How to identify Weathered Igneous Rock

To correctly identify Weathered Igneous Rock (Igneous Rock (likely mafic or intermediate, heavily 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, or mottled due to iron oxidation. Can also be greyish or whitish if feldspars have altered to kaolinite without significant iron staining. Original dark colors of mafic minerals are typically lost.

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 vitreous or sub-vitreous luster of primary minerals is absent.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Friable, earthy, crumbly, or sandy. Original igneous textures (e.g., porphyritic, aphanitic, phaneritic) may be relict but are obscured by alteration. Can feel gritty due to residual quartz or resistant primary minerals, or smooth/soapy 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 (clays, iron oxides) typically form microscopic aggregates or amorphous masses.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Original cleavage planes of primary minerals are generally obliterated. The rock itself does not exhibit macroscopic cleavage, but individual clay minerals may have microscopic basal cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in areas where igneous rocks have been exposed to subaerial weathering for extended periods, particularly in humid tropical and subtropical climates. Common in regolith profiles, road cuts, excavations, and river banks where erosion exposes weathered bedrock.

Key Facts

Hardness

Very soft, typically <3 on Mohs scale (can be scratched with a knife or even fingernail in extreme cases). Original hardness of primary minerals is lost.

Specific Gravity

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

Crystal System

Not applicable to the rock as a whole; individual secondary minerals (clays, iron oxides) have their own crystal systems (e.g., monoclinic for kaolinite, orthorhombic for goethite).

Color

Reddish-brown, yellowish-brown, mottled, greyish, whitish.

Luster

Dull, earthy, powdery.

Transparency

Opaque.

Fracture

Earthy, irregular, crumbly.

Cleavage

Absent at the rock scale; individual clay minerals may exhibit microscopic basal cleavage.

Composition

Dominated by secondary minerals such as clay minerals (kaolinite, smectite, illite), iron oxides/hydroxides (goethite, hematite), aluminum hydroxides (gibbsite), and residual primary minerals (e.g., quartz, resistant feldspar remnants). Original mafic/intermediate igneous rock composition (plagioclase, pyroxene, olivine, amphibole) is largely altered.

Physical characteristics

• Crystal Habit: Not applicable to the rock; secondary minerals form microcrystalline aggregates or amorphous masses.
• Cleavage Type: None at the rock scale.
• Fracture Type: Earthy, irregular, crumbly.
• Tenacity: Friable, brittle.
• Luster Type: Earthy, dull.

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