How to identify Iron Concretion

To correctly identify Iron Concretion (Iron oxide concretion), 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 reddish-brown, dark brown, black, or yellowish-brown due to the presence of various iron oxides (e.g., hematite, goethite, limonite).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes sub-metallic if hematite is dominant.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Fine-grained to sandy, depending on the host rock. Often smooth on the exterior if weathered, but can be rough or botryoidal. Internally, it can be massive, concentric, or show relict sedimentary textures.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Macrocrystalline aggregates of iron oxides cementing host sediment grains. Individual crystals are usually microscopic. Concretions themselves exhibit spherical, ovoid, discoidal, or irregular external forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, as they are aggregates of fine-grained minerals and sediment. Individual iron oxide minerals may have cleavage, but it's not observable in the concretion.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found within sedimentary rock sequences, particularly in sandstones, siltstones, shales, and sometimes limestones. Common in fluvial, deltaic, shallow marine, and eolian (desert) environments where iron-rich fluids interact with permeable sediments.

Key Facts

Hardness

Variable, typically 4-7 on Mohs scale, depending on the cementing iron oxide and the amount of included sediment. Hematite is 5-6, Goethite is 5-5.5.

Specific Gravity

Variable, typically 2.9-4.5, depending on the density of the iron oxides and the proportion of host sediment.

Crystal System

Not applicable to the concretion as a whole. Constituent iron oxides (e.g., hematite: trigonal; goethite: orthorhombic).

Color

Reddish-brown, dark brown, black, yellowish-brown.

Luster

Dull, earthy, sometimes sub-metallic.

Transparency

Opaque.

Fracture

Conchoidal to uneven, depending on the internal structure and grain size.

Cleavage

None (as a concretion).

Composition

Primarily iron oxides (e.g., Fe2O3 as hematite, FeO(OH) as goethite/limonite) cementing detrital grains (e.g., quartz, feldspar) of the host sedimentary rock.

Physical characteristics

• Crystal Habit: Aggregates of microcrystalline to cryptocrystalline iron oxides cementing detrital grains. Concretions themselves are typically spherical, ovoid, discoidal, or irregular.
• Cleavage Type: Not applicable to the concretion.
• Fracture Type: Conchoidal to uneven.
• Tenacity: Brittle.
• Luster Type: Dull to earthy, occasionally sub-metallic.

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