How to identify Weathered Sedimentary Rock

To correctly identify Weathered Sedimentary Rock (Sedimentary Rock (undifferentiated)), 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 reddish-brown, yellowish, or mottled due to oxidation of iron minerals. Can also be bleached or grey if reducing conditions prevail or soluble components are leached. Original rock color may be obscured.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, often lacking the distinct luster of fresh mineral grains due to alteration and the presence of clay minerals or oxides.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Friable, crumbly, soft, or porous. Original clastic textures may be preserved but grains are often less coherent. Can feel gritty (if quartz-rich) or smooth/plastic (if clay-rich). May show relict bedding or original rock fabric.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Original crystal forms of constituent minerals may be degraded or replaced by secondary minerals (e.g., clay minerals, iron oxides) which typically lack distinct macroscopic crystal forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable to the rock as a whole. Individual mineral grains within the weathered rock may exhibit relict cleavage, but the overall rock mass will likely fracture irregularly due to weakened grain boundaries and altered mineralogy.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found at or near the Earth's surface, particularly in areas exposed to atmospheric conditions, water infiltration, and biological activity. Common in outcrops, road cuts, river banks, and any area where bedrock is exposed to weathering processes. Prevalent in humid and tropical climates where chemical weathering is intense, but also occurs in arid and temperate regions.

Key Facts

Hardness

Variable, typically significantly reduced compared to fresh rock (e.g., 1-5 on Mohs scale, depending on the degree of weathering and remaining minerals). Can be easily scratched with a knife or even a fingernail in highly weathered states.

Specific Gravity

Variable, generally lower than fresh rock due to increased porosity and replacement by lighter minerals (e.g., clays). Ranges from approximately 1.5 to 2.5 g/cm³.

Crystal System

Not applicable to the rock as a whole. Constituent minerals may have various crystal systems, but the overall rock is an aggregate of altered and primary grains.

Color

Highly variable, commonly shades of brown, red, yellow, grey, or mottled. Reflects the presence of iron oxides, clay minerals, and organic matter.

Luster

Dull, earthy, or powdery due to the presence of fine-grained alteration products and increased porosity.

Transparency

Opaque to translucent, depending on the constituent minerals and degree of alteration.

Fracture

Irregular, earthy, or conchoidal (if relict quartz grains are prominent). Often crumbles rather than fracturing cleanly.

Cleavage

Not applicable to the rock as a whole. Individual mineral grains may exhibit cleavage, but the rock mass lacks a unified cleavage.

Composition

Highly variable. Consists of primary sedimentary minerals (e.g., quartz, feldspar, calcite, clay minerals) in various stages of alteration, along with secondary minerals formed during weathering (e.g., iron oxides/hydroxides like goethite and hematite, kaolinite, illite, smectite, gypsum, soluble salts). Organic matter may also be present.

Physical characteristics

• Crystal Habit: Not applicable to the rock as a whole. Individual mineral grains may show relict habits or new secondary mineral habits (e.g., platy clays, amorphous oxides).
• Cleavage Type: Not applicable to the rock as a whole. Individual mineral components may exhibit cleavage (e.g., perfect in micas, good in feldspars), but the rock itself does not have a characteristic cleavage.
• Fracture Type: Irregular, earthy, or crumbly. May show relict conchoidal fracture if quartz grains are dominant and still coherent.
• Tenacity: Brittle to friable. Often very weak and easily broken or crumbled by hand, especially in advanced stages of weathering.
• Luster Type: Dull, earthy, or powdery.

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