How to identify Iron Concretion

To correctly identify Iron Concretion (Goethite/Limonite (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

   Typically yellowish-brown, reddish-brown, dark brown, or blackish-brown. The color can vary depending on the degree of hydration and impurities.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, or submetallic. Rarely vitreous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often rough or botryoidal (grape-like) on the exterior. Internally, they can be massive, concentric, or radial. Can be porous or dense.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Macroscopically, they do not exhibit distinct crystal forms, appearing as rounded or irregular masses. Microscopically, goethite is orthorhombic, but limonite is amorphous or cryptocrystalline.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None observed in hand samples due to their cryptocrystalline or amorphous nature. Goethite has perfect {010} cleavage, but it's rarely seen.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in sedimentary rocks (sandstones, shales, limestones), soils, and bog environments. They form in oxidizing conditions where iron-rich solutions are present, often associated with groundwater movement or weathering of iron-bearing minerals.

Key Facts

Hardness

5.0 - 5.5 (Goethite) on Mohs scale; Limonite is variable, often softer (4-5.5)

Specific Gravity

3.3 - 4.3 (Goethite); Limonite is variable, typically 2.7 - 4.3

Crystal System

Orthorhombic (Goethite); Amorphous or cryptocrystalline (Limonite)

Color

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

Luster

Dull, earthy, submetallic

Transparency

Opaque

Fracture

Uneven to conchoidal

Cleavage

None observed in hand samples (Goethite has perfect {010} cleavage, rarely seen)

Composition

Hydrated iron oxide (FeO(OH) for Goethite; FeO(OH)·nH2O for Limonite, a mixture of hydrated iron oxides)

Physical characteristics

• Crystal Habit: Massive, botryoidal, stalactitic, reniform, oolitic, pisolitic, fibrous, radiating, earthy. Concretions are typically rounded to irregular masses.
• Cleavage Type: Not observable in hand specimens of concretions. Goethite has perfect {010} cleavage.
• Fracture Type: Uneven to conchoidal
• Tenacity: Brittle
• Luster Type: Dull, earthy, submetallic

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