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Red sandstone is a clastic sedimentary rock composed predominantly of sand-sized mineral particles or rock fragments. Its distinctive red to reddish-brown color is imparted by the presence of iron oxide minerals, primarily hematite, which acts as a cement or coating on the detrital grains. The primary constituent grains are typically quartz, with lesser amounts of feldspar, mica, and other accessory minerals. The texture can range from fine-grained to coarse-grained, and it often exhibits bedding planes and cross-bedding, indicative of its depositional environment.
How to Identify
- Color
- Predominantly red, reddish-brown, or maroon, due to hematite cement. The intensity of the red color can vary.
- Luster
- Dull to earthy, depending on the degree of cementation and the nature of the iron oxide coatings.
- Texture
- 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.
- Crystal Form
- 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.
- Cleavage
- 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).
- Geological Environment
- 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.
Quick Check
- Color: Red to reddish-brown
- Luster: Dull to earthy
- Streak: Reddish-brown (if sufficient hematite is present and the rock is soft enough to streak)
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.
Formation
Red sandstone forms from the lithification of sand grains, primarily quartz, cemented together by a matrix rich in iron oxides, predominantly hematite (Fe2O3). The iron oxides are typically introduced during or shortly after deposition through the oxidation of iron-bearing minerals in the sediment or groundwater, often in arid or semi-arid oxidizing environments. The red color is a direct result of the ferric iron (Fe3+) content.
Usage
Widely used as a building material, dimension stone, paving stone, and in architectural applications due to its aesthetic appeal and durability. Historically, it has been used for monuments, cathedrals, and residential buildings. Crushed red sandstone can be used as aggregate.
Age Distribution
Ranges from Precambrian to Cenozoic, with notable occurrences in the Devonian (Old Red Sandstone) and Triassic (New Red Sandstone) periods.
Where to Find
United States
Extensive deposits in the American Southwest (e.g., Colorado Plateau, Zion National Park, Grand Canyon), particularly in Triassic and Jurassic formations. Also found in the Appalachian Basin (e.g., Catskill Formation).
United Kingdom
The 'Old Red Sandstone' (Devonian) is prominent in Scotland, Wales, and parts of England. The 'New Red Sandstone' (Permian-Triassic) is found across central and southern England.
Australia
Significant occurrences in central and western Australia, often associated with arid environments.
Canada
Found in various regions, including the Maritimes (e.g., Prince Edward Island).
Finding Tips
Look for Outcrops
Red sandstone often forms prominent cliffs, mesas, and buttes in arid regions due to its resistance to erosion compared to softer shales.
Examine Road Cuts and Quarries
These exposures provide excellent cross-sections of rock layers and can reveal bedding structures and variations in color.
Check for Iron Staining
The presence of red or reddish-brown staining on surrounding rocks or soil can indicate nearby red sandstone deposits.
Observe Depositional Environments
Focus on areas that were historically continental, arid, or shallow marine, as these are common settings for red sandstone formation.
Similar Rocks
Brown Sandstone
Sandstone (rich in goethite/limonite)
Also known as: Ferruginous Sandstone
Arkose
Arkose
Also known as: Feldspathic Sandstone
Quartzite
Quartzite
Also known as: Metamorphosed Sandstone
Scientific Classification
- Mineral Class
- Sedimentary Rock (Clastic)
- Group
- Sandstone
- Crystal System
- Not applicable for the rock; constituent quartz is trigonal.
- Chemical Formula
- Primarily SiO2 (quartz) with Fe2O3 (hematite) cement.
- Composition
- Detrital quartz grains, cemented by iron oxides (hematite), with variable amounts of other minerals.
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