How to identify Limonite

To correctly identify Limonite (Limonite (hydrous iron oxide)), 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

   Typically yellow, yellowish-brown, reddish-brown, or dark brown to black. The color can vary depending on the degree of hydration and impurities.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, or submetallic. Rarely silky in fibrous varieties.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

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

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous or cryptocrystalline; does not form distinct crystals. Often found as pseudomorphs after other iron minerals.

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 ore deposits (gossans), bog iron deposits, lateritic soils, and as a weathering product of iron-bearing rocks in humid climates. Also found as coatings on other minerals and rocks.

Key Facts

Hardness

4 - 5.5 (Mohs scale)

Specific Gravity

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

Crystal System

Amorphous or cryptocrystalline (no defined crystal system)

Color

Yellow, yellowish-brown, reddish-brown, dark brown to black

Luster

Dull, earthy, submetallic

Transparency

Opaque

Fracture

Conchoidal, uneven, earthy

Cleavage

None

Composition

Hydrous iron oxides, primarily goethite (α-FeOOH) and lepidocrocite (γ-FeOOH), with variable water content and impurities.

Physical characteristics

• Crystal Habit: Massive, earthy, botryoidal, stalactitic, mammillary, pisolitic, concretionary, reniform, fibrous, or as pseudomorphs.
• Cleavage Type: None
• Fracture Type: Conchoidal, uneven, earthy
• Tenacity: Brittle to friable
• Luster Type: Dull, earthy, submetallic

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