How to identify Weathered Igneous Rock

To correctly identify Weathered Igneous Rock (Igneous Rock (likely basalt or gabbro, 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, but commonly reddish-brown, yellowish-brown, orange, or mottled due to iron oxidation. Can also be grayish or greenish if less oxidized or if specific clay minerals dominate. Original dark gray/black color of basalt/gabbro is usually lost.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, or clayey. Original vitreous or sub-vitreous luster of primary minerals is absent.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Friable, earthy, clay-rich, granular, or powdery. May retain relict igneous textures (e.g., ophitic, porphyritic) as 'ghost' structures. Can feel gritty due to residual quartz or other resistant minerals, or smooth if dominated by fine clays. Porosity is often high.

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, oxides) typically form microscopic aggregates or cryptocrystalline masses, not distinct macroscopic crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Original mineral cleavage (e.g., pyroxene, feldspar) is generally absent or very poorly preserved in relict grains. The bulk material does not exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in areas where mafic igneous rocks (basalt flows, gabbro intrusions, dikes, sills) have been exposed to prolonged subaerial weathering. Common in tropical and subtropical regions with high rainfall and temperatures, which accelerate chemical weathering. Also found in temperate regions, but often to a lesser degree of alteration. Occurs as part of the regolith profile, overlying fresh bedrock.

Key Facts

Hardness

Very soft, typically 1-3 on Mohs scale (due to clay minerals and oxides). Easily scratched with a fingernail or knife.

Specific Gravity

Variable, generally lower than fresh rock (2.0-2.7 g/cm³), due to hydration and replacement by lighter minerals.

Crystal System

Not applicable to the bulk weathered rock. Original primary minerals were typically monoclinic (pyroxene, amphibole), triclinic (feldspar), or orthorhombic (olivine). Secondary clay minerals are typically monoclinic or triclinic, but form cryptocrystalline aggregates.

Color

Reddish-brown, yellowish-brown, orange, mottled, grayish-green.

Luster

Dull, earthy, clayey.

Transparency

Opaque.

Fracture

Earthy, irregular, conchoidal (if relict quartz is present), or blocky (if indurated).

Cleavage

Absent in the bulk material.

Composition

Dominated by secondary minerals: clay minerals (e.g., kaolinite, smectite, illite), iron oxides/hydroxides (e.g., goethite, hematite), aluminum hydroxides (e.g., gibbsite). May contain relict primary minerals (e.g., quartz, resistant feldspar) and amorphous silica or iron/aluminum gels. Chemical composition reflects the parent mafic rock but with significant loss of mobile elements (Na, K, Ca, Mg, Si) and relative enrichment of immobile elements (Fe, Al, Ti).

Physical characteristics

• Crystal Habit: Not applicable to the bulk rock. Secondary minerals form microcrystalline to cryptocrystalline aggregates.
• Cleavage Type: None in the bulk rock. Original mineral cleavage is destroyed.
• Fracture Type: Earthy, irregular, blocky, or sub-conchoidal.
• Tenacity: Friable, brittle, earthy, sometimes plastic when wet (due to clay content).
• Luster Type: Dull, earthy.

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