How to identify Weathered Igneous Rock

To correctly identify Weathered Igneous Rock (Igneous Rock (likely basalt or diabase, 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, typically reddish-brown, yellowish-brown, orange, or mottled gray/brown. Colors are often indicative of iron oxidation.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, or clay-like. Original mineral luster is usually absent.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Friable, earthy, clayey, sandy, or silty. May retain relict textures like ophitic (diabase) or porphyritic (basalt) but with altered minerals. Often feels soft and crumbles easily.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Original crystal forms are largely obliterated or pseudomorphed by secondary minerals. Relict phenocrysts might be visible but are soft and altered.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No distinct cleavage planes due to mineral alteration. The rock breaks irregularly along relict joint planes or through the altered matrix.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in areas where mafic igneous rocks (basalt flows, diabase sills/dikes) have been exposed to prolonged subaerial weathering. Common in tropical and subtropical regions with high rainfall and temperatures, but also in temperate zones over long geological timescales. Often forms a thick regolith layer above fresh bedrock.

Key Facts

Hardness

Very soft, easily scratched with a fingernail (Mohs hardness < 2.5) to moderately soft (Mohs hardness 2.5-5), depending on the degree of weathering and secondary mineral content. Significantly lower than fresh basalt/diabase (5-6).

Specific Gravity

Highly variable, typically 1.5 to 2.5 g/cm³, significantly lower than fresh basalt/diabase (2.8-3.1 g/cm³) due to porosity and hydration.

Crystal System

Not applicable to the weathered rock as a whole; primary minerals' crystal systems are largely destroyed or pseudomorphed. Secondary clay minerals are typically monoclinic or triclinic.

Color

Reddish-brown, yellowish-brown, orange, mottled gray/brown.

Luster

Dull, earthy.

Transparency

Opaque.

Fracture

Earthy, irregular, or conchoidal in relict fragments.

Cleavage

Absent in the bulk weathered material; relict cleavage may be observed in partially altered primary minerals.

Composition

Predominantly secondary clay minerals (kaolinite, smectite, illite), iron oxides/hydroxides (goethite, hematite), and residual primary minerals (e.g., quartz, highly altered feldspar, pyroxene). May contain aluminum hydroxides (gibbsite) in advanced weathering stages.

Physical characteristics

• Crystal Habit: Not applicable to the rock as a whole. Secondary minerals form cryptocrystalline or microcrystalline aggregates, or earthy masses.
• Cleavage Type: None to poor, due to the destruction of primary mineral structures and the fine-grained nature of secondary minerals.
• Fracture Type: Earthy, irregular, or subconchoidal in relict fragments.
• Tenacity: Friable, brittle, or plastic (if clay-rich).
• Luster Type: Dull, earthy.

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