Weathered Igneous Rock

Description

Weathered igneous rock refers to any igneous rock that has undergone significant alteration due to exposure to the Earth's atmosphere, hydrosphere, and biosphere. This alteration results in changes to its physical appearance, mineralogical composition, and mechanical properties. The original texture and structure of the igneous rock may still be discernible, but the rock will be softer, more friable, and often discolored compared to its fresh counterpart. The degree of weathering can range from superficial discoloration and minor mineral alteration to complete decomposition into soil or saprolite.

How to Identify

Color

Highly variable, often duller than fresh rock. Can be reddish-brown, yellowish, grey, or mottled due to oxidation of iron-bearing minerals (forming goethite, hematite, limonite) or hydration. Leaching can also result in bleached appearances.

Luster

Dull, earthy, or chalky due to the presence of secondary clay minerals and iron oxides. Original vitreous or sub-vitreous luster of primary minerals is often lost.

Texture

Friable, crumbly, or soft. Original interlocking crystalline texture may be preserved but individual grains are easily dislodged. Pitting, etching, and rounding of mineral grains can be observed. May feel gritty (quartz) or smooth/soapy (clays).

Crystal Form

Original crystal forms of primary minerals may be partially preserved but are often corroded, fractured, or replaced by secondary minerals. Secondary minerals (e.g., clays) typically form microscopic aggregates or coatings.

Cleavage

Original cleavage planes of primary minerals may be obscured or weakened. Secondary minerals like clays do not exhibit distinct macroscopic cleavage.

Geological Environment

Found in outcrops, road cuts, river banks, and any surface exposure of igneous bedrock, particularly in humid and temperate to tropical climates where chemical weathering is dominant. Also common in glacial till where igneous clasts have been subjected to weathering.

Key Facts

• Hardness: Highly variable, typically much softer than fresh igneous rock (e.g., 1-5 on Mohs scale, depending on degree of alteration and presence of secondary minerals like clays or quartz).
• Specific Gravity: Variable, generally lower than fresh igneous rock due to hydration and replacement by less dense secondary minerals (e.g., 2.0-2.7 g/cm³).
• Crystal System: Not applicable for the rock as a whole; individual primary minerals retain their original crystal systems, but secondary minerals (e.g., clays) are typically monoclinic or triclinic, often cryptocrystalline.
• Color: Highly variable; often shades of brown, red, yellow, grey, or mottled.
• Luster: Dull, earthy, chalky.
• Transparency: Opaque to translucent (for individual altered grains or secondary minerals).
• Fracture: Irregular, earthy, or conchoidal (if quartz remains dominant).
• Cleavage: Poor to none macroscopically; original cleavage of primary minerals may be obscured or destroyed.
• Composition: Composed of residual primary igneous minerals (e.g., quartz, altered feldspars) and secondary minerals formed by weathering (e.g., clay minerals like kaolinite, smectite, illite; iron oxides/hydroxides like goethite, hematite, limonite; sometimes carbonates or hydroxides like gibbsite). The exact composition depends on the parent igneous rock and the weathering intensity.

Quick Check

• Color: Variable (reddish-brown, yellowish, grey, mottled, bleached)
• Luster: Dull, earthy, chalky
• Streak: Variable, often light brown, yellowish, or white (depending on secondary minerals)

Physical Characteristics

• Crystal Habit: Relict primary mineral habits may be preserved but are often corroded. Secondary minerals typically form fine-grained aggregates, coatings, or pseudomorphs.
• Cleavage Type: Not a defining characteristic of the weathered rock mass. Original mineral cleavages are often compromised.
• Fracture Type: Irregular, earthy, or crumbly. Can be conchoidal if silica is dominant.
• Tenacity: Brittle to friable; often very weak and easily broken.
• Luster Type: Earthy, dull, chalky.

Formation

Weathered igneous rock forms from the physical and chemical breakdown of pre-existing igneous rocks (e.g., granite, basalt, gabbro, rhyolite, andesite). Physical weathering (e.g., freeze-thaw, abrasion, thermal expansion/contraction) breaks the rock into smaller fragments. Chemical weathering (e.g., hydrolysis, oxidation, dissolution, carbonation) alters the primary minerals into secondary minerals (e.g., feldspars to clays, pyroxenes/amphiboles to clays/iron oxides, olivine to serpentine/iddingsite). The degree and type of weathering depend on climate, topography, vegetation, and the original rock's mineralogy and texture.

Usage

Weathered igneous rocks, particularly saprolites, can be used as a source of clay for ceramics, brick making, and as a component in some soils for agriculture. Highly weathered material may be used as fill or sub-base in construction. Lateritic weathering products of mafic igneous rocks can be sources of nickel, cobalt, and iron ores.

Age Distribution

Can be found in any geological period where igneous rocks have been exposed to surface processes.

Where to Find

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Scientific Classification

Mineral Class

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

Group

Igneous rock (altered/weathered)

Crystal System

Not applicable for the rock; constituent minerals vary.

Chemical Formula

Highly variable, reflecting the mix of primary and secondary minerals. No single formula.

Composition

Silicates (quartz, altered feldspars, clay minerals), oxides (iron oxides/hydroxides), sometimes carbonates or hydroxides. The specific elemental composition depends on the parent igneous rock and the degree of chemical alteration.

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