How to identify Weathered Sedimentary Rock

To correctly identify Weathered Sedimentary Rock (Sedimentary rock (likely sandstone or shale, 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 duller or more muted than the fresh rock. Can be reddish-brown (due to iron oxidation), yellowish, grey, or mottled. Original rock colors may be bleached or stained.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, or absent. Original mineral lusters are typically obscured by alteration products.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Friable, crumbly, soft, porous, or earthy. Original clastic textures (e.g., sand grains in sandstone) may be preserved but are easily disaggregated. Shales may become fissile or break down into clay-rich material.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Original crystal forms of constituent minerals (e.g., quartz in sandstone) may be present but are often corroded or obscured by secondary minerals (e.g., clays, iron oxides). No distinct macroscopic crystal forms are typically observed in the weathered mass.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Original rock cleavage (e.g., fissility in shale) may be preserved but is often weakened or destroyed. Minerals within the rock may exhibit their characteristic cleavage, but the rock as a whole will not have a distinct cleavage plane due to disaggregation.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Exposed outcrops, road cuts, river banks, coastal cliffs, arid and humid environments, areas with significant freeze-thaw cycles, and regions with active chemical weathering (e.g., tropical climates).

Key Facts

Hardness

Very soft to moderately soft (typically <3 on Mohs scale), easily scratched with a knife or fingernail, depending on the degree of weathering and original rock type. Can be friable.

Specific Gravity

Variable, generally lower than the fresh rock due to increased porosity and alteration to lighter minerals (e.g., clays). Typically 1.8 - 2.5.

Crystal System

Not applicable to a rock; constituent minerals retain their crystal systems, but the rock as a whole is an aggregate.

Color

Highly variable, often muted or stained (reddish-brown, yellowish, grey, mottled).

Luster

Dull, earthy, or absent.

Transparency

Opaque.

Fracture

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

Cleavage

Absent as a rock property; individual mineral grains may exhibit cleavage, but the weathered rock mass does not.

Composition

Variable, depending on the original sedimentary rock. Typically composed of primary minerals (e.g., quartz, feldspar, clay minerals, calcite) and secondary weathering products (e.g., iron oxides/hydroxides like goethite and limonite, various clay minerals like kaolinite, illite, smectite, gypsum, soluble salts). The original cementing agents (e.g., silica, calcite, iron oxides) may be weakened or dissolved.

Physical characteristics

• Crystal Habit: Not applicable to a rock; constituent minerals have their own habits, but the weathered rock is an aggregate of altered grains and secondary minerals.
• Cleavage Type: Not applicable to a rock; individual minerals may have cleavage, but the weathered rock mass does not exhibit a distinct cleavage.
• Fracture Type: Irregular, earthy, granular, or conchoidal (if silica-rich and still somewhat coherent).
• Tenacity: Friable, brittle, earthy, or crumbly.
• Luster Type: Dull, earthy, or absent.

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