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Limonite is a general term for a mixture of hydrous iron oxide minerals, predominantly Goethite (α-FeO(OH)) and Lepidocrocite (γ-FeO(OH)). It is not a true mineral in the strict sense but rather a mineraloid or a mixture. Goethite, however, is a distinct mineral. Both are characterized by their earthy, yellowish-brown to dark brown color and streak. They are common products of the weathering of other iron-bearing minerals and are often found in sedimentary environments. They are important iron ores, though their iron content is generally lower than that of hematite or magnetite due to the presence of water.
How to Identify
- Color
- Yellowish-brown, reddish-brown, dark brown, blackish-brown. Often has a rusty appearance.
- Luster
- Earthy, dull, submetallic to silky (for botryoidal or stalactitic forms of goethite).
- Texture
- Earthy, porous, often concretionary, botryoidal, stalactitic, or mammillary. Can be massive.
- Crystal Form
- Limonite is amorphous or cryptocrystalline. Goethite forms prismatic, acicular, or tabular crystals, but is more commonly found in massive, botryoidal, reniform, stalactitic, or earthy habits.
- Cleavage
- None for limonite. Goethite has perfect cleavage on {010}, but it is rarely observed in massive forms.
- Geological Environment
- Weathering zones of iron-rich rocks, bog iron deposits, gossans over sulfide ore bodies, sedimentary iron formations, and as concretions in soils and sediments.
Key Facts
- Hardness: Limonite: 4-5.5 (Mohs); Goethite: 5-5.5 (Mohs)
- Specific Gravity: Limonite: 2.7-4.3; Goethite: 3.3-4.3
- Crystal System: Limonite: Amorphous/Cryptocrystalline; Goethite: Orthorhombic
- Color: Yellowish-brown, reddish-brown, dark brown, blackish-brown
- Luster: Earthy, dull, submetallic to silky
- Transparency: Opaque
- Fracture: Conchoidal to uneven, earthy
- Cleavage: Limonite: None; Goethite: Perfect on {010}, but rarely observed
- Composition: Limonite: Hydrous iron oxides (Fe2O3·nH2O), primarily Goethite and Lepidocrocite; Goethite: Iron oxyhydroxide (α-FeO(OH))
Quick Check
- Color: Yellowish-brown to dark brown
- Luster: Earthy, dull, submetallic
- Streak: Yellowish-brown to brownish-yellow
Physical Characteristics
- Crystal Habit: Limonite: Massive, earthy, concretionary, botryoidal, stalactitic. Goethite: Prismatic, acicular, tabular, but commonly massive, botryoidal, reniform, stalactitic, or earthy.
- Cleavage Type: Limonite: None. Goethite: Perfect on {010}, but typically not visible in massive forms.
- Fracture Type: Conchoidal to uneven, earthy.
- Tenacity: Brittle to friable (for earthy varieties).
- Luster Type: Earthy, dull, submetallic, silky (for fibrous goethite).
Formation
Limonite and Goethite typically form as secondary minerals through the weathering and oxidation of primary iron-bearing minerals (e.g., pyrite, magnetite, siderite) in sedimentary environments. They can precipitate from iron-rich solutions in bogs, swamps, and shallow marine settings, forming bog iron ores. They also form as residual deposits from the weathering of iron-rich rocks and as gossans (oxidized caps) over sulfide ore bodies.
Usage
Historically, limonite was a significant iron ore. Today, while still used, it is less preferred than hematite or magnetite due to its lower iron content and higher water content. It is also used as a pigment (ochre) and as a polishing agent. Goethite is a primary component of many iron ores and is a significant source of iron.
Age Distribution
Found in geological formations ranging from Precambrian to Cenozoic, often associated with weathering processes.
Where to Find
Mesabi Range, Minnesota, USA
Historically a major source of iron ore, including significant goethite-rich taconite formations.
Krivoy Rog, Ukraine
One of the world's largest iron ore basins, containing various iron oxides including goethite.
Minas Gerais, Brazil
Known for vast iron ore deposits, including those rich in goethite and hematite.
Pilbara Region, Western Australia
A globally significant iron ore province, with some deposits containing goethite.
Lorraine Basin, France
Historically important for its oolitic iron ores, which are often limonite-rich.
Finding Tips
Look for Rusty Stains
Limonite and goethite are often associated with rusty-colored stains on rocks and in soils, indicating the presence of oxidized iron.
Check Weathering Zones
Explore areas where iron-bearing rocks have undergone significant weathering, such as outcrops, stream beds, and road cuts.
Examine Boggy Areas
Bog iron deposits can be found in swampy or marshy environments where iron has precipitated from groundwater.
Observe Gossans
In mining districts, look for gossans (iron-rich caps) over sulfide ore bodies, which often contain limonite and goethite.
Perform a Streak Test
The characteristic yellowish-brown to brownish-yellow streak is a key diagnostic feature for limonite/goethite, differentiating it from hematite (red streak) and magnetite (black streak).
Similar Rocks
Hematite
Fe2O3
Also known as: Red Iron Ore, Specularite
Magnetite
Fe3O4
Also known as: Magnetic Iron Ore
Siderite
FeCO3
Also known as: Iron Carbonate
Scientific Classification
- Mineral Class
- Oxides and Hydroxides
- Group
- Goethite Group (for Goethite)
- Crystal System
- Limonite: Amorphous/Cryptocrystalline; Goethite: Orthorhombic
- Chemical Formula
- Limonite: FeO(OH)·nH2O (approximate); Goethite: α-FeO(OH)
- Composition
- Hydrous iron oxides (Limonite); Iron oxyhydroxide (Goethite)
Explore Iron Ore (Limonite/Goethite)
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