How to identify Limonite

To correctly identify Limonite (Iron oxyhydroxide), 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

   Yellowish-brown, brown, reddish-brown, dark brown, or blackish-brown. Often has a rusty appearance.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, submetallic, or silky (when fibrous).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Earthy, porous, botryoidal, mammillary, stalactitic, or concretionary. Can be massive or form coatings.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Typically amorphous or cryptocrystalline; does not form distinct crystals. Often forms pseudomorphs after pyrite or other iron minerals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is a mineraloid and typically amorphous.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Oxidized zones of ore deposits, bogs, swamps, shallow marine sediments, lateritic soils, and as a weathering product of iron-rich rocks and minerals.

Key Facts

Hardness

4 to 5.5 on the Mohs scale (variable due to mixed composition)

Specific Gravity

2.7 to 4.3 (variable, typically 3.3 to 3.9)

Crystal System

Amorphous or cryptocrystalline (no defined crystal system)

Color

Yellowish-brown, brown, reddish-brown, dark brown, blackish-brown

Luster

Dull, earthy, submetallic, silky

Transparency

Opaque

Fracture

Conchoidal, uneven, earthy

Cleavage

None

Composition

Hydrated iron(III) oxide, primarily a mixture of goethite and lepidocrocite, often with adsorbed water and impurities. Approximate formula FeO(OH)·nH2O.

Physical characteristics

• Crystal Habit: Massive, earthy, botryoidal, mammillary, stalactitic, concretionary, oolitic, fibrous, or as pseudomorphs.
• Cleavage Type: Absent
• Fracture Type: Conchoidal, uneven, earthy
• Tenacity: Brittle to friable
• Luster Type: Dull, earthy, submetallic, silky

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