How to identify Iron Oxide Concretion

To correctly identify Iron Oxide Concretion (Goethite/Hematite (Iron Oxide)), 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. The specific hue depends on the dominant iron oxide (hematite tends to be redder, goethite browner/yellower) and the presence of other impurities.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes submetallic in denser, more crystalline forms.

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 radial. The texture is generally fine-grained to cryptocrystalline.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Concretionary, botryoidal, reniform, mammillary, stalactitic, or massive. Individual crystals of goethite or hematite are usually microscopic within the concretion.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None observed in the concretionary form due to its aggregate nature. Individual goethite has perfect {010} cleavage, and hematite has no cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in sedimentary rocks such as sandstones, shales, siltstones, and conglomerates. Also prevalent in soils, particularly lateritic soils, and in bog environments (as bog iron ore). They form in environments where iron-rich solutions interact with host sediments, often at redox boundaries.

Key Facts

Hardness

Goethite: 5.0-5.5 on Mohs scale; Hematite: 5.0-6.0 on Mohs scale. The concretion's overall hardness can vary depending on the cementing material and impurities.

Specific Gravity

Goethite: 3.3-4.3; Hematite: 4.9-5.3. Concretions typically range from 2.9 to 4.5, influenced by porosity and non-iron oxide components.

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 concretionary form; Goethite has perfect {010} cleavage, Hematite has none.

Composition

Primarily iron oxides and hydroxides (FeO(OH) and Fe2O3), often mixed with quartz, clay minerals, and other detrital grains.

Physical characteristics

• Crystal Habit: Concretionary, botryoidal, reniform, mammillary, massive. Individual crystals are typically microscopic.
• Cleavage Type: Not applicable for the concretion as a whole. Goethite exhibits perfect {010} cleavage, hematite has no cleavage.
• Fracture Type: Uneven to conchoidal, sometimes splintery.
• Tenacity: Brittle
• Luster Type: Dull, earthy, sometimes submetallic.

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