How to identify Weathered Igneous Rock

To correctly identify Weathered Igneous Rock (Igneous Rock (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 reddish-brown, yellowish, or grayish due to the presence of iron oxides and clay minerals. Original rock colors may be muted or completely obscured. Felsic rocks may become more yellowish or whitish, while mafic rocks often turn reddish-brown.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, or chalky due to the presence of clay minerals and alteration products. Original vitreous or sub-vitreous luster of primary minerals is typically 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 may be observed. Can range from slightly discolored to completely disintegrated into a soil-like material (saprolite) where original rock structures are still visible.

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, rounded, or replaced by secondary minerals. Secondary minerals (e.g., clay minerals) typically form microscopic aggregates or coatings.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Original mineral cleavage planes may be obscured or weakened. The rock as a whole often lacks distinct cleavage, instead exhibiting irregular fracture or crumbling.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found at or near the Earth's surface, particularly in areas exposed to atmospheric conditions, water, and biological activity. Common in outcrops, road cuts, river banks, and any area where igneous bedrock is exposed to weathering processes. More prevalent in humid and tropical climates where chemical weathering is intense.

Key Facts

Hardness

Variable, generally soft (1-5 on Mohs scale) and friable, much lower than the fresh igneous rock.

Specific Gravity

Variable, generally lower than fresh igneous rock (e.g., 2.0-2.6 g/cm³) due to porosity and replacement by lighter secondary minerals.

Crystal System

Not applicable to the rock as a whole; individual relict minerals retain their original crystal systems, while secondary minerals (e.g., clays) are typically monoclinic or triclinic.

Color

Highly variable, often reddish-brown, yellowish, or grayish.

Luster

Dull, earthy, or chalky.

Transparency

Opaque to translucent in individual altered grains; the rock mass is opaque.

Fracture

Irregular, earthy, or crumbly; original conchoidal or uneven fracture of fresh rock is typically lost.

Cleavage

Poor to absent in the rock mass; individual relict minerals may show relict cleavage.

Composition

Composed of primary igneous minerals (e.g., quartz, feldspar, mica, pyroxene, amphibole) in various stages of alteration, along with secondary minerals such as clay minerals (kaolinite, illite, smectite), iron oxides/hydroxides (hematite, goethite, limonite), carbonates (calcite), and sometimes silica (chalcedony, opal).

Physical characteristics

• Crystal Habit: Relict primary mineral habits may be preserved but are often corroded; secondary minerals form fine-grained aggregates or coatings.
• Cleavage Type: Not applicable to the rock mass; individual relict minerals may show their characteristic cleavage (e.g., feldspar: two directions at 90 degrees).
• Fracture Type: Irregular, earthy, crumbly.
• Tenacity: Brittle to friable.
• Luster Type: Dull, earthy.

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