How to identify Vesicular Basalt

To correctly identify Vesicular Basalt (Vesicular Basalt), 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

   Typically dark gray to black, sometimes reddish-brown due to oxidation of iron-bearing minerals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy, sometimes vitreous on fresh surfaces of individual mineral grains.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Vesicular (characterized by abundant gas-bubble cavities), aphanitic (fine-grained matrix), sometimes porphyritic.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Individual crystals (phenocrysts) are typically anhedral to subhedral, often too small to be seen without magnification in the groundmass. Vesicles are typically spherical to ovoid, sometimes elongated due to lava flow.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   No distinct cleavage as a rock; constituent minerals (e.g., pyroxene, plagioclase) exhibit cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Volcanic flows, shield volcanoes, mid-ocean ridges, oceanic islands, continental flood basalts, and any setting where mafic lava erupts subaerially or subaqueously at shallow depths.

Key Facts

Hardness

5-6 on Mohs scale (for constituent minerals, rock itself is friable if highly vesicular)

Specific Gravity

2.0-2.8 (lower than non-vesicular basalt due to porosity, typically 2.7-3.0 for solid basalt)

Crystal System

Not applicable for a rock; constituent minerals are typically monoclinic (pyroxene, plagioclase) or orthorhombic (olivine).

Color

Dark gray, black, sometimes reddish-brown

Luster

Dull, earthy

Transparency

Opaque

Fracture

Conchoidal to irregular, often rough due to vesicles

Cleavage

Absent as a rock; constituent minerals have cleavage.

Composition

Primarily plagioclase feldspar (labradorite to bytownite), pyroxene (augite), and often olivine. Minor amounts of iron-titanium oxides (magnetite, ilmenite) and sometimes volcanic glass.

Physical characteristics

• Crystal Habit: Aphanitic groundmass with occasional porphyritic phenocrysts; vesicles are spherical to elongated.
• Cleavage Type: Not applicable for the rock; constituent minerals exhibit cleavage (e.g., two distinct cleavages at nearly 90 degrees for pyroxene, two at nearly 90 degrees for plagioclase).
• Fracture Type: Irregular to conchoidal, often rough due to the presence of vesicles.
• Tenacity: Brittle
• Luster Type: Dull to earthy

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