How to identify Weathered Igneous Rock

To correctly identify Weathered Igneous Rock (Igneous Rock (highly 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-brown, orange, or mottled due to the presence of iron oxides and hydroxides. Can also be whitish or grayish if clay minerals dominate and iron is leached.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, or powdery due to the presence of clay minerals and iron oxides. Original mineral lusters (vitreous, pearly) are typically absent or significantly diminished.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Friable, crumbly, soft, and often porous. Original igneous textures (e.g., phaneritic, aphanitic, porphyritic) may be partially preserved as 'ghost' textures, but the rock is easily broken apart by hand. May feel gritty due to residual quartz or soft and smooth due to clay minerals.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Original crystal forms of primary minerals are largely destroyed or pseudomorphed by secondary minerals. Secondary minerals (e.g., clay minerals) typically form microscopic, platy, or amorphous aggregates.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Original mineral cleavages are generally obliterated. The rock itself does not exhibit macroscopic cleavage but may break along relict joint planes or exhibit a blocky or granular fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found at or near the Earth's surface, particularly in areas with long-term exposure to humid and warm climates, or in regions with significant tectonic uplift and erosion. Common in tropical and subtropical regions where chemical weathering is intense, forming thick saprolite profiles.

Key Facts

Hardness

Very soft to moderately soft (typically <3 on Mohs scale), easily scratched with a knife or fingernail, depending on the degree of weathering and residual minerals.

Specific Gravity

Lower than fresh igneous rock, typically 1.8 to 2.5 g/cm³ due to increased porosity and replacement by lighter secondary minerals.

Crystal System

Not applicable to the rock as a whole. Constituent secondary minerals (e.g., clay minerals) are typically monoclinic or triclinic, but often occur as cryptocrystalline or amorphous aggregates.

Color

Highly variable, often reddish-brown, yellowish-brown, orange, or mottled.

Luster

Dull, earthy, powdery.

Transparency

Opaque.

Fracture

Earthy, irregular, or granular. May retain relict blocky fracture from original jointing.

Cleavage

None for the rock mass. Original mineral cleavages are destroyed.

Composition

Dominated by secondary minerals such as clay minerals (kaolinite, smectite, illite), iron oxides/hydroxides (goethite, hematite), and residual primary minerals like quartz (if present in the original rock) or resistant heavy minerals. Original feldspars, pyroxenes, amphiboles, and olivine are largely altered or removed.

Physical characteristics

• Crystal Habit: Not applicable to the rock. Secondary minerals form fine-grained aggregates or coatings.
• Cleavage Type: None (for the rock mass).
• Fracture Type: Earthy, irregular, granular.
• Tenacity: Friable, brittle, soft.
• Luster Type: Dull, earthy.

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