How to identify Iron Oxide Concretion

To correctly identify Iron Oxide Concretion (Limonite/Goethite 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 yellow, yellowish-brown, brown, reddish-brown, or dark brown to black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, submetallic, or silky.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often smooth to rough on the exterior, can be botryoidal, mammillary, or irregular. Internally, can be massive, concentric, or porous.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Concretionary, botryoidal, mammillary, stalactitic, reniform, or massive. Individual crystals of goethite are acicular or prismatic but rarely visible without magnification.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None observed in concretions; goethite itself has one perfect cleavage, but it's not typically seen in massive or concretionary forms.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in sedimentary rocks (sandstones, shales, siltstones, limestones), soils, and bog environments. Form in oxidizing conditions where iron-rich groundwater interacts with host sediments.

Key Facts

Hardness

4.5-5.5 on the Mohs scale (for goethite), but can be softer if very earthy or porous.

Specific Gravity

3.3-4.3 (for goethite), variable for concretions depending on porosity and impurities.

Crystal System

Orthorhombic (for goethite); concretions are typically cryptocrystalline to amorphous.

Color

Yellow, yellowish-brown, brown, reddish-brown, dark brown to black.

Luster

Dull, earthy, submetallic, silky.

Transparency

Opaque.

Fracture

Uneven to conchoidal.

Cleavage

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

Composition

Hydrated iron oxides, primarily FeO(OH) (goethite) and amorphous hydrated iron oxides (limonite). Often contains impurities like clay, sand, and organic matter.

Physical characteristics

• Crystal Habit: Concretionary, botryoidal, mammillary, stalactitic, reniform, massive. Microscopic crystals of goethite are acicular or prismatic.
• Cleavage Type: Not typically observed in concretions.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Dull, earthy, submetallic, silky.

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