some cases, as a heavy aggregate.
Age Distribution
Predominantly Precambrian (Banded Iron Formations), but also Phanerozoic (e.g., bog iron, laterites)
Where to Find
Australia (Pilbara region)
World's largest producer of iron ore, primarily from extensive Banded Iron Formations.
Brazil (Minas Gerais)
Significant producer of high-grade hematite ore, often associated with BIFs.
China
Large iron ore reserves, though often lower grade, found in various geological settings.
India
Major producer with significant hematite and goethite deposits.
United States (Mesabi Range, Minnesota)
Historically a major iron ore producing region, primarily from BIFs.
Canada (Labrador Trough)
Important source of iron ore from BIFs.
Finding Tips
Look for Reddish Stains
Iron oxides often stain surrounding rocks and soil a characteristic reddish-brown color, indicating their presence.
Check for Density
Iron ores are typically denser than most common rocks, so a heavy sample for its size can be a good indicator.
Perform a Streak Test
Hematite produces a reddish-brown streak, while goethite produces a yellowish-brown streak. This is a crucial diagnostic test.
Examine for Metallic Luster
Specular hematite has a distinct metallic luster, which can be easily spotted.
Investigate Weathered Zones
Goethite and limonite often form in weathered zones of iron-rich rocks or in boggy environments.
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
- Iron(III) oxide (Hematite), Hydrated iron(III) oxide (Goethite)
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