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Limonite is not a true mineral but a mixture of hydrated iron oxides, primarily goethite. It is characterized by its earthy, often porous or botryoidal habit and its distinctive yellowish-brown to dark brown color. Goethite, a key component of limonite, is a distinct mineral with a well-defined crystal structure. Both are important iron ores and are ubiquitous in weathered iron-rich rocks and soils.
How to Identify
- Color
- Yellowish-brown, reddish-brown, dark brown, or black. Often has a rusty appearance.
- Luster
- Dull, earthy, submetallic, or silky (in fibrous varieties of goethite).
- Texture
- Earthy, porous, botryoidal (grape-like), stalactitic, mammillary, or concretionary. Goethite can be fibrous or massive.
- Crystal Form
- Limonite is amorphous or cryptocrystalline, lacking distinct crystal forms. Goethite forms prismatic, acicular (needle-like), or tabular crystals, but is more commonly found in massive, reniform, botryoidal, or stalactitic habits.
- Cleavage
- Limonite has no cleavage. Goethite has one perfect cleavage {010}, but it is rarely observed in massive forms.
- Geological Environment
- Common in the oxidized zones of iron ore deposits, gossans, lateritic soils, bog iron deposits, and as a weathering product of iron-bearing minerals in sedimentary rocks.
Key Facts
- Hardness: Limonite: 4-5.5 (Mohs); Goethite: 5-5.5 (Mohs)
- Specific Gravity: Limonite: 2.7-4.3 (variable due to porosity and water content); Goethite: 3.3-4.3
- Crystal System: Limonite: Amorphous/Cryptocrystalline; Goethite: Orthorhombic
- Color: Yellowish-brown, reddish-brown, dark brown, black
- Luster: Dull, earthy, submetallic, silky
- Transparency: Opaque
- Fracture: Conchoidal to uneven, earthy
- Cleavage: Limonite: None; Goethite: Perfect on {010}, but rarely observed
- Composition: Hydrous iron oxides, primarily FeO(OH)·nH2O (Limonite) and FeO(OH) (Goethite)
Quick Check
- Color: Yellowish-brown to dark brown
- Luster: Dull, earthy, submetallic
- Streak: Yellowish-brown to brownish-yellow
Physical Characteristics
- Crystal Habit: Limonite: Massive, earthy, botryoidal, stalactitic, concretionary. Goethite: Prismatic, acicular, tabular, massive, reniform, botryoidal, stalactitic.
- Cleavage Type: Limonite: None. Goethite: Perfect on {010}.
- Fracture Type: Conchoidal to uneven, earthy.
- Tenacity: Brittle.
- Luster Type: Dull, earthy, submetallic, silky.
Formation
Limonite is a general term for a mixture of hydrous iron oxide minerals, predominantly goethite (α-FeO(OH)), but also lepidocrocite (γ-FeO(OH)), and ferrihydrite. It forms as a secondary mineral through the weathering and oxidation of primary iron-bearing minerals (e.g., pyrite, siderite, magnetite, hematite) in sedimentary and low-temperature hydrothermal environments. It commonly precipitates from iron-rich waters in bogs, swamps, and shallow marine environments, or as a residual product of lateritic weathering.
Usage
Historically, limonite was a significant iron ore, particularly in bog iron deposits. Today, while still a source of iron, it is less preferred than hematite or magnetite due to its lower iron content and higher water content, which increases processing costs. It is also used as a pigment (ochre) and as a weighting agent in drilling muds.
Age Distribution
Found in geological formations ranging from Precambrian to recent, often associated with weathering processes.
Where to Find
Mesabi Range, Minnesota, USA
Historically a major source of iron ore, including limonite and goethite, derived from the weathering of taconite.
Krivoy Rog, Ukraine
Significant iron ore deposits, including goethite, associated with banded iron formations.
Minas Gerais, Brazil
Vast iron ore deposits, including goethite, often found in lateritic profiles.
Pilbara region, Western Australia
World-class iron ore deposits, with goethite being a significant component in some weathered zones.
Bog Iron Deposits (worldwide)
Found in wetlands, bogs, and swamps where iron-rich groundwater precipitates hydrous iron oxides.
Finding Tips
Look for Rusty Stains
Limonite/goethite often manifests as rusty-colored stains or coatings on other rocks, particularly in areas with iron-rich groundwater or weathered iron-bearing minerals.
Check Weathered Zones
Explore the oxidized caps (gossans) of sulfide ore deposits or areas where iron-rich rocks have undergone significant weathering. Limonite is a common product in these environments.
Investigate Wetlands and Bogs
Bog iron deposits can be found in marshy areas where iron precipitates from water, forming concretions or layers near the surface.
Perform a Streak Test
The characteristic yellowish-brown to brownish-yellow streak is a key diagnostic feature for limonite/goethite, distinguishing it from other iron oxides like hematite (red streak) or magnetite (black streak).
Observe Physical Forms
Look for botryoidal, mammillary, or stalactitic forms, which are common habits for these minerals.
Similar Rocks
Hematite
Fe2O3
Also known as: Red Iron Ore
Magnetite
Fe3O4
Also known as: Magnetic Iron Ore
Siderite
FeCO3
Also known as: Iron Carbonate
Pyrite
FeS2
Also known as: Fool's Gold
Scientific Classification
- Mineral Class
- Oxides and Hydroxides
- Group
- Goethite Group (for Goethite)
- Crystal System
- Limonite: Amorphous/Cryptocrystalline; Goethite: Orthorhombic
- Chemical Formula
- FeO(OH)·nH2O (Limonite, general) / α-FeO(OH) (Goethite)
- Composition
- Hydrated iron oxide
Explore Limonite/Goethite
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