How to identify Limonite

To correctly identify Limonite (Limonite), 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, yellowish-brown, brown, reddish-brown, dark brown to black. Often has a rusty appearance.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, sometimes submetallic to silky in botryoidal or stalactitic forms.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Earthy, porous, often botryoidal, mammillary, stalactitic, or concretionary. Concretions can be smooth or rough on the exterior.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amorphous or cryptocrystalline; does not form distinct crystals. Concretions are typically rounded to irregular masses.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as it is amorphous or cryptocrystalline.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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)

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

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