How to identify Iron Oxide Concretions

To correctly identify Iron Oxide Concretions (Goethite/Hematite), 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 reddish-brown, dark brown, yellowish-brown, or black. Hematite-rich concretions tend to be redder, while goethite-rich ones are browner/yellower.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes submetallic in denser, hematitic varieties.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Smooth to rough exterior, often mammillary or botryoidal. Internally, they can be massive, concentric, or radiating.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Macroscopic crystals are rare; typically cryptocrystalline or amorphous aggregates. Concretions themselves are macroscopic structures, not individual crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None observed in concretions; individual goethite and hematite minerals have distinct cleavage/parting but are rarely visible in concretions.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in sedimentary rocks such as sandstones, shales, siltstones, and limestones. They form in both marine and terrestrial environments where iron-rich fluids are present, often associated with groundwater flow or diagenetic processes.

Key Facts

Hardness

Goethite: 5-5.5 (Mohs). Hematite: 5-6.5 (Mohs). The overall hardness of a concretion can vary depending on its density and cementation.

Specific Gravity

Goethite: 3.3-4.3. Hematite: 4.9-5.3. Concretions will generally be denser than typical sedimentary rocks.

Crystal System

Goethite: Orthorhombic. Hematite: Trigonal. These refer to the individual mineral components, not the concretion as a whole.

Color

Reddish-brown, dark 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 distinct parting on {0001} and {1011}.

Composition

Hydrous iron oxide (Goethite: FeO(OH)) and/or anhydrous iron oxide (Hematite: Fe2O3), often with impurities of silica, clay minerals, and other oxides.

Physical characteristics

• Crystal Habit: Concretions are macroscopic aggregates; individual minerals are typically cryptocrystalline, massive, botryoidal, or stalactitic.
• Cleavage Type: Not applicable for the concretion as a whole. Goethite has perfect {010} cleavage; Hematite has no cleavage but parting on {0001} and {1011}.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Dull, earthy, submetallic.

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