How to identify Concretion with Limonite Core

To correctly identify Concretion with Limonite Core (Concretion (sedimentary rock) with Limonite (iron oxyhydroxide) core), 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

   Exterior: Varies depending on the host rock and cementing material, often light brown, gray, or tan. Core: Yellowish-brown, reddish-brown, dark brown, or black, characteristic of limonite.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Exterior: Dull to earthy. Core: Earthy, dull, sometimes sub-metallic if goethite is well-crystallized.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Exterior: Fine-grained to gritty, depending on the host sediment. Core: Earthy, massive, sometimes botryoidal or mammillary.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Concretions are typically anhedral (lacking distinct crystal faces) and massive. Limonite within the core is microcrystalline to cryptocrystalline, often forming botryoidal or stalactitic aggregates.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None, due to its amorphous or microcrystalline nature and aggregate form.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in clastic sedimentary rocks such as sandstones, siltstones, shales, and mudstones. Also occurs in some limestones. They form in environments where iron-rich fluids can circulate, such as shallow marine, deltaic, fluvial, and lacustrine settings, often associated with redox boundaries.

Key Facts

Hardness

Variable, typically 3.5 to 5.5 on the Mohs scale for the limonite core, but the overall concretion hardness depends on the cementing material and host rock components.

Specific Gravity

2.7 to 4.3 for the limonite core, depending on the specific iron oxyhydroxide composition and porosity. The overall concretion density will vary.

Crystal System

Limonite is an aggregate of cryptocrystalline minerals (primarily goethite, orthorhombic; lepidocrocite, orthorhombic). The concretion itself is an aggregate of various minerals.

Color

Yellowish-brown, reddish-brown, dark brown, black (for the limonite core). Exterior color varies with host rock.

Luster

Earthy, dull, sometimes sub-metallic (for the limonite core).

Transparency

Opaque.

Fracture

Uneven to conchoidal (for the limonite core).

Cleavage

None.

Composition

Core: Hydrated iron oxides (FeO(OH)·nH2O), primarily goethite (α-FeOOH) and lepidocrocite (γ-FeOOH). Concretion: Varies, typically quartz, calcite, clay minerals, cemented by iron oxides/hydroxides.

Physical characteristics

• Crystal Habit: Massive, earthy, botryoidal, mammillary, stalactitic (for limonite). Concretions are typically spherical, ovoid, discoidal, or irregular aggregates.
• Cleavage Type: None.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Earthy to dull.

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