How to identify Weathered Granite

To correctly identify Weathered Granite (Granite (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, often duller than fresh granite. Can be grey, white, pink, red, brown, or yellowish, often with rust-colored staining from oxidized iron-bearing minerals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

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

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   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.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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).

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