How to identify Iron Ore

To correctly identify Iron Ore (Iron Ore (likely Limonite or Goethite with a core of a primary iron mineral)), 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. The streak is typically yellowish-brown to brown.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Earthy, dull, submetallic, or silky (fibrous varieties).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often earthy, porous, botryoidal, stalactitic, mammillary, or concretionary. Can be massive or fibrous.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Goethite forms acicular, prismatic, or tabular crystals, often radiating. Limonite is typically amorphous or cryptocrystalline, forming botryoidal, stalactitic, or earthy masses. The primary iron mineral core (e.g., magnetite, hematite) would retain its characteristic crystal form if unweathered.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Goethite has perfect cleavage on {010} and good cleavage on {100}. Limonite, being a mixture, generally lacks distinct cleavage and exhibits irregular fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in gossans (oxidized caps of sulfide ore deposits), bog iron deposits, lateritic soils, and as weathering products in sedimentary and metamorphic iron formations. Often associated with zones of intense chemical weathering.

Key Facts

Hardness

Goethite: 5-5.5 on Mohs scale. Limonite: Variable, typically 4-5.5 (due to its mixed nature and porosity).

Specific Gravity

Goethite: 3.3-4.3. Limonite: Variable, typically 2.7-4.3 (lower if very porous).

Crystal System

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

Color

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

Luster

Earthy, dull, submetallic, silky.

Transparency

Opaque.

Fracture

Goethite: Uneven to subconchoidal. Limonite: Earthy, irregular.

Cleavage

Goethite: Perfect on {010}, good on {100}. Limonite: None.

Composition

Goethite: Hydrous iron oxide, α-FeO(OH). Limonite: A mixture of hydrated iron oxides, primarily Goethite, Lepidocrocite (γ-FeO(OH)), and ferrihydrite (Fe5HO8·4H2O), often with adsorbed water and impurities. The 'core of a primary iron mineral' implies the presence of unweathered or partially weathered magnetite (Fe3O4), hematite (Fe2O3), or siderite (FeCO3).

Physical characteristics

• Crystal Habit: Goethite: Acicular, prismatic, tabular, radiating, botryoidal, stalactitic, massive. Limonite: Earthy, massive, botryoidal, mammillary, concretionary.
• Cleavage Type: Goethite: Perfect {010}, good {100}. Limonite: Absent.
• Fracture Type: Goethite: Uneven to subconchoidal. Limonite: Earthy, irregular.
• Tenacity: Goethite: Brittle. Limonite: Brittle to friable.
• Luster Type: Earthy, dull, submetallic, silky.

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