How to identify Ferruginous Sandstone with Iron Oxide Concretions

To correctly identify Ferruginous Sandstone with Iron Oxide Concretions (Ferruginous Sandstone with Iron Oxide Concretions), 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, yellowish-brown, or black due to iron oxide cement. Concretions are often darker and more intensely colored than the surrounding matrix.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy in the sandstone matrix; concretions can have a dull to sub-metallic luster if hematite is abundant and well-crystallized.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, sandy texture with visible sand grains (typically fine to coarse-grained, 0.0625 mm to 2 mm). Concretions are typically denser and harder than the surrounding sandstone, with a finer-grained or cryptocrystalline appearance of the cement.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual sand grains are typically sub-angular to rounded. Iron oxide cement is microcrystalline to cryptocrystalline, often forming a pervasive matrix. Concretions exhibit a nodular, spherical, ovoid, or irregular shape, sometimes with internal concentric banding.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No true cleavage in the rock as a whole. Individual mineral grains (e.g., quartz) may show conchoidal fracture. The concretions themselves do not exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in fluvial (river), deltaic, shallow marine, and eolian (wind-blown) sedimentary environments where iron-rich fluids are present during or after deposition. Often associated with paleosols (ancient soils) or zones of groundwater fluctuation.

Key Facts

Hardness

Sandstone matrix: 6-7 (quartz grains), overall rock 3-5. Concretions: 5-6.5 (due to iron oxides)

Specific Gravity

2.6-3.5, depending on the proportion and type of iron oxide cement. Higher iron content leads to higher specific gravity.

Crystal System

Quartz: Trigonal. Hematite: Trigonal. Goethite: Orthorhombic. Limonite: Amorphous/cryptocrystalline.

Color

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

Luster

Dull to earthy; concretions can be sub-metallic.

Transparency

Opaque.

Fracture

Irregular to conchoidal (quartz grains), uneven to splintery (rock as a whole).

Cleavage

None in the rock; individual quartz grains may show conchoidal fracture.

Composition

Predominantly quartz (SiO2) sand grains cemented by iron oxides (Fe2O3 - hematite, FeO(OH) - goethite, hydrated iron oxides - limonite). Minor amounts of feldspar, lithic fragments, and other accessory minerals may be present.

Physical characteristics

• Crystal Habit: Sand grains are detrital, typically sub-angular to rounded. Iron oxides form microcrystalline to cryptocrystalline cement. Concretions are nodular, spherical, ovoid, or irregular masses.
• Cleavage Type: Not applicable to the rock as a whole. Individual quartz grains exhibit conchoidal fracture.
• Fracture Type: Uneven to splintery in the sandstone matrix; conchoidal to uneven in concretions.
• Tenacity: Brittle.
• Luster Type: Earthy to dull for the sandstone matrix; sub-metallic to dull for the concretions.

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