How to identify Iron Oxide Concretion

To correctly identify Iron Oxide Concretion (Goethite/Limonite), 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 black. The color can vary depending on the specific iron oxyhydroxide present and impurities.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Earthy to dull, sometimes submetallic on fresh fracture surfaces, especially for denser varieties of goethite.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often smooth on the exterior, but can be rough or botryoidal. Internally, they can be massive, concentric, or radiating fibrous. The texture is generally non-crystalline to microcrystalline.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Concretions are typically anhedral (lacking well-formed crystal faces) and massive, botryoidal, reniform, or stalactitic. Goethite itself is orthorhombic but rarely forms macroscopic crystals in concretions, usually appearing as cryptocrystalline or fibrous aggregates.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None observed in concretions due to their cryptocrystalline or amorphous nature. Goethite itself has one perfect cleavage {010} but this is not typically visible in concretionary forms.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in sedimentary rocks such as sandstones, shales, siltstones, and unconsolidated sediments (e.g., glacial tills, alluvial deposits). They form in environments where iron-rich waters interact with organic matter or changes in redox potential, such as swamps, bogs, shallow marine settings, and groundwater-saturated zones.

Key Facts

Hardness

5.0 - 5.5 (Mohs scale) for goethite; limonite is a mixture and can vary, but generally similar or slightly lower.

Specific Gravity

3.3 - 4.3 for goethite; limonite typically 2.7 - 4.3, depending on porosity and composition.

Crystal System

Orthorhombic for goethite; limonite is amorphous to cryptocrystalline.

Color

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

Luster

Earthy, dull, submetallic.

Transparency

Opaque.

Fracture

Uneven to conchoidal.

Cleavage

None observed in concretions; goethite has one perfect cleavage {010} but rarely seen.

Composition

Hydrous iron oxides, primarily FeO(OH) for goethite, often mixed with other iron oxyhydroxides and impurities for limonite.

Physical characteristics

• Crystal Habit: Massive, botryoidal, reniform, stalactitic, fibrous, radiating, or earthy aggregates within concretions.
• Cleavage Type: Not applicable for concretions; goethite has perfect {010} cleavage.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Earthy, dull, submetallic.

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