How to identify Iron Concretion

To correctly identify Iron Concretion (Goethite/Hematite 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, yellowish-brown, or black. The color can vary depending on the specific iron oxide/hydroxide present (e.g., goethite is often yellowish-brown, hematite is reddish-brown to black).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes submetallic on fresh surfaces, especially for hematite-rich varieties.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to rough exterior. Internally, they can be massive, concentric (banded), or radial. Often denser and harder than the surrounding host rock.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Macroscopic crystals are rare; typically cryptocrystalline or amorphous aggregates. May show botryoidal, reniform, or stalactitic habits within cavities.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None observed in concretionary form due to their cryptocrystalline nature. Individual goethite or hematite crystals would exhibit cleavage, but not the concretion as a whole.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in clastic sedimentary rocks such as sandstones, siltstones, shales, and sometimes limestones. They form in both marine and terrestrial environments where iron-rich fluids are present and redox conditions fluctuate.

Key Facts

Hardness

Variable, typically 4.5 to 5.5 on Mohs scale (for goethite) or 5 to 6 (for hematite), but can be lower if porous or mixed with softer minerals.

Specific Gravity

Variable, typically 3.3 to 4.3 (for goethite) or 4.9 to 5.3 (for hematite), generally higher than the host rock.

Crystal System

Goethite: Orthorhombic; Hematite: Trigonal. However, concretions are typically cryptocrystalline or amorphous aggregates.

Color

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

Luster

Dull, earthy, submetallic.

Transparency

Opaque.

Fracture

Uneven to conchoidal.

Cleavage

None observed in concretionary form.

Composition

Hydrous iron oxide (Goethite: FeO(OH)) and anhydrous iron oxide (Hematite: Fe2O3), often mixed with clay, quartz, and other detrital minerals.

Physical characteristics

• Crystal Habit: Concretionary, botryoidal, reniform, massive, radial, concentric. Individual crystals are microscopic.
• Cleavage Type: Absent in concretionary form.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Dull, earthy, submetallic.

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