How to identify Red Sandstone

To correctly identify Red Sandstone (Sandstone (rich in iron oxides)), 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

   Predominantly red, reddish-brown, or maroon, due to hematite cement. The intensity of the red color can vary.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, depending on the degree of cementation and the nature of the iron oxide coatings.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, granular, and gritty to the touch. Individual sand grains are typically visible and palpable. Grain size ranges from 0.0625 mm to 2 mm.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual mineral grains (e.g., quartz) are typically anhedral to subhedral, reflecting their detrital origin. The rock itself does not exhibit a macroscopic crystal form.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No true cleavage as a rock. It breaks along grain boundaries or bedding planes. Individual mineral grains within the sandstone may exhibit cleavage (e.g., feldspar).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in continental depositional environments such as fluvial (river), eolian (desert dunes), and lacustrine (lake) settings, particularly in arid or semi-arid climates where oxidizing conditions favor the formation of ferric iron oxides. Also found in shallow marine environments where iron-rich sediments are exposed to oxygen.

Key Facts

Hardness

Variable, typically 6-7 on Mohs scale for quartz grains, but the overall rock hardness is lower (3-5) due to the cement and porosity. Can be scratched by a steel file.

Specific Gravity

2.2 - 2.8 g/cm3, depending on porosity and mineral composition.

Crystal System

Not applicable for the rock as a whole; constituent quartz grains are trigonal.

Color

Red, reddish-brown, maroon.

Luster

Dull, earthy.

Transparency

Opaque as a rock; individual quartz grains can be translucent.

Fracture

Irregular to conchoidal (for individual quartz grains), granular for the rock.

Cleavage

None as a rock; individual mineral grains may exhibit cleavage.

Composition

Predominantly quartz (SiO2) grains, cemented by iron oxides (primarily hematite, Fe2O3), with minor amounts of feldspar, mica, and other detrital minerals.

Physical characteristics

• Crystal Habit: Clastic, granular; individual grains are typically anhedral to subhedral.
• Cleavage Type: None (as a rock).
• Fracture Type: Granular, irregular.
• Tenacity: Brittle.
• Luster Type: Dull to earthy.

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