How to identify Weathered Igneous Rock

To correctly identify Weathered Igneous Rock (Igneous rock (likely basalt or andesite) with weathering rind), 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

   Outer rind: Reddish-brown, yellowish-brown, orange, light gray, or white. Inner core: Dark gray to black (basalt) or medium to dark gray (andesite).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy on the rind due to alteration products; sub-vitreous to dull on the fresh core.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Rind: Often porous, friable, earthy, or powdery. Core: Aphanitic (fine-grained) to porphyritic, sometimes vesicular.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual crystals within the rind are typically altered and not well-formed. Fresh core may show phenocrysts (e.g., plagioclase, pyroxene) in a fine-grained groundmass.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No macroscopic cleavage in the rock mass. Individual mineral grains within the fresh core may exhibit cleavage (e.g., pyroxene, plagioclase). The altered minerals in the rind typically lack distinct cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in volcanic terrains, lava flows, dikes, sills, and volcanic ash deposits exposed to surface weathering processes. Common in regions with active or past volcanism, ranging from oceanic islands to continental arcs and rift zones. The development of a weathering rind is ubiquitous in subaerial environments.

Key Facts

Hardness

Rind: 1-4 (Mohs) due to clay minerals and iron oxides; Core: 5-6 (Mohs) for fresh basalt/andesite.

Specific Gravity

Rind: 1.8-2.5 (variable due to porosity and alteration); Core: 2.7-3.0 for fresh basalt/andesite.

Crystal System

Not applicable to the rock as a whole; constituent minerals in the fresh core are typically monoclinic (pyroxene, plagioclase) or orthorhombic (olivine). Alteration products in the rind are often amorphous or poorly crystalline.

Color

Outer rind: Reddish-brown, yellowish-brown, orange, light gray; Inner core: Dark gray to black (basalt) or medium to dark gray (andesite).

Luster

Dull to earthy on the rind; sub-vitreous to dull on the fresh core.

Transparency

Opaque.

Fracture

Rind: Earthy, irregular, or conchoidal (if silicified); Core: Conchoidal to irregular.

Cleavage

Absent in the rock mass; present in individual mineral grains within the fresh core (e.g., pyroxene, plagioclase).

Composition

Rind: Dominated by secondary minerals such as clay minerals (smectite, kaolinite), iron oxides/hydroxides (goethite, hematite), and sometimes silica or carbonates. Core: Primarily plagioclase feldspar, pyroxene, olivine (in basalt), and amphibole/biotite (in andesite), with accessory minerals like magnetite and apatite.

Physical characteristics

• Crystal Habit: Not applicable to the rock as a whole. Fresh core minerals are typically anhedral to subhedral; rind minerals are often cryptocrystalline or amorphous.
• Cleavage Type: Not applicable to the rock mass. Constituent minerals in the fresh core exhibit various cleavages (e.g., two at ~90 degrees for pyroxene, two at ~60/120 degrees for amphibole, perfect basal for plagioclase).
• Fracture Type: Earthy, irregular, or sub-conchoidal in the rind; conchoidal to irregular in the fresh core.
• Tenacity: Friable to brittle in the rind; brittle in the fresh core.
• Luster Type: Dull to earthy in the rind; sub-vitreous to dull in the fresh core.

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