How to identify Weathered Rock

To correctly identify Weathered Rock (Indeterminate 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, often discolored compared to fresh rock. Common colors include reds, browns, yellows (due to iron oxides), grays, and whites. Can be mottled.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, or variable depending on the secondary minerals present. Original mineral lusters may be obscured.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Friable, crumbly, porous, or softened. Original rock texture may be partially or completely obliterated. Can range from granular to clay-rich.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Original crystal forms may be degraded or replaced by secondary minerals (e.g., clay minerals, amorphous iron oxides).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Original mineral cleavage may be preserved in relict grains, but the overall rock mass often lacks coherent cleavage due to alteration and breakdown.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found at or near the Earth's surface, in outcrops, road cuts, stream banks, and any area exposed to atmospheric and biological agents. Common in humid tropical and temperate regions where chemical weathering is intense, but also in arid regions where physical weathering dominates.

Key Facts

Hardness

Highly variable, generally much softer than the parent rock (e.g., 1-5 on Mohs scale, depending on degree of weathering and secondary minerals).

Specific Gravity

Highly variable, generally lower than the parent rock due to porosity and replacement by lighter minerals (e.g., 1.5-2.8).

Crystal System

Not applicable to the rock as a whole; individual relict minerals retain their original crystal systems, while secondary minerals (e.g., clays) have their own.

Color

Variable, commonly reds, browns, yellows, grays, whites.

Luster

Dull, earthy, or variable.

Transparency

Opaque to translucent, depending on constituent minerals.

Fracture

Irregular, earthy, or conchoidal in relict quartz grains.

Cleavage

Absent in the bulk rock; relict minerals may show cleavage.

Composition

Highly variable, consisting of relict primary minerals, newly formed secondary minerals (e.g., clay minerals, iron oxides, hydroxides), and dissolved components. Chemical composition reflects the parent rock and the extent of chemical alteration.

Physical characteristics

• Crystal Habit: Not applicable to the rock as a whole; individual minerals may show relict or newly formed habits.
• Cleavage Type: Absent in the bulk rock; relict minerals may exhibit their characteristic cleavage.
• Fracture Type: Irregular, earthy, or granular.
• Tenacity: Friable, brittle, or earthy.
• Luster Type: Dull, earthy, or variable.

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