How to identify Weathered Sedimentary Rock

To correctly identify Weathered Sedimentary Rock (Sedimentary rock (likely sandstone or siltstone) with weathering rind), 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

   The weathering rind typically exhibits colors different from the fresh interior, often reddish-brown, yellowish-orange, dark brown, or sometimes bleached white/gray. The interior will reflect the original rock's color (e.g., light gray, tan, white for quartz-rich sandstone).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy in the weathered rind due to the presence of clay minerals, iron oxides, and altered surfaces. The fresh interior may show a dull to vitreous luster depending on mineralogy and cement.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The rind can be more friable (crumbly) or, conversely, harder (case-hardened) than the interior. It may show pitting, exfoliation, or a smoother, more compact surface. The interior will have the characteristic clastic texture of sandstone (gritty, sand-sized grains) or siltstone (smooth, fine-grained, slightly gritty).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual mineral grains within the sedimentary rock are typically anhedral to subhedral. Secondary minerals within the rind (e.g., iron oxides, clay minerals) may form microcrystalline aggregates or coatings.

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 exhibit cleavage, but this is rarely observable in hand samples of fine-grained sedimentary rocks. The rock may fracture along bedding planes or joints.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Exposed outcrops, cliffs, riverbeds, glacial erratics, archaeological sites, and any surface where sedimentary rocks are subjected to subaerial weathering. Common in arid, semi-arid, temperate, and tropical climates.

Key Facts

Hardness

Varies significantly. The rind can be softer (1-3 on Mohs scale if clay-rich or leached) or harder (up to 6-7 if case-hardened by silica or iron oxide cementation) than the fresh interior (typically 6-7 for quartz-rich sandstone/siltstone).

Specific Gravity

Varies depending on the original rock composition and the degree/type of weathering. Generally 2.2 - 2.8 g/cm³.

Crystal System

Not applicable to the rock as a whole. Constituent minerals (e.g., quartz, feldspar) have their own crystal systems.

Color

Highly variable, as described above.

Luster

Dull to earthy.

Transparency

Opaque.

Fracture

Irregular to conchoidal (if silica-cemented) in the fresh rock; often friable or granular in the weathered rind.

Cleavage

None for the rock; individual mineral grains may exhibit cleavage.

Composition

Primarily quartz, feldspar, and clay minerals, with varying amounts of lithic fragments, micas, and heavy minerals. The weathering rind will have an enriched or depleted concentration of certain elements and secondary minerals like iron oxides (hematite, goethite), manganese oxides, and various clay minerals (kaolinite, illite, smectite).

Physical characteristics

• Crystal Habit: Clastic, detrital grains for the primary rock. Secondary minerals in the rind may form microcrystalline coatings or aggregates.
• Cleavage Type: Absent in the rock mass.
• Fracture Type: Irregular, granular, or conchoidal.
• Tenacity: Brittle, but can be friable in the weathered rind.
• Luster Type: Dull to earthy.

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