How to identify Sandstone with Iron Staining and Lichen

To correctly identify Sandstone with Iron Staining and Lichen (Sandstone (sedimentary rock) with iron oxides/hydroxides and various lichen species), check each of these features in order. Many rocks and minerals look alike, so cross-reference multiple properties before deciding.

Step-by-step identification

1. 1

   Color

   Overall color and any zoning or banding

   Dominantly reddish-brown, orange, yellow, or dark brown due to iron oxides/hydroxides. Lichens add patches of green, gray, yellow, orange, or black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy for the iron-stained matrix; individual quartz grains may show vitreous luster. Lichens are typically matte or dull.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual sand grains are typically anhedral to subhedral. Iron oxides/hydroxides are amorphous or microcrystalline coatings. Lichens do not exhibit crystal forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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.

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