How to identify Porphyritic Basalt

To correctly identify Porphyritic Basalt (Igneous rock, extrusive, 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

   Typically dark gray to black. Phenocrysts can be lighter (plagioclase) or greenish (olivine).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

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

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 exhibit euhedral to subhedral crystal forms (e.g., tabular plagioclase, prismatic pyroxene, equant 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., pyroxene, plagioclase) exhibit characteristic cleavage, but the rock as a whole does not show rock cleavage. Phenocrysts of plagioclase show two cleavages at nearly 90 degrees; pyroxene shows two cleavages at nearly 90 degrees; olivine has poor cleavage.

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 feldspar and pyroxene)

Specific Gravity

2.8-3.1 g/cm³

Crystal System

Monoclinic (pyroxene, plagioclase), Orthorhombic (olivine)

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 uneven

Cleavage

Poor to distinct in phenocrysts (e.g., plagioclase, pyroxene), absent in groundmass

Composition

Predominantly plagioclase feldspar (labradorite), pyroxene (augite), olivine, and sometimes minor 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, equant olivine); groundmass is anhedral and microcrystalline.
• Cleavage Type: Phenocrysts of plagioclase exhibit two cleavages at nearly 90 degrees; pyroxene exhibits two cleavages at nearly 90 degrees; olivine has poor cleavage. The rock itself does not exhibit cleavage.
• Fracture Type: Conchoidal to uneven, depending on the grain size and presence of phenocrysts.
• Tenacity: Brittle
• Luster Type: Dull to sub-vitreous for the groundmass; vitreous to pearly for phenocrysts.

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