Weathered Igneous Rock

Description

Weathered igneous rock, particularly mafic or intermediate types, represents a spectrum of alteration from slightly discolored to completely decomposed material. Initially, primary minerals like feldspars, pyroxenes, and olivine begin to alter. Feldspars hydrolyze to clay minerals, pyroxenes and olivine oxidize and hydrolyze to iron oxides/hydroxides and clays. The rock loses its original crystalline texture and hardness, becoming friable, earthy, and often discolored (red, brown, yellow) due to the presence of iron oxides. Original rock structures, such as relict phenocrysts or flow textures, may still be discernible, but the rock's integrity is significantly compromised. In extreme cases, it can form saprolite, where the rock is completely decomposed but retains its original rock fabric, or laterite, a highly oxidized and leached soil-like material.

How to Identify

Color

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.

Luster

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.

Texture

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.

Crystal Form

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.

Cleavage

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.

Geological Environment

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.

Quick Check

• Color: Reddish-brown, yellowish-brown, mottled, or greyish/whitish
• Luster: Dull, earthy, powdery
• Streak: Variable, often light brown, yellowish, or whitish (depending on dominant secondary minerals)

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.

Formation

Weathered igneous rock forms from the physical and chemical breakdown of primary igneous rocks (e.g., basalt, gabbro, andesite, diorite) due to exposure to atmospheric conditions, water, and biological activity. Chemical weathering, particularly hydrolysis and oxidation, is dominant in humid climates, altering primary minerals like feldspars, pyroxenes, and olivine into secondary minerals such as clays (kaolinite, smectite, illite), iron oxides/hydroxides (goethite, hematite), and sometimes carbonates. Physical weathering (e.g., freeze-thaw, abrasion) breaks the rock into smaller fragments. The degree of weathering depends on climate, topography, time, and the original rock's mineralogy and texture.

Usage

Heavily weathered igneous rocks, particularly saprolites, are often used as a source of aggregate for road construction (sub-base material), fill material, and as a component in some low-grade construction materials. Lateritic weathered igneous rocks can be sources of bauxite (aluminum ore) or nickel laterite deposits. In some agricultural regions, weathered igneous rocks contribute to soil formation, providing nutrients and improving soil structure.

Age Distribution

Can occur in any geological period where igneous rocks are exposed to surface processes, from Precambrian to Cenozoic.

Where to Find

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Scientific Classification

Mineral Class

Not a single mineral; a rock composed of various secondary minerals.

Group

Weathered Rock (specifically, altered igneous rock)

Crystal System

Not applicable to the rock; constituent minerals vary.

Chemical Formula

Highly variable, complex mixture of hydrated silicates, oxides, and hydroxides (e.g., Al2Si2O5(OH)4 for kaolinite, FeO(OH) for goethite).

Composition

Predominantly hydrous aluminum silicates (clays), iron oxides/hydroxides, and potentially aluminum hydroxides, with residual primary minerals.

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