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Iron-rich sandstone is a clastic sedimentary rock characterized by a significant proportion of iron oxide or hydroxide minerals, which act as a cementing agent and/or detrital grains. This iron content imparts distinctive reddish-brown, yellow, or purplish hues to the rock. The primary framework grains are typically quartz, but feldspar and lithic fragments can also be present. The degree of iron enrichment can vary widely, from a subtle staining to a dominant cementing material that significantly increases the rock's density and hardness.
How to Identify
- Color
- Typically reddish-brown, orange, yellow, or purplish due to iron oxides (hematite, goethite). The color can be uniform or mottled.
- Luster
- Dull to earthy, depending on the abundance and type of iron cement. Quartz grains may exhibit a vitreous luster.
- Texture
- Clastic, granular, with sand-sized grains (0.0625 mm to 2 mm). Grains are typically visible to the naked eye. Can range from fine-grained to coarse-grained. May feel gritty.
- Crystal Form
- Individual mineral grains (e.g., quartz) are typically anhedral to subhedral. Iron oxide cement forms a microcrystalline to amorphous matrix around the grains.
- Cleavage
- No rock cleavage. Individual mineral grains (e.g., feldspar) may show cleavage, but the rock as a whole does not. Fracture is typically irregular or conchoidal through the cement and grains.
- Geological Environment
- Fluvial (river), deltaic, shallow marine, eolian (desert dunes), and lacustrine (lake) environments where iron-bearing solutions are present or iron-rich sediments are deposited. 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) if the cement is weaker or higher if the iron cement is very strong and pervasive.
- Specific Gravity: Variable, typically 2.6 to 3.0 g/cm³, higher than pure quartz sandstone (2.65 g/cm³) due to the denser iron minerals.
- Crystal System: Not applicable for the rock as a whole. Constituent minerals (e.g., quartz - trigonal, hematite - trigonal, goethite - orthorhombic) have their own crystal systems.
- Color: Reddish-brown, orange, yellow, purplish, often mottled.
- Luster: Dull to earthy, with individual quartz grains possibly vitreous.
- Transparency: Opaque.
- Fracture: Irregular to conchoidal, depending on the cement and grain packing.
- Cleavage: None for the rock. Individual mineral grains may exhibit cleavage.
- Composition: Predominantly quartz (SiO₂) grains, cemented by iron oxides (e.g., hematite - Fe₂O₃, goethite - FeO(OH)) or hydroxides (e.g., limonite - hydrated iron oxides). May also contain feldspar, lithic fragments, and other accessory minerals.
Quick Check
- Color: Reddish-brown, orange, yellow, purplish
- Luster: Dull to earthy
- Streak: Reddish-brown (if hematite is dominant), yellowish-brown (if goethite/limonite is dominant), or white (if iron content is low and quartz dominates the streak)
Physical Characteristics
- Crystal Habit: Clastic, granular texture with sand-sized grains. Iron cement forms a microcrystalline to amorphous matrix.
- Cleavage Type: Not applicable for the rock. Individual mineral grains may show cleavage.
- Fracture Type: Irregular to conchoidal.
- Tenacity: Brittle.
- Luster Type: Dull to earthy.
Formation
Iron-rich sandstone forms from the lithification of sand-sized clastic sediments (primarily quartz) that have been cemented by iron oxides or hydroxides. The iron minerals (e.g., hematite, goethite, limonite) can be introduced during deposition as detrital grains, or more commonly, precipitated from iron-bearing solutions circulating through the pore spaces of the sand after deposition (diagenesis). The presence of oxygenated water is crucial for the oxidation of iron and the formation of these cementing minerals. Environments include fluvial, deltaic, shallow marine, and eolian settings where iron is available from weathering of source rocks.
Usage
Historically, some iron-rich sandstones have been used as low-grade iron ore, particularly if the iron content is sufficiently high. More commonly, they are used as building stone, aggregate, and in some cases, as a source of pigment (ochre). The durability and distinctive color make them attractive for architectural applications.
Age Distribution
Found throughout the geological record, from Precambrian to Cenozoic, wherever conditions for iron oxidation and clastic sedimentation co-occurred.
Where to Find
Colorado Plateau, USA
Extensive formations like the Navajo Sandstone and Entrada Sandstone exhibit significant iron staining, particularly in areas like Zion National Park and Arches National Park, giving them their characteristic red and orange colors.
Western Australia
The Hamersley Basin contains vast deposits of banded iron formations (BIFs) and associated iron-rich sedimentary rocks, though these are often more iron-rich than typical ferruginous sandstone.
Nubian Sandstone Aquifer System, North Africa
Large areas of iron-stained sandstones are found across Egypt, Sudan, Libya, and Chad, forming significant aquifer systems.
Devonian Old Red Sandstone, UK
Widespread in parts of Scotland, Wales, and England, these sandstones are often rich in hematite, giving them a distinctive red color.
Finding Tips
Look for Reddish Hues
The most obvious indicator is the characteristic red, orange, or brown coloration. This color is often more pronounced on weathered surfaces.
Examine Grain Size and Texture
Confirm it's a sandstone by observing sand-sized grains. The iron cement may make the rock feel denser and harder than typical quartz sandstone.
Check for Iron Staining
Iron oxides can sometimes be seen as a coating on individual grains or as a pervasive cement. A streak test (on an unglazed porcelain plate) might yield a reddish-brown streak if hematite is abundant.
Consider the Geological Context
Look for it in sedimentary sequences, particularly those deposited in oxidizing environments or near sources of iron-rich minerals.
Similar Rocks
Quartz Sandstone
Quartz Arenite
Also known as: Arenite
Ironstone
Ironstone
Also known as: Siderite Ironstone, Hematitic Ironstone
Laterite
Laterite
Also known as: Bauxite (if aluminum-rich)
Scientific Classification
- Mineral Class
- Sedimentary Rock (Clastic)
- Group
- Sandstone
- Crystal System
- Not applicable for the rock.
- Chemical Formula
- Primarily SiO₂ (quartz) + Fe₂O₃ (hematite) or FeO(OH) (goethite/limonite) + other minor constituents.
- Composition
- Quartz grains cemented by iron oxides/hydroxides. Variable proportions of other detrital minerals.
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