How to identify Concretion in Sandstone

To correctly identify Concretion in Sandstone (Sandstone with Iron Oxide 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

   Concretions are typically reddish-brown, dark brown, or black due to iron oxides. The surrounding sandstone can be various shades of tan, yellow, white, or grey.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy in the concretion, similar to the host sandstone, but can be slightly more vitreous if silica-cemented or sub-metallic if hematite-rich.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The concretion is generally denser and harder than the surrounding sandstone, often with a finer grain size due to complete cementation. The host sandstone is clastic, granular, and friable to well-cemented.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Concretions do not exhibit external crystal form; they are massive or concentrically layered aggregates of cemented grains. The cementing minerals (e.g., hematite, goethite) are typically cryptocrystalline or microcrystalline.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No cleavage in the concretion itself, as it is a cemented aggregate. The constituent minerals (e.g., quartz grains) do not show cleavage in this context.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Sedimentary basins, particularly in fluvial, deltaic, shallow marine, and eolian sandstone deposits. They are common in formations where groundwater flow and chemical conditions favored the precipitation of iron minerals.

Key Facts

Hardness

Variable, typically 5-6.5 on Mohs scale for the iron oxide cement, but the overall hardness is influenced by the quartz grains (7) and the degree of cementation. Generally harder than the host sandstone.

Specific Gravity

2.6-3.8, depending on the density of the cementing minerals (e.g., hematite ~5.2, goethite ~3.3-4.3) and the porosity of the concretion.

Crystal System

Not applicable for the concretion as a whole. The cementing minerals (e.g., hematite is trigonal, goethite is orthorhombic) have their own crystal systems, but are typically cryptocrystalline within the concretion.

Color

Concretion: Reddish-brown, dark brown, black. Sandstone: Tan, yellow, white, grey.

Luster

Dull to earthy.

Transparency

Opaque.

Fracture

Conchoidal to irregular, often breaking across sand grains.

Cleavage

None.

Composition

Primarily quartz (SiO2) sand grains cemented by iron oxides/hydroxides (e.g., hematite Fe2O3, goethite FeO(OH)). Minor amounts of other minerals may be present depending on the host rock and fluid chemistry.

Physical characteristics

• Crystal Habit: Massive, nodular, spherical, ovoid, discoidal, or irregular aggregates. Internal structure can be massive or concentrically layered.
• Cleavage Type: None.
• Fracture Type: Conchoidal to irregular.
• Tenacity: Brittle.
• Luster Type: Dull to earthy.

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