How to identify Iron-rich Sandstone

To correctly identify Iron-rich Sandstone (Ferruginous Sandstone), 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, orange, yellow, or purplish due to iron oxide/hydroxide cement. Colors can vary based on the specific iron mineral (e.g., hematite for red, goethite/limonite for yellow/brown).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, depending on the abundance and type of iron cement and the degree of weathering. Quartz grains will have a vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, granular. Sand-sized grains (0.0625 mm to 2 mm) are visible and often feel gritty. The texture can range from fine-grained to coarse-grained. The cementation can make it well-indurated or friable.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual sand grains (e.g., quartz) are typically anhedral to subhedral. The iron cement forms a microcrystalline to amorphous coating or infilling between grains.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No true cleavage planes for the rock as a whole. Individual mineral grains (e.g., feldspar) may exhibit cleavage, but the rock breaks irregularly.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Forms in various sedimentary environments where iron is available and conditions are oxidizing, such as fluvial (river), deltaic, shallow marine, eolian (desert), and lacustrine (lake) settings. Often associated with paleosols (ancient soils) or zones of intense weathering.

Key Facts

Hardness

Variable, typically 3-7 on Mohs scale, depending on the degree of cementation and the hardness of the cementing minerals. Quartz grains are 7.

Specific Gravity

2.65 - 3.0, higher than typical quartz sandstone due to the denser iron minerals.

Crystal System

Not applicable for the rock as a whole. Constituent minerals have their own crystal systems (e.g., quartz is trigonal, hematite is trigonal, goethite is orthorhombic).

Color

Reddish-brown, orange, yellow, purplish.

Luster

Dull to earthy.

Transparency

Opaque.

Fracture

Irregular to conchoidal (for individual quartz grains).

Cleavage

None (for the rock).

Composition

Predominantly quartz (SiO2) grains, cemented by iron oxides (e.g., hematite, Fe2O3) and/or iron hydroxides (e.g., goethite, FeO(OH); limonite, a mixture of hydrated iron oxides).

Physical characteristics

• Crystal Habit: Clastic, granular. Cement is microcrystalline to amorphous.
• Cleavage Type: None.
• Fracture Type: Irregular to subconchoidal.
• Tenacity: Brittle.
• Luster Type: Dull to earthy.

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