How to identify Weathered Rock Fragment

To correctly identify Weathered Rock Fragment (Indeterminate Sedimentary or Metamorphic Rock), 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, depending on the original rock and the degree/type of weathering. Can be shades of brown, red, yellow (due to iron oxides), grey, black, or mottled. Often duller than fresh rock.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, or sometimes slightly waxy if clay minerals are abundant. Rarely vitreous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Clastic, often rough or granular. May show signs of pitting, etching, or rounding if transported. Internal texture (foliation, bedding) may be preserved but often degraded.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Not applicable for a rock fragment; individual mineral grains within the fragment may show anhedral to subhedral forms, but weathering often obscures these.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not applicable for a rock fragment. The original rock may have had cleavage (e.g., slaty cleavage in slate, schistosity in schist), but weathering can degrade or obscure it. Individual mineral grains within the fragment may exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in any environment where weathering occurs: soils, regolith, riverbeds, glacial tills, beaches, desert pavements, and incorporated into clastic sedimentary rocks (e.g., conglomerates, sandstones, breccias).

Key Facts

Hardness

Variable, depends on the original rock and the degree of weathering. Can range from very soft (e.g., weathered shale) to hard (e.g., weathered quartzite). Often softer than the fresh parent rock.

Specific Gravity

Variable, typically between 2.0 and 2.8 g/cm³, depending on the original rock and the extent of alteration (e.g., hydration, dissolution).

Crystal System

Not applicable for a rock fragment; refers to individual minerals.

Color

Highly variable, often muted or discolored due to weathering.

Luster

Dull to earthy.

Transparency

Opaque to translucent at thin edges.

Fracture

Irregular, conchoidal (if siliceous), splintery, or blocky, depending on the original rock and weathering effects.

Cleavage

Not applicable for a rock fragment; refers to individual minerals or rock fabric.

Composition

Heterogeneous, consisting of various minerals (e.g., quartz, feldspar, micas, clay minerals, iron oxides) and rock fragments derived from either sedimentary or metamorphic parent rocks. May include secondary minerals formed during weathering.

Physical characteristics

• Crystal Habit: Not applicable for a rock fragment.
• Cleavage Type: Not applicable for a rock fragment; individual minerals may exhibit cleavage.
• Fracture Type: Irregular, conchoidal, splintery, or blocky.
• Tenacity: Brittle to friable, depending on the degree of weathering and cementation (if part of a sedimentary rock).
• Luster Type: Dull, earthy.

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