How to identify Limonite/Goethite

To correctly identify Limonite/Goethite (Hydrous iron oxides (e.g., 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

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

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, submetallic, or silky (in fibrous varieties of goethite).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Earthy, porous, botryoidal (grape-like), stalactitic, mammillary, or concretionary. Goethite can be fibrous or massive.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Limonite is amorphous or cryptocrystalline, lacking distinct crystal forms. Goethite forms prismatic, acicular (needle-like), or tabular crystals, but is more commonly found in massive, reniform, botryoidal, or stalactitic habits.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Limonite has no cleavage. Goethite has one perfect cleavage {010}, but it is rarely observed in massive forms.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in the oxidized zones of iron ore deposits, gossans, lateritic soils, bog iron deposits, and as a weathering product of iron-bearing minerals in sedimentary rocks.

Key Facts

Hardness

Limonite: 4-5.5 (Mohs); Goethite: 5-5.5 (Mohs)

Specific Gravity

Limonite: 2.7-4.3 (variable due to porosity and water content); Goethite: 3.3-4.3

Crystal System

Limonite: Amorphous/Cryptocrystalline; Goethite: Orthorhombic

Color

Yellowish-brown, reddish-brown, dark brown, black

Luster

Dull, earthy, submetallic, silky

Transparency

Opaque

Fracture

Conchoidal to uneven, earthy

Cleavage

Limonite: None; Goethite: Perfect on {010}, but rarely observed

Composition

Hydrous iron oxides, primarily FeO(OH)·nH2O (Limonite) and FeO(OH) (Goethite)

Physical characteristics

• Crystal Habit: Limonite: Massive, earthy, botryoidal, stalactitic, concretionary. Goethite: Prismatic, acicular, tabular, massive, reniform, botryoidal, stalactitic.
• Cleavage Type: Limonite: None. Goethite: Perfect on {010}.
• Fracture Type: Conchoidal to uneven, earthy.
• Tenacity: Brittle.
• Luster Type: Dull, earthy, submetallic, silky.

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