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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 hydrous iron oxide. These minerals often occur together, especially in weathered deposits. Concretions and nodules of iron ore are common, forming through the precipitation of iron oxides around a nucleus in sedimentary environments. These can range from small, pea-sized ooids to large, irregular masses. High-grade iron ores are typically rich in hematite, while lower-grade ores often contain significant goethite and other iron hydroxides, sometimes collectively referred to as limonite.
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 fibrous, botryoidal, stalactitic, or earthy, commonly forming concretions and nodules.
- Crystal Form
- Hematite: Rhombohedral crystals, tabular, massive, botryoidal. Goethite: Orthorhombic crystals, but typically massive, fibrous, or botryoidal.
- 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
- Hematite: Found in Banded Iron Formations (BIFs), hydrothermal veins, contact metamorphic deposits, and as a weathering product. Goethite: Common in lateritic soils, bog iron deposits, gossans (weathered sulfide deposits), and as concretions in sedimentary 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, red; Goethite: Yellowish-brown, dark brown, black
- Luster: Hematite: Metallic, submetallic, dull, earthy; Goethite: Adamantine, silky, dull, earthy
- Transparency: Opaque
- Fracture: Hematite: Uneven to subconchoidal; Goethite: Uneven
- Cleavage: Hematite: None (parting common); Goethite: Perfect on {010} (rarely observed)
- Composition: Hematite: Iron(III) oxide (Fe2O3); Goethite: Iron(III) oxyhydroxide (FeO(OH))
Quick Check
- Color: Hematite: Steel-gray to red; 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: Prismatic, acicular, fibrous, botryoidal, stalactitic, massive, earthy, oolitic, concretionary.
- Cleavage Type: Hematite: None, 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 those composed of hematite and goethite, form through a variety of geological processes. Banded Iron Formations (BIFs), the largest source of iron ore, are Precambrian sedimentary rocks formed by the precipitation of iron oxides and silica from ancient oceans, likely due to the oxygenation of the atmosphere by photosynthetic organisms. Later enrichment of BIFs through supergene processes (weathering and leaching) can form high-grade hematite deposits. Goethite and other hydrous iron oxides often form in surficial environments through the weathering of iron-bearing minerals, accumulating as concretions, nodules, or bog iron deposits in swamps and shallow marine settings. Hydrothermal processes can also deposit iron oxides.
Usage
Primarily used for the production of iron and steel, which are fundamental to construction, manufacturing, infrastructure, and transportation. Other uses include pigments (ochre), heavy media separation, and as a polishing agent (rouge).
Age Distribution
Precambrian to Cenozoic, with major deposits in the Precambrian (BIFs)
Where to Find
Pilbara Region, Western Australia
One of the world's largest iron ore provinces, dominated by high-grade hematite deposits derived from enriched BIFs.
Minas Gerais, Brazil
Significant deposits of high-grade hematite (itabirite) and goethite, often associated with BIFs.
Lake Superior Region, USA and Canada
Historically and currently a major source of iron ore, primarily from BIFs (e.g., Mesabi Range).
Krivoy Rog, Ukraine
Large deposits of high-grade hematite and magnetite associated with Precambrian BIFs.
Mauritania
Significant iron ore deposits, including hematite and goethite, often in BIFs.
Finding Tips
Look for Reddish Stains
Iron oxides, especially hematite, will often stain surrounding rocks and soil a distinctive reddish-brown color. This can be a good indicator of iron ore presence.
Check for Density
Iron ore minerals are relatively dense. Picking up a sample and feeling its weight can help distinguish it from lighter, non-ore minerals.
Perform a Streak Test
Hematite produces a characteristic reddish-brown streak, while goethite produces a yellowish-brown streak. This is a crucial diagnostic test.
Examine Concretions and Nodules
In sedimentary environments, look for rounded to irregular, often heavy, dark brown to black concretions or nodules, which are frequently composed of goethite or hematite.
Use a Magnet (for Magnetite, but also for weakly magnetic hematite)
While hematite and goethite are generally not strongly magnetic, some varieties of hematite can be weakly magnetic. Magnetite (Fe3O4), often associated with iron ore, is strongly magnetic and will readily attract a magnet.
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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