How to identify Concretion

To correctly identify Concretion (Sedimentary Concretion), 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

   Highly variable, depending on the cementing agent and host rock. Can be gray, brown, black, reddish-brown, yellowish, or white.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes vitreous if composed of quartz (e.g., chert nodules).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically fine-grained to microcrystalline, often smooth on the exterior, but can be rough or botryoidal. Internally, they can be massive, layered, or septarian (cracked and filled with secondary minerals).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Generally anhedral to subhedral, forming a massive, compact aggregate. Individual crystals are usually microscopic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Absent in the concretion as a whole, though constituent minerals may exhibit cleavage (e.g., calcite).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in shales, siltstones, sandstones, limestones, and mudstones. They form in various depositional environments, including marine, lacustrine, and fluvial settings, where conditions allow for localized mineral precipitation.

Key Facts

Hardness

Variable, typically 3 to 7 on Mohs scale, depending on cementing agent (e.g., calcite 3, quartz 7, siderite 3.5-4.5). Generally harder than the host rock.

Specific Gravity

Variable, typically 2.6 to 3.0, depending on composition (e.g., calcite 2.71, quartz 2.65, siderite 3.8-3.9).

Crystal System

Not applicable to the concretion as a whole; constituent minerals have their own crystal systems (e.g., calcite is trigonal, quartz is trigonal).

Color

Highly variable, reflecting the cementing mineral and impurities.

Luster

Dull, earthy, sometimes vitreous (if siliceous).

Transparency

Opaque.

Fracture

Conchoidal (if siliceous), irregular, or splintery.

Cleavage

Absent in the concretion as a whole.

Composition

Primarily composed of a cementing mineral (e.g., calcite, quartz/chalcedony, siderite, goethite, pyrite) and trapped host sediment particles (e.g., sand, silt, clay).

Physical characteristics

• Crystal Habit: Massive, compact, often spherical, ovoid, discoidal, or irregular shapes. Can exhibit concentric layering or radial structures internally.
• Cleavage Type: None for the concretion as a whole.
• Fracture Type: Irregular to conchoidal (especially in siliceous types).
• Tenacity: Brittle.
• Luster Type: Dull to earthy, sometimes vitreous.

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