Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Limonite is a general term for a mixture of amorphous or poorly crystalline hydrous iron oxide minerals, primarily goethite (α-FeOOH) and lepidocrocite (γ-FeOOH), often with adsorbed water and other impurities. It is not a true mineral in the strict sense because it lacks a definite crystal structure and chemical composition. It is characterized by its earthy, dull appearance and yellow to brown color. It is a common product of the weathering of iron-rich rocks and minerals.
How to Identify
- Color
- Typically yellow, yellowish-brown, reddish-brown, or dark brown to black. The color can vary depending on the degree of hydration and impurities.
- Luster
- Dull, earthy, or submetallic. Rarely silky in fibrous varieties.
- Texture
- Earthy, massive, botryoidal, stalactitic, mammillary, or concretionary. Can also be porous or pisolitic.
- Crystal Form
- Amorphous or cryptocrystalline; does not form distinct crystals. Often found as pseudomorphs after other iron minerals.
- Cleavage
- None, due to its amorphous or cryptocrystalline nature.
- Geological Environment
- Common in oxidized zones of ore deposits (gossans), bog iron deposits, lateritic soils, and as a weathering product of iron-bearing rocks in humid climates. Also found as coatings on other minerals and rocks.
Key Facts
- Hardness: 4 - 5.5 (Mohs scale)
- Specific Gravity: 2.7 - 4.3 (variable, depending on composition and porosity)
- Crystal System: Amorphous or cryptocrystalline (no defined crystal system)
- Color: Yellow, yellowish-brown, reddish-brown, dark brown to black
- Luster: Dull, earthy, submetallic
- Transparency: Opaque
- Fracture: Conchoidal, uneven, earthy
- Cleavage: None
- Composition: Hydrous iron oxides, primarily goethite (α-FeOOH) and lepidocrocite (γ-FeOOH), with variable water content and impurities.
Quick Check
- Color: Yellow, brown, reddish-brown, dark brown to black
- Luster: Dull, earthy, submetallic
- Streak: Yellowish-brown
Physical Characteristics
- Crystal Habit: Massive, earthy, botryoidal, stalactitic, mammillary, pisolitic, concretionary, reniform, fibrous, or as pseudomorphs.
- Cleavage Type: None
- Fracture Type: Conchoidal, uneven, earthy
- Tenacity: Brittle to friable
- Luster Type: Dull, earthy, submetallic
Formation
Limonite forms as a secondary product from the weathering and oxidation of primary iron-bearing minerals (e.g., pyrite, siderite, magnetite, hematite) in the presence of water and oxygen. It is a common component of gossans (oxidized caps of ore deposits), lateritic soils, and bog iron deposits.
Usage
Historically, limonite was a significant iron ore, particularly for early iron smelting. It is also used as a pigment (ochre) in paints, ceramics, and cosmetics. In some regions, it is used as a natural fertilizer due to its iron content.
Age Distribution
Found in geological formations of all ages where iron-bearing rocks have been exposed to weathering.
Where to Find
Mesabi Range, Minnesota, USA
Historically a major source of iron ore, including limonite, formed from the weathering of taconite.
Krivoy Rog, Ukraine
Significant iron ore deposits, where limonite is a common component of the weathered iron formations.
Lorraine, France
Famous for its oolitic iron ores, which are largely composed of limonite.
Cuba
Extensive lateritic nickel-cobalt-iron deposits where limonite is a major constituent.
Brazil
Large deposits of lateritic iron ore, often rich in limonite, particularly in the 'Iron Quadrangle' region.
Finding Tips
Look for Weathered Iron Deposits
Limonite is a secondary mineral, so search in areas where primary iron-bearing minerals (like pyrite, siderite, or magnetite) have undergone significant weathering and oxidation. Gossans (iron-stained caps over sulfide ore bodies) are excellent places to look.
Check Boggy or Swampy Areas
Bog iron deposits, which are accumulations of limonite in wetlands, are common. Look for reddish-brown to yellowish-brown crusts or nodules in these environments.
Examine Lateritic Soils
In tropical and subtropical regions, lateritic soils, which are rich in iron and aluminum oxides, often contain significant amounts of limonite.
Observe Color and Streak
The characteristic yellow-brown streak is a key identifier. Even if the specimen appears dark, its streak will be yellowish-brown.
Test for Hardness
Limonite is relatively soft (4-5.5 on Mohs scale), so it can be scratched by a steel file but will scratch copper.
Similar Rocks
Goethite
Goethite
Also known as: Alpha-FeOOH
Hematite
Hematite (Fe2O3)
Also known as: Iron Ore
Jarosite
Jarosite (KFe3(SO4)2(OH)6)
Also known as: Potassium Iron Sulfate Hydroxide
Siderite
Siderite (FeCO3)
Also known as: Iron Carbonate
Scientific Classification
- Mineral Class
- Oxides and Hydroxides (specifically, a mineraloid mixture of hydrous iron oxides)
- Group
- Iron Hydroxides
- Crystal System
- Amorphous or cryptocrystalline
- Chemical Formula
- FeO(OH)·nH2O (generalized, as it's a mixture)
- Composition
- Hydrous iron oxides, predominantly goethite and lepidocrocite, with variable amounts of adsorbed water and other impurities. The iron content can range from 40% to 60%.
Explore Limonite
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.