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Limonite is a general term for a mixture of hydrated 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 significant iron ores, characterized by their earthy, yellowish-brown to dark brown colors and often botryoidal, stalactitic, or earthy habits. They are products of the oxidation and hydration of other iron minerals.
How to Identify
- Color
- Yellowish-brown, reddish-brown, dark brown, blackish-brown. Often has a rusty appearance.
- Luster
- Dull, earthy, submetallic, silky (in fibrous varieties).
- Texture
- Earthy, porous, botryoidal, stalactitic, mammillary, concretionary, or massive.
- Crystal Form
- Limonite is amorphous or cryptocrystalline. Goethite forms acicular, prismatic, or tabular crystals, often radiating or fibrous, but more commonly massive, reniform, or stalactitic.
- Cleavage
- None for Limonite. Goethite has perfect cleavage on {010} and good on {100}.
- Geological Environment
- Oxidized zones of ore deposits (gossans), bog iron deposits, lateritic soils, sedimentary iron formations, as concretions in sandstones and shales, and as a weathering product of iron-rich rocks.
Key Facts
- Hardness: 4.0-5.5 (Goethite), variable (Limonite, often softer, 1-5.5)
- Specific Gravity: 3.3-4.3 (Goethite), 2.7-4.3 (Limonite, variable due to porosity and impurities)
- Crystal System: Orthorhombic (Goethite), Amorphous/Cryptocrystalline (Limonite)
- Color: Yellowish-brown, reddish-brown, dark brown, blackish-brown
- Luster: Dull, earthy, submetallic, silky
- Transparency: Opaque
- Fracture: Uneven, conchoidal (Goethite), earthy (Limonite)
- Cleavage: Perfect on {010}, good on {100} (Goethite); None (Limonite)
- Composition: Hydrated iron oxide (Goethite: α-FeO(OH); Limonite: a mixture, primarily Goethite and Lepidocrocite)
Quick Check
- Color: Yellowish-brown to dark brown
- Luster: Dull, earthy, submetallic
- Streak: Yellowish-brown to brownish-yellow
Physical Characteristics
- Crystal Habit: Acicular, prismatic, tabular, radiating, fibrous, massive, reniform, stalactitic, botryoidal, earthy, concretionary.
- Cleavage Type: Perfect on {010}, good on {100} (Goethite); None (Limonite).
- Fracture Type: Uneven to subconchoidal (Goethite); Earthy (Limonite).
- Tenacity: Brittle.
- Luster Type: Dull, earthy, submetallic, silky.
Formation
Limonite and Goethite typically form as secondary minerals through the weathering of primary iron-bearing minerals (e.g., pyrite, magnetite, hematite, siderite) in oxidizing environments. They are common in gossans, bog iron deposits, and as cementing agents in sandstones and conglomerates. Goethite can also precipitate directly from iron-rich waters.
Usage
Primarily as an iron ore for steel production. Historically used as a pigment (ochre) and as a polishing agent.
Age Distribution
Can form throughout geological time, often associated with weathering processes and sedimentary environments.
Where to Find
Mesabi Range, Minnesota, USA
Historically a major source of iron ore, including taconite (a low-grade iron formation) which, after processing, yields iron concentrates. Limonite and Goethite are found in weathered zones.
Krivoy Rog, Ukraine
One of the world's largest iron ore basins, containing significant deposits of various iron ores, including hydrated forms.
Carajás Mine, Pará, Brazil
Known for its high-grade hematite, but also contains significant amounts of hydrated iron oxides in weathered profiles.
Pilbara Region, Western Australia
A globally significant iron ore province, primarily hematite, but Goethite and Limonite are present as weathering products and in some ore bodies.
Bog Iron Deposits (various locations worldwide)
Formed in wetlands and bogs where iron-rich groundwater precipitates hydrated iron oxides. Common in temperate and boreal regions.
Finding Tips
Look for Rusty Stains
Limonite and Goethite often manifest as rusty-brown stains or coatings on other rocks, indicating the presence of iron oxidation.
Check Weathered Zones
Explore the oxidized caps (gossans) of sulfide ore deposits or areas where iron-rich rocks have undergone significant weathering.
Investigate Bogs and Wetlands
Bog iron deposits can be found in marshy areas where iron has precipitated from groundwater, forming concretions or layers.
Examine Sedimentary Rocks
Limonite and Goethite can act as cementing agents in sandstones and conglomerates, or form concretions within shales and mudstones.
Perform a Streak Test
A 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).
Similar Rocks
Hematite
Fe2O3
Also known as: Red Iron Ore, Kidney 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-Diaspore Group (for Goethite)
- Crystal System
- Orthorhombic (Goethite); Amorphous/Cryptocrystalline (Limonite)
- Chemical Formula
- α-FeO(OH) (Goethite); FeO(OH)·nH2O (Limonite, generalized)
- Composition
- Hydrated iron oxide. Goethite is iron oxyhydroxide. Limonite is a mixture of hydrated iron oxides, often including Goethite, Lepidocrocite, and amorphous ferric hydroxides.
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