Weathered Igneous Rock

Description

Weathered igneous rock, specifically heavily weathered basalt or diabase, is characterized by its friable, earthy, or clay-rich texture, often retaining relict structures of the original rock (e.g., relict phenocrysts, joint patterns). Its color typically ranges from reddish-brown, yellowish-brown, to mottled gray, reflecting the oxidation of iron-bearing minerals and the formation of clay minerals. The original hard, dense rock is transformed into a soft, easily crumbled material. Primary minerals are largely altered, replaced by secondary minerals such as clay minerals (kaolinite, smectite), iron oxides/hydroxides (goethite, hematite), and sometimes aluminum hydroxides (gibbsite) in tropical climates. The specific gravity and hardness are significantly reduced compared to the parent rock.

How to Identify

Color

Highly variable, typically reddish-brown, yellowish-brown, orange, or mottled gray/brown. Colors are often indicative of iron oxidation.

Luster

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

Texture

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.

Crystal Form

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

Cleavage

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

Geological Environment

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.

Quick Check

• Color: Reddish-brown, yellowish-brown, mottled gray/brown
• Luster: Dull, earthy
• Streak: Variable, often brownish-red to yellowish-brown (due to iron oxides)

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.

Formation

Weathered igneous rock forms from the in-situ decomposition of primary igneous rocks, predominantly mafic types like basalt and diabase, through physical and chemical weathering processes. Physical weathering (e.g., freeze-thaw, abrasion, exfoliation) breaks the rock into smaller fragments, increasing surface area. Chemical weathering (e.g., hydrolysis, oxidation, dissolution) then alters the primary minerals. For basalt and diabase, common primary minerals like plagioclase feldspar, pyroxene (augite), and olivine are susceptible to hydrolysis, forming clay minerals (kaolinite, smectite, illite) and iron oxides/hydroxides (goethite, hematite). Oxidation of iron-bearing minerals contributes to reddish-brown coloration. The degree of weathering depends on climate (temperature, precipitation), topography, vegetation, and time.

Usage

Heavily weathered igneous rocks, particularly saprolites, are often used as a source of aggregate for road construction (sub-base material), fill material, and sometimes as a low-grade ore for certain elements if enriched (e.g., nickel laterites). In some agricultural regions, weathered volcanic soils (andisols) derived from basalt are highly fertile. However, their engineering properties (low strength, high compressibility, high plasticity) often limit their use in structural foundations without significant stabilization.

Age Distribution

Can occur in any geological period where mafic igneous rocks (basalt, diabase) are present, from Precambrian to Cenozoic.

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Scientific Classification

Mineral Class

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

Group

Not applicable; classified as a residual soil or saprolite.

Crystal System

Not applicable to the rock; constituent secondary minerals have their own crystal systems.

Chemical Formula

Highly variable, complex mixture of hydrated silicates, oxides, and hydroxides. No single formula.

Composition

Hydrated aluminum silicates (clay minerals), iron oxides (Fe₂O₃), iron hydroxides (FeO(OH)), residual silica (SiO₂), and minor amounts of other elements depending on the parent rock.

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