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Limonite is a general term for a mixture of hydrous iron oxide minerals, with goethite (α-FeO(OH)) being the most common and often dominant component. Other minerals that can be present include lepidocrocite (γ-FeO(OH)), ferrihydrite, and amorphous hydrous iron oxides. It is characterized by its earthy, often porous or botryoidal habit, and its distinctive yellowish-brown to dark brown color and streak. It is a secondary mineral, forming under ambient conditions, and is never found as primary igneous or metamorphic rock-forming mineral.
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. Can also be massive or fibrous.
- Crystal Form
- Goethite forms acicular (needle-like), fibrous, bladed, or tabular crystals, but is most commonly found in massive, botryoidal, reniform, or stalactitic aggregates. Limonite, being a mixture, does not have a distinct crystal form but rather an aggregate habit.
- Cleavage
- Goethite has perfect cleavage on {010}, but it is rarely observed in massive or fine-grained aggregates. Limonite exhibits no cleavage.
- Geological Environment
- Weathering zones of iron-rich rocks and sulfide deposits, bog and swamp deposits, lateritic soils, and as cementing material in sandstones and conglomerates.
Key Facts
- Hardness: Goethite: 5-5.5 on Mohs scale. Limonite: Varies, typically 4-5.5 (due to its aggregate nature and impurities).
- Specific Gravity: Goethite: 3.3-4.3. Limonite: Varies, typically 2.7-4.3 (depending on porosity and composition).
- Crystal System: Goethite: Orthorhombic. Limonite: Amorphous to cryptocrystalline (not a true mineral with a defined crystal system).
- Color: Yellowish-brown, reddish-brown, dark brown, black.
- Luster: Dull, earthy, submetallic, silky.
- Transparency: Opaque.
- Fracture: Uneven, conchoidal (for goethite), earthy.
- Cleavage: Goethite: Perfect on {010}, but rarely seen. Limonite: None.
- Composition: Goethite: Hydrous iron oxide (α-FeO(OH)). Limonite: A mixture of hydrous iron oxides, predominantly goethite, with variable amounts of water and impurities.
Quick Check
- Color: Yellowish-brown to dark brown/black
- Luster: Dull, earthy, submetallic
- Streak: Yellowish-brown to brownish-yellow
Physical Characteristics
- Crystal Habit: Goethite: Acicular, fibrous, bladed, tabular, massive, botryoidal, reniform, stalactitic. Limonite: Massive, earthy, porous, concretionary, mammillary.
- Cleavage Type: Goethite: Perfect {010}. Limonite: None.
- Fracture Type: Goethite: Uneven to subconchoidal. Limonite: Earthy.
- Tenacity: Brittle.
- Luster Type: Dull, earthy, submetallic, silky.
Formation
Limonite and goethite form primarily as secondary minerals through the weathering of iron-bearing minerals (e.g., pyrite, magnetite, hematite, siderite) in oxidizing environments. They can precipitate from iron-rich waters in bogs, swamps, and shallow marine environments, forming bog iron ores. They also form as gossans (iron hats) above sulfide ore deposits and as lateritic soils in tropical and subtropical regions.
Usage
Historically, limonite was a significant iron ore. Today, goethite-rich iron ores are still mined, though often blended with higher-grade hematite. It is also used as a pigment (ochre) in paints, ceramics, and cosmetics. Goethite is a component of some soils and plays a role in environmental remediation due to its adsorptive properties.
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 goethite-rich taconite formations.
Lorraine, France
Famous for its oolitic iron ores, which are often rich in goethite.
Krivoy Rog, Ukraine
Significant iron ore deposits, including those with goethite.
Minas Gerais, Brazil
Known for vast iron ore deposits, including lateritic goethite ores.
Pilbara Region, Western Australia
World-class iron ore province, with some deposits containing goethite.
Bog Iron Deposits
Found in temperate and boreal regions worldwide, in bogs, swamps, and shallow lakes where iron precipitates from groundwater.
Finding Tips
Look for Rusty Stains
Limonite/goethite often forms as a result of weathering and oxidation, so look for rusty-colored stains on rocks, soil, and in water bodies.
Check Weathered Outcrops
Exposed rock outcrops, especially those containing iron-bearing minerals, are good places to find limonite/goethite as a weathering product or gossan.
Explore Bogs and Swamps
Bog iron deposits can be found in wetland environments where iron-rich groundwater emerges and oxidizes.
Examine Lateritic Soils
In tropical and subtropical regions, look for reddish-brown, iron-rich soils (laterites) which often contain significant amounts of goethite.
Observe Streak Color
The characteristic yellowish-brown to brownish-yellow streak is a key diagnostic feature. Rub the specimen on an unglazed porcelain streak plate.
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
Pyrite
FeS2
Also known as: Fool's Gold
Scientific Classification
- Mineral Class
- Oxides and Hydroxides
- Group
- Goethite Group (for Goethite)
- Crystal System
- Orthorhombic (for Goethite)
- Chemical Formula
- Goethite: α-FeO(OH). Limonite: FeO(OH)·nH2O (general representation for a mixture)
- Composition
- Hydrous iron oxide. Goethite is specifically alpha-iron(III) oxyhydroxide. Limonite is a mixture of various hydrous iron(III) oxides, often including goethite, lepidocrocite, and ferrihydrite, with adsorbed water.
Explore Limonite/Goethite (Hydrous Iron Oxides)
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