How to identify Iron Ore (Limonite/Goethite)

To correctly identify Iron Ore (Limonite/Goethite) (Limonite/Goethite (Iron Oxide/Hydroxide)), 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, silky (in fibrous varieties).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

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

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Goethite forms acicular, prismatic, or tabular crystals, often radiating or fibrous. Limonite is typically amorphous or cryptocrystalline, forming botryoidal, stalactitic, or massive aggregates.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Goethite has perfect cleavage on {010} and good cleavage on {100}, but it is rarely observed due to its fine-grained or massive habit. Limonite exhibits no cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in gossans (weathered caps of sulfide deposits), lateritic soils, bog iron deposits, sedimentary iron formations, and as an alteration product in various rock types. It is common in oxidized zones of ore deposits and as a cementing agent in sandstones.

Key Facts

Hardness

Goethite: 5-5.5 (Mohs); Limonite: 4-5.5 (Mohs, variable due to mixture and porosity)

Specific Gravity

Goethite: 3.3-4.3; Limonite: 2.7-4.3 (variable, often lower due to porosity)

Crystal System

Goethite: Orthorhombic; Limonite: Amorphous to cryptocrystalline (no defined crystal system)

Color

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

Luster

Earthy, dull, submetallic, silky

Transparency

Opaque

Fracture

Uneven, conchoidal (Goethite), earthy (Limonite)

Cleavage

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

Composition

Goethite: Hydrous iron oxide (α-FeOOH); Limonite: A mixture of hydrated iron oxides, primarily goethite and lepidocrocite, with variable water content and impurities.

Physical characteristics

• Crystal Habit: Goethite: Acicular, prismatic, tabular, radiating, fibrous, botryoidal, stalactitic, massive. Limonite: Massive, earthy, botryoidal, mammillary, concretionary, oolitic.
• Cleavage Type: Goethite: Perfect {010}, good {100}; Limonite: None
• Fracture Type: Goethite: Uneven to subconchoidal; Limonite: Earthy, uneven
• Tenacity: Brittle
• Luster Type: Earthy, dull, submetallic, silky

Explore Iron Ore (Limonite/Goethite)