How to identify Weathered Sedimentary Rock

To correctly identify Weathered Sedimentary Rock (Sedimentary rock (unspecified type, likely sandstone or conglomerate)), 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 than fresh rock. Can be reddish, yellowish, brownish due to iron oxides; grayish, whitish, or mottled depending on original composition and weathering products (e.g., clays, leached minerals).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Typically dull, earthy, or powdery due to the presence of clay minerals, iron oxides, and fragmented surfaces. Original mineral luster (e.g., vitreous quartz) may be obscured.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Friable, crumbly, soft, or porous. Grains may be easily dislodged. Original clastic texture (e.g., sand grains, pebbles) may be preserved but with weakened cementation. Can feel gritty, earthy, or clayey.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Original crystal forms of constituent minerals (e.g., quartz, feldspar) are typically preserved but may be corroded or fractured. New secondary minerals (e.g., clays) may form as microscopic aggregates, not distinct crystals visible to the naked eye.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Cleavage of individual mineral grains (e.g., feldspar, mica) may be observed if the rock is not too heavily weathered. However, the overall rock mass typically lacks macroscopic cleavage planes due to disaggregation and alteration.

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. Common in outcrops, road cuts, river banks, and any area undergoing erosion. Often forms the regolith or saprolite layer above bedrock.

Key Facts

Hardness

Highly variable, generally softer than the unweathered rock. Can range from 1 (talc-like) to 7 (quartz), but the overall rock mass will be significantly weaker.

Specific Gravity

Variable, generally lower than the unweathered rock due to increased porosity and presence of lighter secondary minerals (e.g., clays). Typically 1.8-2.5 g/cm3.

Crystal System

Not applicable to the rock as a whole; refers to constituent minerals (e.g., quartz is trigonal, feldspars are triclinic/monoclinic). Secondary minerals like clays are typically monoclinic or triclinic, often forming cryptocrystalline aggregates.

Color

Highly variable, often duller, reddish, yellowish, brownish, grayish, or mottled.

Luster

Dull, earthy, powdery.

Transparency

Opaque to translucent, depending on the degree of weathering and mineral composition.

Fracture

Irregular, earthy, or conchoidal if individual quartz grains are fractured. The overall rock mass often crumbles rather than fracturing cleanly.

Cleavage

Absent at the rock scale; individual mineral grains may exhibit cleavage (e.g., feldspar, mica).

Composition

Composed of original sedimentary rock minerals (e.g., quartz, feldspar, clay minerals, calcite, rock fragments) along with secondary weathering products such as various clay minerals (kaolinite, illite, smectite), iron oxides/hydroxides (hematite, goethite), and sometimes residual resistant minerals.

Physical characteristics

• Crystal Habit: Not applicable to the rock as a whole. Constituent minerals may show their characteristic habits, but often corroded or fragmented. Secondary minerals form fine-grained aggregates.
• Cleavage Type: Not applicable to the rock as a whole. Individual mineral grains may exhibit perfect, good, or poor cleavage.
• Fracture Type: Irregular, earthy, or crumbly. May show conchoidal fracture if quartz grains are dominant and intact.
• Tenacity: Brittle to friable. The rock mass is generally much less tenacious than its unweathered counterpart.
• Luster Type: Dull, earthy, powdery.

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