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Limonite is not a true mineral but rather a mixture of hydrated iron oxide minerals, primarily goethite (FeO(OH)) and lepidocrocite (γ-FeO(OH)), often with amorphous ferric oxyhydroxides. It is characterized by its earthy, dull luster and various shades of yellow, brown, and reddish-brown. Limonite concretions are typically rounded, nodular, or irregular masses that form within sedimentary rocks or soils. They can range in size from a few millimeters to several meters and often exhibit concentric layering or internal structures reflecting their growth process. Limonite is a common product of weathering and is ubiquitous in many geological environments.
How to Identify
- Color
- Yellow, yellowish-brown, brown, reddish-brown, dark brown to black. Often has a rusty appearance.
- Luster
- Dull, earthy, sometimes submetallic to silky in botryoidal or stalactitic forms.
- Texture
- Earthy, porous, often botryoidal, mammillary, stalactitic, or concretionary. Concretions can be smooth or rough on the exterior.
- Crystal Form
- Amorphous or cryptocrystalline; does not form distinct crystals. Concretions are typically rounded to irregular masses.
- Cleavage
- None, as it is amorphous or cryptocrystalline.
- Geological Environment
- Common in oxidized zones of ore deposits, bogs, swamps, marshes, lateritic soils, and as a weathering product of iron-bearing rocks and minerals. Found as coatings, stains, and concretions in sedimentary rocks.
Key Facts
- Hardness: 4 to 5.5 (Mohs scale)
- Specific Gravity: 2.7 to 4.3 (variable due to porosity and composition)
- Crystal System: Amorphous or cryptocrystalline (composed of microcrystalline goethite and lepidocrocite, which are orthorhombic)
- Color: Yellow, yellowish-brown, brown, reddish-brown, dark brown to black
- Luster: Dull, earthy, sometimes submetallic or silky
- Transparency: Opaque
- Fracture: Conchoidal, uneven, earthy
- Cleavage: None
- Composition: Hydrated iron oxides, primarily FeO(OH)·nH2O (a mixture of goethite, lepidocrocite, and amorphous ferric oxyhydroxides)
Quick Check
- Color: Yellow, brown, reddish-brown
- Luster: Dull, earthy
- Streak: Yellowish-brown
Physical Characteristics
- Crystal Habit: Massive, earthy, botryoidal, mammillary, stalactitic, concretionary, oolitic, pisolitic
- Cleavage Type: None
- Fracture Type: Conchoidal, uneven, earthy
- Tenacity: Brittle
- Luster Type: Dull, earthy, submetallic, silky
Formation
Limonite is a secondary mineraloid formed by the hydration and oxidation of primary iron-bearing minerals (e.g., pyrite, siderite, magnetite, hematite) under surficial conditions. It commonly forms in swamps, bogs, and other wetland environments where iron-rich waters interact with organic matter and oxygen, leading to the precipitation of hydrated iron oxides. It can also form as a weathering product in iron ore deposits or as a cement in sedimentary rocks. Concretions specifically form when iron oxides precipitate around a nucleus (e.g., a shell fragment, a grain of sand, or organic matter) within a porous sediment, growing outwards in concentric layers.
Usage
Historically, limonite was a significant iron ore, particularly bog iron. It has been used as a pigment (ochre) since prehistoric times, providing yellow, brown, and reddish-brown colors. It is also used as a polishing agent and, in some cases, as a soil amendment. Limonite concretions are often collected for their aesthetic and geological interest.
Age Distribution
Forms continuously in surficial environments; can be found in deposits ranging from recent to ancient geological periods.
Where to Find
Mesabi Range, Minnesota, USA
Known for extensive iron ore deposits, where limonite forms as a weathering product and in associated soils.
Krivoy Rog, Ukraine
Major iron ore region with significant occurrences of hydrated iron oxides.
Pilbara Region, Western Australia
World-class iron ore province where limonite is a common component of weathered iron formations.
Various bog iron deposits worldwide
Found in wetland environments across many continents, including North America, Europe, and Asia.
Sedimentary basins globally
Commonly found as concretions in sandstones, shales, and conglomerates, often associated with fossiliferous horizons.
Finding Tips
Look for Rusty Stains
Limonite often manifests as rusty-brown or yellowish-brown stains on rocks, soil, and in water bodies, indicating its presence.
Check Wetland Environments
Bogs, swamps, and marshy areas are prime locations for 'bog iron' deposits, which are largely composed of limonite.
Examine Weathered Outcrops
Look for limonite in the oxidized zones (gossans) above sulfide ore deposits or as a weathering product of iron-rich rocks.
Search Sedimentary Rocks
Concretions are frequently found in sedimentary layers, particularly in sandstones and shales. Look for rounded or irregular masses that stand out from the surrounding rock.
Perform a Streak Test
Limonite consistently produces a yellowish-brown streak, which is a key diagnostic property to distinguish it from other iron oxides like hematite (red streak).
Similar Rocks
Hematite
Fe2O3
Also known as: Red Iron Ore
Goethite
FeO(OH)
Also known as: Alpha-FeOOH
Jarosite
KFe3(SO4)2(OH)6
Also known as: Potassium Iron Sulfate Hydroxide
Siderite
FeCO3
Also known as: Iron Carbonate
Scientific Classification
- Mineral Class
- Oxides and Hydroxides
- Group
- Hydroxides (specifically, a mineraloid mixture of hydrated iron oxides)
- Crystal System
- Amorphous (constituent minerals goethite and lepidocrocite are orthorhombic)
- Chemical Formula
- FeO(OH)·nH2O (generalized, as it's a mixture)
- Composition
- Hydrated iron oxides, predominantly goethite and lepidocrocite, with variable water content and impurities.
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