Iron-rich Sedimentary Rock

Sedimentary Rock

Hematitic Sandstone or Siltstone

Also known as: Hematitic Sandstone, Hematitic Siltstone, Ferruginous Sandstone, Ferruginous Siltstone

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Description

Iron-rich sedimentary rocks, specifically hematitic sandstones and siltstones, are clastic sedimentary rocks characterized by a significant proportion of iron oxide cement, predominantly hematite, which imparts a distinctive red, reddish-brown, or purplish color. The detrital grains are typically quartz, feldspar, and rock fragments, ranging in size from sand (0.0625 mm to 2 mm) to silt (0.0039 mm to 0.0625 mm). The iron oxide cement can range from a minor component to a dominant binder, influencing the rock's hardness, density, and overall appearance. The presence of hematite indicates oxidizing conditions during or after deposition.

How to Identify

Color: Typically red, reddish-brown, purplish-red, or maroon due to the presence of hematite. The intensity of the color depends on the concentration of iron oxides.
Luster: Dull to earthy, reflecting the fine-grained nature of the hematite cement and detrital grains. Rarely sub-metallic if hematite is present as specularite.
Texture: Clastic, ranging from arenaceous (sandy) to silty. Grains are typically sub-angular to rounded. Can be well-sorted or poorly sorted. The cement is often pervasive, filling pore spaces.
Crystal Form: Individual hematite crystals are typically microscopic and anhedral, forming a pervasive cement. Detrital grains retain their original clastic forms.
Cleavage: No rock cleavage. Individual mineral grains (e.g., quartz, feldspar) may exhibit their characteristic cleavage, but the rock as a whole does not.
Geological Environment: Formed in various depositional environments where iron-rich fluids were present and oxidizing conditions prevailed. This includes fluvial (river), deltaic, shallow marine, and lacustrine (lake) settings. Often associated with continental red beds, which indicate arid to semi-arid climates.

Key Facts

Hardness: Variable, typically 5.5 to 7 on the Mohs scale, depending on the proportion and crystallinity of quartz and hematite cement. Pure hematite is 5-6.
Specific Gravity: Variable, typically 2.6 to 3.5. Higher iron content leads to higher specific gravity. Pure quartz sandstone is ~2.65, pure hematite is ~5.26.
Crystal System: Hematite (the primary iron mineral) is trigonal. Detrital grains (e.g., quartz) are hexagonal.
Color: Red, reddish-brown, purplish-red, maroon.
Luster: Dull to earthy, sometimes sub-metallic if specular hematite is present.
Transparency: Opaque.
Fracture: Conchoidal to irregular in quartz grains; generally irregular to splintery for the rock as a whole.
Cleavage: None for the rock. Hematite has no cleavage. Quartz has no cleavage.
Composition: Primarily quartz (SiO2) and/or other detrital silicates (e.g., feldspar, rock fragments) cemented by hematite (Fe2O3). Minor amounts of other iron oxides/hydroxides (e.g., goethite) may be present.

Quick Check

Color: Red, reddish-brown, purplish-red
Luster: Dull to earthy
Streak: Reddish-brown to cherry-red

Physical Characteristics

Crystal Habit: Detrital grains are clastic (rounded to angular). Hematite cement is typically microcrystalline to cryptocrystalline, forming a pervasive matrix.
Cleavage Type: Absent in the rock. Individual detrital minerals may exhibit cleavage (e.g., feldspar).
Fracture Type: Irregular to conchoidal (for quartz grains).
Tenacity: Brittle.
Luster Type: Earthy to dull.

Formation

Formed by the accumulation and lithification of detrital grains (sand or silt) cemented by iron oxides, primarily hematite (Fe2O3). The iron oxides can be precipitated directly from iron-rich waters during or shortly after deposition (syngenetic/early diagenetic) or introduced later by circulating iron-bearing fluids (epigenetic). The red color is indicative of oxidized iron.

Usage

Historically used as a low-grade iron ore, especially if the hematite content is sufficiently high. Also used as building stone, aggregate, and in some cases, as a pigment source. Its durability and distinctive color make it suitable for decorative purposes.

Age Distribution

Common throughout the geological record, particularly abundant in Proterozoic and Phanerozoic successions where iron-rich conditions prevailed during deposition.

Where to Find

Colorado Plateau, USA

Extensive red bed sequences, including hematitic sandstones, are prominent in national parks like Zion, Bryce Canyon, and Grand Canyon, dating from the Permian to Jurassic periods.

Western Australia

While famous for Banded Iron Formations, some Proterozoic and Phanerozoic successions also contain significant hematitic sandstones and siltstones.

Devonian Old Red Sandstone, UK

A classic example of continental red beds, rich in iron oxides, found across parts of Scotland, Wales, and England.

Brazil

Various sedimentary basins contain iron-rich clastic rocks, particularly in regions with extensive iron ore deposits.

India

Sedimentary sequences associated with iron ore provinces often include hematitic sandstones and siltstones.

Finding Tips

Look for Red Beds

Target geological formations known as 'red beds,' which are extensive sequences of red-colored sedimentary rocks, often indicative of arid or semi-arid continental depositional environments and the presence of iron oxides.

Examine Outcrops

Look for distinctive reddish-brown to purplish-red outcrops in road cuts, river banks, cliffs, and quarries. The color is a primary indicator.

Check for Hardness and Density

Hematite cement can make the rock harder and denser than typical sandstone or siltstone. Test with a steel knife (Mohs 5.5) to see if it scratches easily (if not, it's harder than 5.5).

Perform a Streak Test

Rub a piece of the rock on an unglazed porcelain streak plate. A reddish-brown to cherry-red streak is characteristic of hematite, even if the rock itself appears darker.

Observe Grain Size

Determine if the rock is primarily composed of sand-sized grains (gritty feel) or silt-sized grains (smooth but not plastic when wet). This distinguishes sandstone from siltstone.

Similar Rocks

Banded Iron Formation

Also known as: BIF

Laterite

Also known as: Ferricrete

Red Shale

Also known as: Hematitic Shale

Jasper

Also known as: Red Chert

Scientific Classification

Mineral Class: Oxide (for hematite cement); Silicate (for detrital quartz/feldspar)
Group: Sedimentary Rock (Clastic)
Crystal System: Trigonal (for hematite); Hexagonal (for quartz)
Chemical Formula: SiO2 (quartz) + Fe2O3 (hematite) + other detrital minerals
Composition: Quartz, feldspar, rock fragments, and other detrital minerals cemented by hematite (Fe2O3) and sometimes other iron oxides/hydroxides.

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