How to identify Weathered Igneous Rock

To correctly identify Weathered Igneous Rock (Igneous Rock (undifferentiated, 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 reddish-brown, yellowish, or mottled due to the presence of iron oxides/hydroxides (e.g., goethite, hematite) and clay minerals. Original rock colors may be muted or completely obscured.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, or chalky due to the presence of secondary minerals and fine-grained alteration products. Original vitreous or pearly lusters of primary minerals are typically lost.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Friable, crumbly, or earthy. Original interlocking crystalline texture may be preserved as a 'ghost' texture in saprolite, but individual mineral grains are often softened, discolored, or replaced by clay. Can range from slightly discolored and fractured to completely disintegrated into a soil-like mass.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Original crystal forms of primary minerals are typically degraded or destroyed. Secondary minerals (e.g., clays) may form microscopic aggregates or coatings, but macroscopic crystal forms are rare.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Original mineral cleavages are often obscured or destroyed by alteration. The rock itself may exhibit no distinct cleavage, instead fracturing irregularly or crumbling.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in outcrops of igneous rocks exposed to the Earth's surface, particularly in humid and tropical climates where chemical weathering is intense. Common in road cuts, quarries, river banks, and any area where bedrock is exposed to atmospheric conditions. Often forms a regolith layer overlying fresh bedrock.

Key Facts

Hardness

Highly variable, typically much softer than fresh igneous rock, ranging from 1-5 on Mohs scale depending on the degree of alteration and secondary minerals present. Can be easily scratched with a knife or even a fingernail in highly weathered states.

Specific Gravity

Variable, generally lower than fresh igneous rock due to the replacement of dense primary minerals with less dense secondary minerals (e.g., clays) and increased porosity. Ranges from approximately 1.8 to 2.6 g/cm³.

Crystal System

Not applicable to the rock as a whole. Original primary minerals had specific crystal systems, but these are often destroyed or replaced. Secondary minerals (e.g., clays) have their own crystal systems (e.g., monoclinic, triclinic, hexagonal).

Color

Highly variable, commonly reddish-brown, yellowish, orange, or mottled due to iron oxides/hydroxides and clay minerals. Can also be bleached white or gray.

Luster

Dull, earthy, or chalky.

Transparency

Opaque.

Fracture

Irregular, earthy, or crumbly. Original conchoidal or uneven fractures of primary minerals are typically lost.

Cleavage

No distinct rock cleavage. Original mineral cleavages are often destroyed or obscured.

Composition

Composed of altered primary igneous minerals (e.g., feldspars, quartz, micas, pyroxenes, amphiboles) and a significant proportion of secondary minerals such as clay minerals (kaolinite, illite, smectite), iron oxides/hydroxides (goethite, hematite, limonite), aluminum hydroxides (gibbsite), and sometimes carbonates or silica polymorphs. The exact composition depends on the original igneous rock and the weathering intensity.

Physical characteristics

• Crystal Habit: Not applicable to the rock as a whole. Original igneous textures are degraded; secondary minerals typically form fine-grained aggregates or coatings.
• Cleavage Type: None for the rock. Original mineral cleavages are largely destroyed.
• Fracture Type: Irregular, earthy, or crumbly.
• Tenacity: Friable, brittle, or earthy. Much less tenacious than fresh igneous rock.
• Luster Type: Dull, earthy.

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