How to identify Iron Ore (Limonite/Goethite)

To correctly identify Iron Ore (Limonite/Goethite) (Limonite/Goethite), 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, blackish-brown. Often has a rusty appearance.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Earthy, dull, submetallic to silky (for botryoidal or stalactitic forms of goethite).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

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

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Limonite is amorphous or cryptocrystalline. Goethite forms prismatic, acicular, or tabular crystals, but is more commonly found in massive, botryoidal, reniform, stalactitic, or earthy habits.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None for limonite. Goethite has perfect cleavage on {010}, but it is rarely observed in massive forms.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Weathering zones of iron-rich rocks, bog iron deposits, gossans over sulfide ore bodies, sedimentary iron formations, and as concretions in soils and sediments.

Key Facts

Hardness

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

Specific Gravity

Limonite: 2.7-4.3; Goethite: 3.3-4.3

Crystal System

Limonite: Amorphous/Cryptocrystalline; Goethite: Orthorhombic

Color

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

Luster

Earthy, dull, submetallic to silky

Transparency

Opaque

Fracture

Conchoidal to uneven, earthy

Cleavage

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

Composition

Limonite: Hydrous iron oxides (Fe2O3·nH2O), primarily Goethite and Lepidocrocite; Goethite: Iron oxyhydroxide (α-FeO(OH))

Physical characteristics

• Crystal Habit: Limonite: Massive, earthy, concretionary, botryoidal, stalactitic. Goethite: Prismatic, acicular, tabular, but commonly massive, botryoidal, reniform, stalactitic, or earthy.
• Cleavage Type: Limonite: None. Goethite: Perfect on {010}, but typically not visible in massive forms.
• Fracture Type: Conchoidal to uneven, earthy.
• Tenacity: Brittle to friable (for earthy varieties).
• Luster Type: Earthy, dull, submetallic, silky (for fibrous goethite).

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