Weathered Granite

Description

Weathered granite is a granite that has undergone significant alteration due to surficial processes. It typically retains the original granitic texture (phaneritic, equigranular to porphyritic) but with minerals showing signs of degradation. Feldspars may appear dull, chalky, or stained, and can be easily scratched or crumbled. Mafic minerals (biotite, hornblende) may be oxidized, appearing reddish-brown or yellowish. Quartz grains usually remain intact and vitreous. The rock's overall coherence is reduced, ranging from slightly friable to completely disaggregated into a sandy, clay-rich material (grus or saprolite). The color can vary widely depending on the original granite and the extent and type of weathering, often exhibiting shades of brown, yellow, red, or grey.

How to Identify

Color

Highly variable, often duller than fresh granite. Can be grey, white, pink, red, brown, or yellowish, often with rust-colored staining from oxidized iron-bearing minerals.

Luster

Dull to earthy for altered feldspars and mafic minerals; vitreous for quartz. Overall luster is significantly reduced compared to fresh granite.

Texture

Phaneritic (coarse-grained) relict texture, but individual mineral grains may be friable, crumbly, or easily dislodged. Can range from slightly softened rock to a granular, sandy-clayey material (grus).

Crystal Form

Original anhedral to subhedral crystal forms of quartz, feldspar, and mafic minerals are generally preserved, but their integrity is compromised by weathering.

Cleavage

Cleavage planes in feldspars and micas may be visible but are often obscured by alteration products (e.g., clay minerals) and may appear dull or chalky. The rock as a whole may lack distinct cleavage due to disaggregation.

Geological Environment

Found in outcrop areas of granite intrusions, particularly in regions with prolonged exposure to surface weathering processes. Common in areas with moderate to high rainfall and temperature, leading to deep weathering profiles. Often forms the regolith overlying fresh granite bedrock.

Key Facts

• Hardness: Variable, significantly reduced compared to fresh granite. Feldspars may be 2-6 (Mohs), quartz 7. Overall rock coherence is low.
• Specific Gravity: Variable, generally lower than fresh granite (2.6-2.7 g/cm³) due to the formation of lower-density clay minerals and increased porosity. Can range from 2.0 to 2.6 g/cm³.
• Crystal System: Original minerals (quartz, feldspar) retain their respective crystal systems (trigonal for quartz, triclinic/monoclinic for feldspar), but the rock as a whole is a disaggregated aggregate.
• Color: Grey, white, pink, red, brown, yellow, often mottled or stained.
• Luster: Dull, earthy, vitreous (for quartz).
• Transparency: Opaque to translucent for individual altered minerals; rock is opaque.
• Fracture: Irregular to granular, often crumbling.
• Cleavage: Poor to absent in the bulk rock; relict cleavage may be observed in individual, partially altered mineral grains (e.g., feldspar, mica).
• Composition: Primarily quartz, altered feldspars (e.g., kaolinite, illite, smectite replacing plagioclase and K-feldspar), and altered mafic minerals (e.g., chlorite, iron oxides replacing biotite, hornblende). Minor accessory minerals may also be present.

Quick Check

• Color: Dull, often stained (brown, yellow, red), less vibrant than fresh granite.
• Luster: Dull to earthy for altered minerals, vitreous for quartz.
• Streak: White to light grey (for quartz and feldspar), reddish-brown for iron oxides.

Physical Characteristics

• Crystal Habit: Relict anhedral to subhedral granular texture, but individual grains are often friable or disaggregated.
• Cleavage Type: Not applicable to the bulk rock; relict cleavage in individual minerals is poor to distinct.
• Fracture Type: Irregular, granular, crumbling.
• Tenacity: Brittle, friable, easily disaggregated.
• Luster Type: Dull, earthy, vitreous (for quartz).

Formation

Weathered granite forms from the in-situ chemical and physical breakdown of primary granite. Chemical weathering, particularly hydrolysis of feldspars to clay minerals (e.g., kaolinite, illite, smectite), is a dominant process. Biotite and amphibole may alter to chlorite or iron oxides. Quartz is generally resistant but can be physically dislodged. Physical weathering (e.g., freeze-thaw, thermal expansion/contraction, exfoliation) also contributes by creating fractures and disaggregating the rock. The degree of weathering depends on climate (temperature, precipitation), topography, vegetation, and the original mineralogy and texture of the granite. In humid, warm climates, chemical weathering is more pronounced, leading to deep saprolite profiles.

Usage

Weathered granite, particularly decomposed granite (DG) or grus, is widely used as a natural aggregate for pathways, driveways, landscaping, and as a base material for construction. It provides good drainage and a natural aesthetic. Finer fractions can be used as a soil amendment to improve drainage and provide mineral nutrients. In some cases, highly weathered granite can be a source of clay minerals.

Age Distribution

Weathered granite can be found wherever granite intrusions exist, ranging from Precambrian to Cenozoic in age. The weathering process itself is ongoing and can occur over geological timescales.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Not a single mineral; a rock composed of silicates (quartz, feldspars, micas, amphiboles) and their alteration products (clay minerals, iron oxides).

Group

Igneous rock (plutonic) that has undergone significant weathering.

Crystal System

Not applicable to the rock as a whole; constituent minerals have their own crystal systems.

Chemical Formula

Variable, reflecting the complex mixture of primary minerals and their alteration products. Dominated by SiO2, Al2O3, K2O, Na2O, CaO, FeO, Fe2O3, MgO, H2O.

Composition

Quartz, altered K-feldspar (orthoclase, microcline), altered plagioclase feldspar, altered biotite, altered hornblende, muscovite, and secondary minerals such as kaolinite, illite, smectite, chlorite, and various iron oxides (e.g., goethite, hematite).

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