How to identify Iron-rich Sandstone

To correctly identify Iron-rich Sandstone (Sandstone (ferruginous)), 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 deep red due to the presence of iron oxide cements (hematite, goethite, limonite). The color can be uniform or mottled.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, depending on the abundance and type of iron oxide 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 palpable. Can range from fine-grained to coarse-grained. Often feels gritty.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual quartz grains are typically anhedral to subhedral. Iron oxide cements are microcrystalline or amorphous, coating or filling interstitial spaces between grains.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No true cleavage for the rock as a whole. Individual mineral grains (e.g., quartz) may exhibit conchoidal fracture, while the rock itself will fracture irregularly.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Formed in various sedimentary environments where sand deposition and iron-rich fluid circulation occur, including fluvial (river), eolian (desert), deltaic, and shallow marine settings. Often associated with oxidizing conditions.

Key Facts

Hardness

Variable, typically 6-7 on Mohs scale for quartz grains, but the overall rock hardness can be lower (3-6) depending on the cement type and abundance. Iron oxide cements themselves are generally 5-6.

Specific Gravity

2.65 - 2.95 g/cm³ (higher than typical sandstone due to iron content)

Crystal System

Not applicable for the rock as a whole; quartz is trigonal, iron oxides vary (e.g., hematite is trigonal, goethite is orthorhombic).

Color

Red, reddish-brown, orange, yellow, brown.

Luster

Dull to earthy, with individual quartz grains exhibiting vitreous luster.

Transparency

Opaque (rock), individual quartz grains can be translucent to transparent.

Fracture

Irregular to conchoidal (for quartz grains).

Cleavage

None (for the rock).

Composition

Predominantly quartz (SiO2) grains, cemented by iron oxides and hydroxides (e.g., hematite Fe2O3, goethite FeO(OH), limonite FeO(OH)·nH2O). May contain minor amounts of feldspar, lithic fragments, and other accessory minerals.

Physical characteristics

• Crystal Habit: Clastic, granular; individual quartz grains are typically anhedral to subhedral. Iron oxide cement forms microcrystalline coatings or pore fillings.
• Cleavage Type: None (rock); quartz grains exhibit conchoidal fracture rather than cleavage.
• Fracture Type: Irregular to subconchoidal.
• Tenacity: Brittle.
• Luster Type: Dull to earthy (overall rock); vitreous (quartz grains).

Explore Iron-rich Sandstone