How to identify Limonite

To correctly identify Limonite (FeO(OH)·nH2O), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Yellow-brown, reddish-brown, dark brown, or black. Often has a characteristic earthy yellow-brown color.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Earthy, dull, sometimes submetallic in botryoidal or stalactitic forms.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Earthy, porous, often botryoidal, mammillary, stalactitic, or concretionary. Can be massive or pisolitic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous or cryptocrystalline; does not form distinct crystals. Occurs as earthy masses, concretions, stalactites, or coatings.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, due to its amorphous or cryptocrystalline nature.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in oxidized zones of iron ore deposits, gossans, bog iron deposits, lateritic soils, and as a weathering product of iron-bearing minerals in sedimentary and metamorphic rocks. Forms in both terrestrial and aquatic environments where iron-rich solutions are exposed to oxygen.

Key Facts

Hardness

4 to 5.5 on the Mohs scale (variable due to its composite nature)

Specific Gravity

2.7 to 4.3 (variable, depending on composition and porosity)

Crystal System

Amorphous or cryptocrystalline (no defined crystal system)

Color

Yellow-brown, reddish-brown, dark brown, black

Luster

Earthy, dull, sometimes submetallic

Transparency

Opaque

Fracture

Conchoidal, uneven, earthy

Cleavage

None

Composition

Hydrated iron oxides, primarily goethite (FeO(OH)) and lepidocrocite (γ-FeO(OH)), with variable water content and impurities.

Physical characteristics

• Crystal Habit: Massive, earthy, botryoidal, mammillary, stalactitic, concretionary, pisolitic, oolitic, reniform, coatings.
• Cleavage Type: None
• Fracture Type: Conchoidal, uneven, earthy
• Tenacity: Brittle to friable
• Luster Type: Earthy, dull, sometimes submetallic

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