How to identify Porphyritic Basalt

To correctly identify Porphyritic Basalt (Basalt (porphyritic texture)), 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

   Dark gray to black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to sub-vitreous in the groundmass; phenocrysts can exhibit vitreous to pearly luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Porphyritic, with visible phenocrysts (typically 1 mm to several cm) set in an aphanitic (fine-grained) to glassy groundmass.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Phenocrysts often show euhedral to subhedral crystal forms (e.g., tabular plagioclase, prismatic pyroxene, anhedral to subhedral olivine). Groundmass crystals are anhedral and microscopic.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Minerals within the rock (e.g., plagioclase, pyroxene) exhibit characteristic cleavage, but the rock itself does not have a rock cleavage. Fracture is typically conchoidal to irregular.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Volcanic flows, dikes, sills, and volcanic cones in various tectonic settings including mid-ocean ridges, oceanic islands, continental rifts, and subduction zones (back-arc basins).

Key Facts

Hardness

6-7 (Mohs scale, for constituent minerals like plagioclase and pyroxene; bulk rock hardness can vary)

Specific Gravity

2.8-3.1 g/cm³

Crystal System

Monoclinic (pyroxene), Triclinic (plagioclase), Orthorhombic (olivine) - refers to constituent minerals

Color

Dark gray to black

Luster

Dull to sub-vitreous (groundmass), vitreous to pearly (phenocrysts)

Transparency

Opaque (rock), translucent to transparent (individual phenocrysts)

Fracture

Conchoidal to irregular

Cleavage

Good in constituent minerals (e.g., plagioclase, pyroxene), but not a rock cleavage

Composition

Mafic; primarily plagioclase feldspar (labradorite), pyroxene (augite), olivine, and iron-titanium oxides. Silica content typically 45-52 wt% SiO₂.

Physical characteristics

• Crystal Habit: Phenocrysts are typically euhedral to subhedral (e.g., tabular plagioclase, prismatic pyroxene, anhedral olivine). Groundmass is aphanitic.
• Cleavage Type: Perfect to good in constituent minerals (e.g., plagioclase: 2 directions at ~90°; pyroxene: 2 directions at ~90°).
• Fracture Type: Conchoidal to irregular, depending on grain size and presence of phenocrysts.
• Tenacity: Brittle
• Luster Type: Dull to sub-vitreous for the fine-grained groundmass; phenocrysts can exhibit vitreous to pearly luster.

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