How to identify Weathered Sedimentary Rock

To correctly identify Weathered Sedimentary Rock (Sedimentary Rock (likely sandstone or siltstone with iron staining)), 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, yellowish-brown, orange, or ochre due to iron oxyhydroxide staining. Original rock colors (e.g., grey, white) may be visible in less weathered areas or as relict patches.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, often non-lustrous due to the porous nature and presence of earthy iron oxides.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically clastic, ranging from fine-grained (siltstone) to medium-grained (sandstone). Grains may be poorly cemented and friable, easily crumbling. Can feel gritty (sandstone) or smooth/silky (siltstone). Pitting, solution features, and weathering rinds are common.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual mineral grains (e.g., quartz) are typically anhedral to subhedral, detrital. Secondary iron minerals often form amorphous coatings or microcrystalline aggregates, not distinct crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No true cleavage in the rock mass. Individual mineral grains within the rock may exhibit cleavage (e.g., feldspar), but the rock as a whole fractures irregularly. Weathering can create planes of weakness that mimic bedding or jointing.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Exposed outcrops, road cuts, river banks, arid and semi-arid regions, humid tropical environments (where chemical weathering is intense), and any area where sedimentary rocks are subjected to subaerial weathering processes.

Key Facts

Hardness

Variable, typically low (1-5 on Mohs scale) for the weathered matrix, but individual quartz grains can be 7. Overall rock integrity is compromised.

Specific Gravity

Variable, typically 2.0-2.6 g/cm³ for the bulk rock, lower than unweathered counterparts due to increased porosity and less dense weathering products.

Crystal System

Not applicable to the rock as a whole. Constituent minerals have their own crystal systems (e.g., quartz is trigonal, iron oxides are various).

Color

Reddish-brown, yellowish-brown, orange, ochre.

Luster

Dull, earthy.

Transparency

Opaque.

Fracture

Irregular, conchoidal (for individual quartz grains), or granular (for the rock mass).

Cleavage

None for the rock mass. Individual mineral grains may exhibit cleavage.

Composition

Primarily quartz (SiO₂), feldspar, and rock fragments, cemented by silica, calcite, or clay minerals. Secondary components include iron oxyhydroxides (e.g., goethite FeO(OH), hematite Fe₂O₃, limonite - a mixture of hydrated iron oxides), clay minerals (e.g., kaolinite, illite), and sometimes gypsum or other evaporites.

Physical characteristics

• Crystal Habit: Not applicable to the rock. Constituent minerals are typically anhedral to subhedral detrital grains. Secondary iron minerals form earthy masses or coatings.
• Cleavage Type: None for the rock mass. Individual mineral grains may show cleavage (e.g., feldspar: perfect basal and prismatic).
• Fracture Type: Irregular to granular, often friable.
• Tenacity: Brittle, often friable or crumbly.
• Luster Type: Dull, earthy.

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