How to identify Weathered Igneous Rock

To correctly identify Weathered Igneous Rock (Igneous Rock (undifferentiated)), 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 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.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   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.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   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).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   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.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

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

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   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.

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.

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