How to identify Iron Ore

To correctly identify Iron Ore (Iron Ore (likely Limonite or 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

   Dull, earthy, submetallic, silky (in fibrous varieties).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Earthy, porous, botryoidal, stalactitic, mammillary, concretionary, or massive.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Limonite is amorphous or cryptocrystalline. Goethite forms acicular, prismatic, or tabular crystals, often radiating or fibrous, but more commonly massive, reniform, or stalactitic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None for Limonite. Goethite has perfect cleavage on {010} and good on {100}.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Oxidized zones of ore deposits (gossans), bog iron deposits, lateritic soils, sedimentary iron formations, as concretions in sandstones and shales, and as a weathering product of iron-rich rocks.

Key Facts

Hardness

4.0-5.5 (Goethite), variable (Limonite, often softer, 1-5.5)

Specific Gravity

3.3-4.3 (Goethite), 2.7-4.3 (Limonite, variable due to porosity and impurities)

Crystal System

Orthorhombic (Goethite), Amorphous/Cryptocrystalline (Limonite)

Color

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

Luster

Dull, earthy, submetallic, silky

Transparency

Opaque

Fracture

Uneven, conchoidal (Goethite), earthy (Limonite)

Cleavage

Perfect on {010}, good on {100} (Goethite); None (Limonite)

Composition

Hydrated iron oxide (Goethite: α-FeO(OH); Limonite: a mixture, primarily Goethite and Lepidocrocite)

Physical characteristics

• Crystal Habit: Acicular, prismatic, tabular, radiating, fibrous, massive, reniform, stalactitic, botryoidal, earthy, concretionary.
• Cleavage Type: Perfect on {010}, good on {100} (Goethite); None (Limonite).
• Fracture Type: Uneven to subconchoidal (Goethite); Earthy (Limonite).
• Tenacity: Brittle.
• Luster Type: Dull, earthy, submetallic, silky.

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