How to identify Porphyritic Basalt

To correctly identify Porphyritic Basalt (Basalt with phenocrysts), 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 greenish-black. The phenocrysts may be lighter (plagioclase) or greenish (olivine) against the dark groundmass.

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 (aphanitic) or sometimes glassy groundmass. The groundmass crystals are generally less than 1 mm, while phenocrysts are typically 1 mm or larger.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

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

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not typically observable in the fine-grained groundmass. Phenocrysts of plagioclase exhibit two good cleavages at nearly 90 degrees; pyroxene has two good cleavages at approximately 90 degrees; olivine has poor cleavage or none.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Associated with volcanic eruptions, mid-ocean ridges, oceanic islands, continental rifts, and large igneous provinces. It forms in environments where mafic magma undergoes a two-stage cooling process.

Key Facts

Hardness

6-7 on Mohs scale (for constituent minerals like plagioclase and pyroxene; overall rock hardness is similar)

Specific Gravity

2.8-3.0 g/cm³

Crystal System

Monoclinic (pyroxene), Triclinic (plagioclase), Orthorhombic (olivine) for phenocrysts; groundmass is typically too fine to determine.

Color

Dark gray, black, greenish-black

Luster

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

Transparency

Opaque (rock), translucent to opaque (phenocrysts)

Fracture

Conchoidal to uneven

Cleavage

Not distinct in groundmass; phenocrysts show characteristic cleavages (e.g., plagioclase: 2 directions at ~90°; pyroxene: 2 directions at ~90°)

Composition

Mafic, dominated by plagioclase feldspar (labradorite to bytownite), pyroxene (augite), and often olivine. Minor iron-titanium oxides (magnetite, ilmenite). Silica content typically 45-52 wt% SiO2.

Physical characteristics

• Crystal Habit: Phenocrysts are typically euhedral to subhedral (e.g., tabular plagioclase, prismatic pyroxene, anhedral to subhedral olivine). Groundmass is aphanitic.
• Cleavage Type: Not applicable to the rock as a whole; individual phenocrysts exhibit mineral-specific cleavages.
• Fracture Type: Conchoidal to uneven, depending on grain size and mineralogy.
• Tenacity: Brittle
• Luster Type: Dull to sub-vitreous for the groundmass; phenocrysts can be vitreous (plagioclase) or greasy (olivine).

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