Iron-Rich Sandstone

Sedimentary Rock

Sedimentary rock, predominantly quartz with iron oxide cement/veining

Also known as: Ferruginous Sandstone, Red Sandstone, Brown Sandstone (depending on iron oxide type)

Got a photo? Identify rocks instantly with the Rockby app

Description

Sandstone with iron oxide is a clastic sedimentary rock characterized by its dominant quartz sand grains and the presence of iron oxide minerals acting as cement, matrix, or forming veins. The iron oxides impart characteristic red, brown, yellow, or orange hues to the rock. The texture can range from fine-grained to coarse-grained, and the degree of cementation by iron oxides can vary, affecting the rock's hardness and durability. The primary mineralogy is quartz (SiO2), with varying amounts of feldspar, lithic fragments, and accessory minerals, all bound by iron oxide minerals (e.g., hematite Fe2O3, goethite FeO(OH), limonite FeO(OH)·nH2O).

How to Identify

Color: Typically red, reddish-brown, brown, yellow, or orange due to the iron oxide content. The specific hue depends on the type and hydration state of the iron oxide (e.g., hematite for red, goethite/limonite for yellow/brown).
Luster: Dull to earthy, reflecting the luster of the quartz grains and the earthy nature of the iron oxide cement.
Texture: Clastic, granular, with visible sand grains (typically 0.0625 mm to 2 mm in diameter). Can be fine-grained, medium-grained, or coarse-grained. May feel gritty to the touch.
Crystal Form: Individual quartz grains are typically sub-angular to rounded. Iron oxides occur as amorphous or microcrystalline coatings, cements, or vein fillings, not typically as macroscopic crystals.
Cleavage: No rock cleavage. Individual quartz grains exhibit conchoidal fracture, but the rock as a whole does not show cleavage planes.
Geological Environment: Common in continental depositional environments such as fluvial (river), eolian (desert), and deltaic settings. Also found in shallow marine environments where oxidizing conditions prevail. Often associated with ancient red bed sequences.

Key Facts

Hardness: Variable, typically 6-7 on Mohs scale for quartz grains, but the overall rock hardness can be lower (3-6) depending on the strength and abundance of the iron oxide cement.
Specific Gravity: 2.65 - 2.95 g/cm³ (higher if significant iron oxide content, lower if porous).
Crystal System: Quartz is trigonal; iron oxides (hematite, goethite) are trigonal and orthorhombic, respectively. The rock itself is an aggregate of mineral grains.
Color: Red, reddish-brown, brown, yellow, orange.
Luster: Dull to earthy.
Transparency: Opaque (rock), individual quartz grains can be translucent to transparent.
Fracture: Irregular to conchoidal (for individual quartz grains). The rock typically breaks along grain boundaries or through the cement.
Cleavage: None (for the rock). Quartz has no cleavage.
Composition: Predominantly quartz (SiO2) grains, cemented by iron oxides (e.g., hematite Fe2O3, goethite FeO(OH), limonite FeO(OH)·nH2O). May contain minor amounts of feldspar, mica, and other accessory minerals.

Quick Check

Color: Red, reddish-brown, brown, yellow, orange
Luster: Dull to earthy
Streak: Reddish-brown (for hematite-rich), yellowish-brown (for goethite/limonite-rich)

Physical Characteristics

Crystal Habit: Clastic, granular; quartz grains are typically anhedral to subhedral, rounded to sub-angular. Iron oxides form microcrystalline coatings or amorphous masses.
Cleavage Type: Absent in the rock.
Fracture Type: Irregular to granular.
Tenacity: Brittle.
Luster Type: Dull to earthy.

Formation

Sandstone with iron oxide forms from the lithification of sand grains, predominantly quartz, cemented together by iron oxides (e.g., hematite, goethite, limonite). The iron oxides can be introduced during deposition as detrital grains, precipitated from iron-rich pore waters during diagenesis, or formed by the oxidation of iron-bearing minerals within the sandstone after deposition. This often occurs in oxidizing environments, such as continental settings (fluvial, eolian) or shallow marine environments.

Usage

Historically used as a building stone, paving material, and for decorative purposes due to its attractive colors. Some iron-rich sandstones can be minor sources of iron ore if the iron content is sufficiently high and concentrated. Also used as aggregate and in some cases, for abrasive applications.

Age Distribution

Found throughout the geological record, from Precambrian to Cenozoic, wherever conditions for sandstone deposition and subsequent iron oxidation or precipitation occurred.

Where to Find

Colorado Plateau, USA

Extensive red sandstone formations (e.g., Navajo Sandstone, Entrada Sandstone) are prominent, colored by hematite.

Western Australia

Many iron ore deposits are associated with banded iron formations and lateritic weathering, which can produce iron-rich sandstones.

United Kingdom

Old Red Sandstone (Devonian) and New Red Sandstone (Permian-Triassic) are widespread, characterized by their reddish hues from iron oxides.

Brazil

Lateritic soils and iron-rich sedimentary sequences are common, leading to the formation of ferruginous sandstones.

Finding Tips

Look for Red Beds

Iron-rich sandstones are often part of 'red bed' sequences, which are thick successions of sedimentary rocks deposited in oxidizing continental environments. Look for outcrops with prominent reddish, brownish, or yellowish coloration.

Examine Outcrops

Observe road cuts, river banks, and cliff faces where sedimentary layers are exposed. The characteristic color and granular texture will be evident.

Check for Hardness and Grain Size

Test the hardness with a steel knife (quartz is harder, iron oxide cement can be softer). Feel the texture to determine grain size. The presence of iron oxide cement can make the rock more resistant to weathering than uncemented sandstone.

Consider Associated Minerals

While primarily quartz and iron oxides, look for other detrital minerals like feldspar, mica, or heavy minerals, which can provide clues about the source area.

Similar Rocks

Quartzite

Also known as: Metasandstone

Arkose

Also known as: Feldspathic Sandstone

Ironstone

Also known as: Siderite-rich rock, Oolitic Ironstone

Scientific Classification

Mineral Class: Sedimentary Rock (Clastic)
Group: Sandstone
Crystal System: Not applicable for a rock; constituent minerals are trigonal (quartz, hematite) and orthorhombic (goethite).
Chemical Formula: SiO2 (for quartz) + Fe2O3 (hematite) / FeO(OH) (goethite) / FeO(OH)·nH2O (limonite) + other minor constituents.
Composition: Silica (quartz) and various iron oxides, with minor amounts of other detrital minerals.

Explore Iron-Rich Sandstone

Identification Localities & Collecting Look-alikes

Identify rocks anywhere

Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.

Download on theApp Store Get it onGoogle Play Continue onWeb

Open in app