How to identify Goethite Concretion

To correctly identify Goethite Concretion (FeO(OH)), 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

   Dark brown, yellowish-brown, reddish-brown, blackish-brown. Often has a rusty appearance.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, submetallic, silky (especially in fibrous varieties).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often rough or granular on the exterior. Internally, can be massive, botryoidal, reniform, stalactitic, or concentrically layered.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Goethite itself is orthorhombic, but in concretions, it rarely forms distinct crystals visible to the naked eye. It typically occurs as cryptocrystalline, massive, fibrous, or radiating aggregates within the concretionary structure.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Goethite has perfect cleavage on {010}, but this is rarely observed in cryptocrystalline or massive concretionary forms.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Sedimentary rocks (sandstones, shales, limestones), soils, bogs, swamps, lateritic soils, gossans (weathered sulfide deposits), and as a weathering product of iron-rich minerals in various rock types.

Key Facts

Hardness

5.0 - 5.5 (Mohs scale)

Specific Gravity

3.3 - 4.3 (variable due to porosity and impurities)

Crystal System

Orthorhombic (for the mineral goethite)

Color

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

Luster

Adamantine, submetallic, dull, earthy, silky

Transparency

Opaque

Fracture

Uneven to subconchoidal

Cleavage

Perfect on {010}, but rarely observed in massive forms

Composition

Iron oxyhydroxide (FeO(OH))

Physical characteristics

• Crystal Habit: Typically massive, botryoidal, reniform, stalactitic, fibrous, radiating, or earthy. In concretions, it forms cryptocrystalline to microcrystalline aggregates.
• Cleavage Type: Perfect on {010}, but often not apparent in concretionary forms.
• Fracture Type: Uneven to subconchoidal, sometimes splintery in fibrous varieties.
• Tenacity: Brittle
• Luster Type: Adamantine to submetallic (on fresh surfaces of well-crystallized material), dull to earthy (most common in concretions), silky (in fibrous varieties).

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