How to identify Iron Concretion in Shale

To correctly identify Iron Concretion in Shale (Iron oxide/hydroxide concretion in shale), 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, yellowish-brown, dark brown, or black due to various iron oxides/hydroxides. Can be grey to brownish-grey if siderite is dominant and unweathered.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes sub-metallic if hematite is prominent.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Fine-grained to cryptocrystalline, often massive. Can be granular or show concentric banding. The surface may be smooth or botryoidal. Often much harder and denser than the surrounding shale.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Macroscopic crystal forms are rare; typically anhedral aggregates forming nodular or discoidal masses. Septarian concretions show internal cracks filled with secondary minerals (e.g., calcite, quartz, barite).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No distinct cleavage in the concretionary mass itself, though individual mineral components may have cleavage (e.g., siderite).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found embedded within fine-grained sedimentary rocks, predominantly shales, mudstones, and siltstones. They are common in marine and lacustrine environments where anoxic conditions can lead to iron mobilization and subsequent precipitation.

Key Facts

Hardness

Variable, typically 3.5 to 6.5 on the Mohs scale, depending on the dominant iron mineral (e.g., siderite ~3.5-4.5, goethite ~5-5.5, hematite ~5.5-6.5). Generally harder than the host shale.

Specific Gravity

Variable, typically 2.9 to 4.5, depending on mineral composition. Significantly denser than typical shale (2.0-2.8).

Crystal System

Individual mineral components have their own crystal systems (e.g., siderite: trigonal; goethite: orthorhombic; hematite: trigonal). The concretion as a whole is an aggregate and does not have a single crystal system.

Color

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

Luster

Dull, earthy, sometimes sub-metallic.

Transparency

Opaque.

Fracture

Uneven to conchoidal, sometimes splintery.

Cleavage

None for the concretionary mass; individual mineral components may exhibit cleavage (e.g., siderite has perfect rhombohedral cleavage).

Composition

Primarily iron oxides (e.g., goethite, hematite), iron hydroxides (e.g., limonite), and/or iron carbonates (e.g., siderite), mixed with varying amounts of clay minerals, quartz, and organic matter from the host shale.

Physical characteristics

• Crystal Habit: Nodular, discoidal, spherical, ovoid, or irregular masses; often massive or concentrically banded. Septarian concretions show internal polygonal cracks.
• Cleavage Type: Not applicable to the concretion as a whole. Individual mineral components may have cleavage (e.g., siderite: perfect rhombohedral).
• Fracture Type: Uneven, conchoidal, or splintery.
• Tenacity: Brittle.
• Luster Type: Dull, earthy, sub-metallic.

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