How to identify Weathered Sedimentary Rock

To correctly identify Weathered Sedimentary Rock (Sedimentary rock (unspecified type, heavily weathered)), 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

   Highly variable, often dull and earthy. Can be reddish-brown, yellowish, grey, or mottled, depending on the original rock and the presence of iron oxides or organic matter.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Earthy to dull, rarely vitreous or pearly due to the breakdown of original mineral grains and the presence of clay minerals.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Friable, crumbly, granular, or powdery. Original clastic textures may be obscured. May feel gritty or clayey depending on the dominant weathering products.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for the rock as a whole. Individual mineral grains, if still identifiable, may show relict crystal forms, but weathering products (e.g., clay minerals) are typically microscopic and anhedral.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable for the rock as a whole. Original bedding planes may be preserved but are often weakened or obscured. Individual mineral grains may exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Exposed outcrops, road cuts, river banks, coastal cliffs, and any surface environment where sedimentary rocks are subjected to prolonged physical and chemical weathering. Common in humid tropical and temperate climates.

Key Facts

Hardness

Very low (1-3 on Mohs scale) for heavily weathered material, easily scratched or crumbled by a fingernail or knife. Original mineral hardnesses are largely irrelevant for the bulk weathered rock.

Specific Gravity

Variable, generally lower than the unweathered parent rock due to increased porosity and presence of lighter weathering products (e.g., clay minerals). Typically ranges from 1.8 to 2.5.

Crystal System

Not applicable for the rock as a whole. Constituent minerals may have various crystal systems.

Color

Highly variable, often dull and earthy; reddish-brown, yellowish, grey, or mottled.

Luster

Earthy to dull.

Transparency

Opaque.

Fracture

Irregular, earthy, or crumbly. Original fracture patterns may be obscured.

Cleavage

Not applicable for the rock as a whole. Original bedding planes may be preserved but are often weakened.

Composition

Highly variable, depending on the original sedimentary rock and the degree/type of weathering. Typically consists of residual quartz, clay minerals (kaolinite, illite, smectite), iron oxides/hydroxides (goethite, hematite), and possibly relict feldspars, micas, or other resistant minerals. Organic matter may also be present.

Physical characteristics

• Crystal Habit: Not applicable for the bulk rock. Individual mineral grains may be anhedral to subhedral.
• Cleavage Type: Not applicable for the bulk rock. Original bedding planes may be preserved but are often weakened.
• Fracture Type: Earthy, irregular, or crumbly.
• Tenacity: Friable, brittle, easily disaggregated.
• Luster Type: Earthy to dull.

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