How to identify Iron Oxide Concretion

To correctly identify Iron Oxide Concretion (Goethite/Hematite (Fe2O3 H2O / Fe2O3)), 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 dark reddish-brown, brown, yellowish-brown, or black. Hematite-rich concretions tend to be redder.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes submetallic on fresh fracture surfaces, especially for hematite.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often smooth on the exterior, but can be botryoidal, mammillary, or irregular. Internally, it can be massive, concentric, or radiating. May show evidence of original sedimentary layering or fossil inclusions.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Concretionary, nodular, botryoidal, mammillary, reniform. Individual crystals of goethite are acicular or fibrous, while hematite can be platy or granular, but these are rarely visible without magnification in concretions.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None observed in concretions due to their cryptocrystalline or microcrystalline nature. Goethite has perfect {010} cleavage, and hematite has no cleavage but exhibits parting on {0001} and {1011}, which are not typically seen in concretions.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in clastic sedimentary rocks such as sandstones, siltstones, and shales, as well as in lateritic soils and bog iron deposits. They form in environments where iron-rich fluids interact with host sediments, often associated with groundwater flow, diagenesis, and weathering processes.

Key Facts

Hardness

Goethite: 5-5.5; Hematite: 5-6.5 (on Mohs scale). Concretions can vary depending on the proportion of iron oxides and included sediment.

Specific Gravity

Goethite: 3.3-4.3; Hematite: 4.9-5.3. Concretions typically range from 2.9 to 4.5, depending on porosity and mineral content.

Crystal System

Goethite: Orthorhombic; Hematite: Trigonal.

Color

Dark reddish-brown, brown, yellowish-brown, black.

Luster

Dull, earthy, submetallic.

Transparency

Opaque.

Fracture

Uneven to conchoidal.

Cleavage

None observed in concretions; Goethite has perfect {010} cleavage, Hematite has no cleavage but parting.

Composition

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

Physical characteristics

• Crystal Habit: Concretionary, nodular, botryoidal, mammillary, reniform; microcrystalline to cryptocrystalline aggregates.
• Cleavage Type: Not applicable for concretions as a whole; individual goethite crystals have perfect {010} cleavage.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Dull, earthy, submetallic.

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