How to identify Iron-rich Sandstone

To correctly identify Iron-rich Sandstone (Sedimentary rock, likely quartz sandstone with iron oxide cement), 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 deep red, reddish-brown, orange, yellow, to brown, depending on the specific iron oxide/hydroxide cement (hematite for red, goethite/limonite for yellow/brown/orange).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, reflecting the nature of the sand grains and the iron oxide cement. Individual quartz grains may exhibit a vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, arenaceous (sandy). Grains are typically visible and feel gritty. Can be fine-grained to coarse-grained. The cement can make the rock feel dense and hard.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for the rock as a whole. Individual quartz grains are anhedral to subhedral. Iron oxide cement typically occurs as microcrystalline coatings or pore fillings, not macroscopic crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   None for the rock as a whole. Individual quartz grains lack cleavage but exhibit conchoidal fracture. The iron oxide cement does not exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Fluvial (river), deltaic, beach, shallow marine, eolian (desert dunes), and lacustrine (lake) environments where sand deposition occurs and iron-bearing minerals are present and subject to oxidation. Often associated with oxidizing conditions in the depositional or post-depositional environment.

Key Facts

Hardness

Variable, typically 6-7 on Mohs scale for quartz grains, but the overall rock hardness is influenced by the cement. Well-cemented varieties can be 5-7, while poorly cemented ones can be much softer (3-4).

Specific Gravity

Variable, typically 2.65-2.95, depending on the proportion and type of iron oxide cement. Higher iron content increases density.

Crystal System

Not applicable for the rock. Quartz is trigonal. Iron oxides (hematite) are trigonal, goethite is orthorhombic.

Color

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

Luster

Dull to earthy.

Transparency

Opaque for the rock. Individual quartz grains can be translucent to transparent.

Fracture

Irregular to conchoidal (for quartz grains).

Cleavage

None.

Composition

Predominantly quartz (SiO2) grains, cemented by iron oxides and hydroxides (e.g., hematite Fe2O3, goethite FeO(OH), limonite FeO(OH)·nH2O). Minor amounts of other detrital minerals (feldspar, micas) and clay minerals may be present.

Physical characteristics

• Crystal Habit: Not applicable for the rock. Quartz grains are detrital. Iron oxides form microcrystalline coatings or amorphous masses.
• Cleavage Type: None.
• Fracture Type: Irregular to conchoidal.
• Tenacity: Brittle.
• Luster Type: Dull to earthy.

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