Sandstone with Iron Staining and Lichen

Sedimentary Rock (modified by biological and chemical weathering)

Sandstone (sedimentary rock) with iron oxides/hydroxides and various lichen species

Also known as: Lichen-covered Iron-stained Sandstone

Got a photo? Identify rocks instantly with the Rockby app

Description

This rock is primarily sandstone, a clastic sedimentary rock composed of sand-sized mineral grains (0.0625 mm to 2 mm), typically quartz, feldspar, and rock fragments, cemented together. The defining characteristics are the pervasive reddish, yellowish, or brownish coloration due to iron oxide/hydroxide staining, and the presence of various lichen species growing on its exposed surfaces. The iron staining can be uniform, patchy, or form Liesegang banding. Lichens appear as crusty, leafy, or bushy growths, varying in color (green, gray, yellow, orange, black) depending on the species and environmental conditions.

How to Identify

Color: Dominantly reddish-brown, orange, yellow, or dark brown due to iron oxides/hydroxides. Lichens add patches of green, gray, yellow, orange, or black.
Luster: Dull to earthy for the iron-stained matrix; individual quartz grains may show vitreous luster. Lichens are typically matte or dull.
Texture: Clastic, granular, sand-sized grains (0.0625-2 mm) are visible and palpable. Can range from fine-grained to coarse-grained. Lichens add a crusty, foliose, or fruticose texture to the surface.
Crystal Form: Individual sand grains are typically anhedral to subhedral. Iron oxides/hydroxides are amorphous or microcrystalline coatings. Lichens do not exhibit crystal forms.
Cleavage: No true cleavage for the rock as a whole. Individual mineral grains (e.g., feldspar) may show cleavage, but the rock breaks along grain boundaries or cement. Lichens have no cleavage.
Geological Environment: Exposed outcrops, cliffs, desert environments, coastal areas, riverbeds, and any terrestrial environment where sandstone is exposed to weathering and biological colonization. Often found in arid to semi-arid regions where iron oxidation is prominent, and in areas with stable rock surfaces for lichen growth.

Key Facts

Hardness: Variable, typically 6-7 for quartz grains, but the overall rock hardness depends on cementation (can be 2-7). Lichens are soft and easily scraped.
Specific Gravity: 2.2-2.8 g/cm³ (for sandstone), lichens have negligible impact on bulk specific gravity.
Crystal System: Sandstone is a rock, not a single mineral. Quartz is hexagonal. Iron oxides/hydroxides can be amorphous or crystalline (e.g., hematite is trigonal, goethite is orthorhombic). Lichens are biological organisms.
Color: Reddish-brown, orange, yellow, with green, gray, yellow, orange, black lichen patches.
Luster: Dull to earthy (rock), matte (lichen).
Transparency: Opaque (rock), lichens are opaque.
Fracture: Conchoidal to irregular (for quartz grains), granular (for the rock).
Cleavage: None (for the rock).
Composition: Primarily quartz (SiO₂), with varying amounts of feldspar, rock fragments, and clay minerals. Cemented by silica, calcite, or iron oxides/hydroxides (Fe₂O₃, FeO(OH)). Lichens are composed of fungal hyphae and algal/cyanobacterial cells.

Quick Check

Color: Reddish-brown to orange with patches of green, gray, yellow, or black (lichen).
Luster: Dull to earthy (rock matrix), matte (lichen).
Streak: White to light brown (for the sandstone matrix), reddish-brown to yellowish-brown (if significant iron oxide is scraped). Lichens do not produce a streak.

Physical Characteristics

Crystal Habit: Granular (sandstone), amorphous/microcrystalline (iron oxides), biological growth forms (lichens: crustose, foliose, fruticose).
Cleavage Type: Not applicable for the rock as a whole.
Fracture Type: Granular to irregular.
Tenacity: Brittle (sandstone), lichens are flexible when wet, brittle when dry.
Luster Type: Dull to earthy.

Formation

Sandstone forms from the lithification of sand-sized grains, primarily quartz, through compaction and cementation. Iron staining results from the oxidation of iron-bearing minerals (e.g., pyrite, biotite, amphiboles) within the sandstone or from iron-rich groundwater percolating through the rock. The iron oxides/hydroxides (e.g., hematite, goethite, limonite) precipitate and coat the sand grains or fill pore spaces. Lichens are symbiotic organisms (algae/cyanobacteria and fungi) that colonize exposed rock surfaces, deriving nutrients from the rock and atmosphere, and contributing to both physical and chemical weathering.

Usage

While the sandstone itself has historical uses in construction, the presence of iron staining and lichen generally indicates an exposed, weathered surface. Lichens are important bioindicators of air quality and environmental conditions. Iron staining can sometimes enhance the aesthetic appeal for decorative purposes or landscaping, but the rock's structural integrity might be compromised by extensive weathering.

Age Distribution

Sandstones can range from Precambrian to Cenozoic. Iron staining can occur at any post-depositional stage. Lichen colonization typically occurs on exposed surfaces over decades to centuries.

Where to Find

Colorado Plateau, USA

Extensive sandstone formations (e.g., Navajo Sandstone, Entrada Sandstone) exhibit prominent iron staining and diverse lichen communities due to arid conditions and long-term exposure.

Australian Outback

Many sandstone outcrops and rock formations display intense iron staining (e.g., 'Uluru' - Arkose, but similar staining processes) and various lichen species adapted to harsh conditions.

Garden of the Gods, Colorado, USA

Spectacular red sandstone formations with visible iron staining and lichen growth on exposed surfaces.

Coastal Cliffs (e.g., UK, France)

Sandstone cliffs exposed to marine aerosols and weathering often show iron staining and robust lichen colonization.

Finding Tips

Look for Exposed Surfaces

This rock type is a product of surface weathering and biological activity, so it will be found on exposed rock faces, boulders, and outcrops rather than deep underground.

Identify Iron Staining

Search for reddish, orange, or yellowish discoloration that is not uniform but often follows bedding planes, fractures, or forms Liesegang rings. This staining is distinct from the primary color of the sandstone.

Observe Lichen Growth

Look for crusty, leafy, or bushy growths on the rock surface. Lichens come in many colors (green, gray, yellow, orange, black) and textures. They are often more prevalent on stable, undisturbed surfaces.

Consider the Environment

Areas with a history of iron-rich groundwater, fluctuating water tables, or long-term exposure to atmospheric conditions are prime locations.

Similar Rocks

Unstained Sandstone

Arenite

Also known as: Clean Sandstone

Iron-rich Shale

Argillite (with high iron content)

Also known as: Red Shale

Laterite

Also known as: Iron-rich Soil

Bio-weathered Granite

Granite (with lichen colonization)

Also known as: Lichen-covered Granite

Scientific Classification

Mineral Class: Sedimentary Rock (clastic) with secondary mineral alteration and biological colonization.
Group: Siliciclastic Sedimentary Rock (Arenite) with Iron Oxides/Hydroxides and Lichen.
Crystal System: Not applicable for the rock as a whole. Constituent minerals vary.
Chemical Formula: Primarily SiO₂ (quartz), Fe₂O₃, FeO(OH) (iron oxides/hydroxides), plus organic compounds from lichens.
Composition: Quartz, feldspar, rock fragments, clay minerals, iron oxides/hydroxides (hematite, goethite, limonite), and organic matter from various lichen species.

Explore Sandstone with Iron Staining and Lichen

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