How to identify Porphyritic Basalt

To correctly identify Porphyritic Basalt (Igneous Rock, Extrusive, Mafic, 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, sometimes with a greenish or brownish tint due to alteration of mafic minerals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to sub-vitreous in the groundmass; phenocrysts can exhibit vitreous (plagioclase) or greasy (olivine) luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Porphyritic, characterized by large, visible crystals (phenocrysts) embedded in a fine-grained to aphanitic (microscopic crystals) or sometimes 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 interlocking.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Phenocrysts of plagioclase exhibit two good cleavages at nearly 90 degrees; pyroxene has two good cleavages at nearly 90 degrees; olivine typically has poor cleavage or conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Volcanic flows (lava flows), sills, dikes, and shallow intrusions 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)

Specific Gravity

2.8-3.1 g/cm³

Crystal System

Monoclinic (pyroxene), Triclinic (plagioclase), Orthorhombic (olivine) - refers to the crystal systems of the constituent minerals.

Color

Dark gray to black

Luster

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

Transparency

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

Fracture

Conchoidal to uneven (groundmass), conchoidal (olivine), splintery (pyroxene)

Cleavage

Good in plagioclase (2 directions at ~90°), good in pyroxene (2 directions at ~90°), poor in olivine

Composition

Predominantly plagioclase feldspar (labradorite to bytownite), pyroxene (augite), and often olivine. Minor amounts of magnetite, ilmenite, and apatite may be present. Silica content typically 45-52 wt% SiO₂.

Physical characteristics

• Crystal Habit: Phenocrysts are typically euhedral to subhedral, often tabular (plagioclase) or prismatic (pyroxene). Groundmass crystals are anhedral and interlocking.
• Cleavage Type: Perfect to good in phenocrysts (e.g., plagioclase, pyroxene), generally absent in the fine-grained groundmass.
• Fracture Type: Conchoidal to uneven in the groundmass; conchoidal in olivine phenocrysts.
• Tenacity: Brittle
• Luster Type: Dull to sub-vitreous for the groundmass; vitreous for plagioclase phenocrysts; greasy for olivine phenocrysts.

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