How to identify Iron-stained Sandstone

To correctly identify Iron-stained Sandstone (Sandstone with iron oxide/hydroxide staining), 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

   Ranges from vibrant reds (hematite) to oranges, yellows, and browns (goethite, limonite). The color is often uniform but can also be mottled or banded.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Typically dull to earthy, reflecting the luster of the iron oxide/hydroxide coatings. The underlying quartz grains may exhibit a vitreous luster if visible.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, granular, with visible sand grains. Can be fine- to coarse-grained. The iron minerals often fill pore spaces, reducing porosity.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   The iron oxides/hydroxides typically occur as microcrystalline coatings or amorphous masses within the pore spaces, not as distinct macroscopic crystals. The primary sand grains (e.g., quartz) retain their original detrital forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No inherent cleavage for the rock as a whole. Individual mineral grains (e.g., feldspar) may exhibit cleavage, but the iron cement does not.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in terrestrial and shallow marine sedimentary environments where iron-rich fluids are present. Often associated with arid or semi-arid climates, ancient river systems, deltas, and coastal plains. Can also form in areas with significant weathering of iron-bearing source rocks.

Key Facts

Hardness

Variable, typically 6-7 on Mohs scale for quartz grains, but overall rock hardness can be lower if poorly cemented, or higher if well-cemented by iron oxides. Iron oxides themselves range from 5-6 (hematite) to 5-5.5 (goethite).

Specific Gravity

2.65 - 2.95 (higher than pure quartz sandstone due to denser iron oxides)

Crystal System

Not applicable for the rock as a whole. Quartz is trigonal; hematite is trigonal; goethite is orthorhombic.

Color

Red, orange, brown, yellow

Luster

Dull, earthy, sometimes sub-vitreous for exposed quartz grains

Transparency

Opaque

Fracture

Conchoidal to irregular for individual quartz grains; granular for the rock as a whole.

Cleavage

None for the rock; individual mineral grains may exhibit cleavage (e.g., feldspar).

Composition

Predominantly quartz (SiO2) grains, with varying amounts of feldspar, rock fragments, and accessory minerals. Cementing material is primarily iron oxides (e.g., hematite - Fe2O3) and/or iron hydroxides (e.g., goethite - FeO(OH), limonite - hydrated iron oxides).

Physical characteristics

• Crystal Habit: Detrital grains (e.g., sub-angular to rounded quartz) cemented by microcrystalline to amorphous iron oxides/hydroxides.
• Cleavage Type: Not applicable for the rock; individual mineral grains may show cleavage.
• Fracture Type: Granular to irregular.
• Tenacity: Brittle.
• Luster Type: Dull to earthy.

Explore Iron-stained Sandstone