How to identify Iron Concretion

To correctly identify Iron Concretion (Iron oxide/hydroxide (e.g., Goethite, Hematite) with Quartz), 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, black, or yellowish-brown, depending on the specific iron oxide/hydroxide minerals present and their hydration state.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes submetallic if hematite is dominant and well-crystallized.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Often granular due to incorporated quartz, but can be smooth to botryoidal on external surfaces. Internally, concentric layering may be visible.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Macroscopic crystal forms are rare; typically massive, botryoidal, reniform, or spherical/ovoid aggregates. Microcrystalline iron oxides/hydroxides cement quartz grains.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No distinct cleavage due to its composite nature and fine-grained, often amorphous to microcrystalline iron mineral components. Quartz grains within the concretion retain their own fracture properties.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found within sedimentary rock sequences, particularly sandstones, shales, and siltstones. Common in fluvial, lacustrine, and shallow marine environments where diagenetic processes are active. Often exposed on eroded surfaces as resistant remnants.

Key Facts

Hardness

Variable, typically 4-7 on the Mohs scale, depending on the proportion and type of iron oxide/hydroxide and quartz content. Quartz is 7, hematite 5-6, goethite 5-5.5.

Specific Gravity

Variable, typically 2.9-4.5, depending on the density of the iron minerals and the amount of incorporated quartz and porosity. Hematite is ~5.2, Goethite ~4.3.

Crystal System

Not applicable for the concretion as a whole, which is an aggregate. Individual iron minerals (e.g., hematite - trigonal, goethite - orthorhombic) and quartz (trigonal) have their own systems.

Color

Reddish-brown, dark brown, black, yellowish-brown.

Luster

Dull, earthy, submetallic.

Transparency

Opaque.

Fracture

Irregular to conchoidal, depending on the mineralogy and texture.

Cleavage

None (for the concretion as a whole).

Composition

Iron oxides (e.g., Fe2O3 - Hematite) and/or iron hydroxides (e.g., FeO(OH) - Goethite, often hydrated as Limonite), cemented with varying amounts of detrital quartz (SiO2) and other host rock minerals.

Physical characteristics

• Crystal Habit: Massive, botryoidal, reniform, spherical, ovoid, discoidal, irregular aggregates. Internal structure often concentric.
• Cleavage Type: None.
• Fracture Type: Irregular to conchoidal.
• Tenacity: Brittle.
• Luster Type: Dull, earthy, submetallic.

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