How to identify Weathered Igneous Rock

To correctly identify Weathered Igneous Rock (Igneous Rock), 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. Common colors include reddish-brown, yellowish-brown, gray, white, or mottled, due to the presence of iron oxides, hydroxides, and clay minerals. Original mineral colors may be faded or altered.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull, earthy, or chalky, contrasting with the vitreous or sub-vitreous luster of fresh feldspars and quartz, or the metallic/submetallic luster of fresh mafic minerals. Some relict minerals may retain their original luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Friable, crumbly, porous, or soft. Original crystalline or glassy texture may be obscured or replaced by a granular, earthy, or clayey texture. Relict textures like phenocrysts or flow structures might still be visible, but the matrix is altered. Can range from slightly discolored to completely decomposed saprolite where original rock fabric is preserved but minerals are altered.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Original crystal forms of primary minerals may be partially or completely destroyed. Secondary minerals (e.g., clay minerals) typically form microscopic, platy, or amorphous aggregates. Relict phenocrysts might show signs of etching or alteration rims.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Original mineral cleavages may be obscured or destroyed. Secondary minerals like clays do not exhibit macroscopic cleavage in the same way as primary silicates. The rock itself may break irregularly or along planes of weakness introduced by weathering.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in surficial environments where igneous rocks are exposed to atmospheric and hydrological processes. Common in areas with long-term tectonic stability, humid climates, and moderate to high temperatures, which promote chemical weathering. Also found in arid regions where physical weathering dominates.

Key Facts

Hardness

Highly variable, generally much softer than fresh igneous rock (e.g., 1-5 on Mohs scale, depending on the degree of weathering and secondary minerals present).

Specific Gravity

Variable, often lower than fresh igneous rock due to increased porosity and replacement by lighter secondary minerals (e.g., 1.8-2.5 g/cm³).

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, or amorphous.

Color

Variable, often duller, with reds, browns, yellows, grays, whites.

Luster

Dull, earthy, chalky.

Transparency

Opaque to translucent.

Fracture

Irregular, earthy, crumbly, or conchoidal (if relict quartz is present).

Cleavage

Poor to absent in the rock mass; relict minerals may show original cleavage, but secondary minerals typically do not exhibit macroscopic cleavage.

Composition

Dominated by secondary minerals such as clay minerals (kaolinite, illite, smectite), iron oxides/hydroxides (goethite, hematite), aluminum hydroxides (gibbsite), and residual primary minerals (e.g., quartz, muscovite, resistant heavy minerals). Original silicates (feldspars, pyroxenes, amphiboles, micas) are partially or completely altered.

Physical characteristics

• Crystal Habit: Not applicable to the rock as a whole. Relict primary minerals may show original habits; secondary minerals typically form cryptocrystalline or amorphous aggregates.
• Cleavage Type: Not applicable to the rock mass. Relict primary minerals may show their characteristic cleavage (e.g., feldspar: 2 directions at 90 degrees; amphibole: 2 directions at 56/124 degrees).
• Fracture Type: Irregular, earthy, crumbly, or blocky.
• Tenacity: Brittle to friable.
• Luster Type: Dull, earthy, chalky.

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