How to identify Porphyritic Basalt

To correctly identify Porphyritic Basalt (Porphyritic 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 greenish-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, characterized by larger, well-formed crystals (phenocrysts) set within a fine-grained to aphanitic (or sometimes glassy) groundmass. Phenocrysts are typically 1 mm to several centimeters in size, while the groundmass crystals are microscopic.

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

   Minerals within the rock (e.g., plagioclase, pyroxene) exhibit characteristic cleavage planes, but the rock as a whole does not have a rock cleavage. Fracture is more common.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Associated with volcanic eruptions, lava flows, dikes, sills, and shallow intrusions in both continental and oceanic settings. Common in mid-ocean ridges, oceanic islands, and continental rift zones.

Key Facts

Hardness

6-7 (Mohs scale, for constituent minerals like plagioclase and pyroxene; overall rock hardness is similar but can vary with groundmass crystallinity)

Specific Gravity

2.8-3.1 g/cm³

Crystal System

Monoclinic (pyroxene), Triclinic (plagioclase), Orthorhombic (olivine) for phenocrysts; groundmass is typically microcrystalline or amorphous.

Color

Dark gray, black, greenish-black

Luster

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

Transparency

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

Fracture

Conchoidal to uneven

Cleavage

Not a rock cleavage; constituent minerals exhibit cleavage (e.g., plagioclase: 2 directions at ~90°, pyroxene: 2 directions at ~90°)

Composition

Predominantly plagioclase feldspar (labradorite to bytownite), pyroxene (augite), and often olivine. Minor amounts of magnetite, ilmenite, and sometimes volcanic glass.

Physical characteristics

• Crystal Habit: Phenocrysts are typically euhedral to subhedral, often tabular (plagioclase) or prismatic (pyroxene). Groundmass is anhedral and interlocking.
• Cleavage Type: Not applicable to the rock as a whole; mineral components exhibit good to perfect cleavage.
• Fracture Type: Conchoidal to uneven, depending on the crystallinity of the groundmass.
• Tenacity: Brittle
• Luster Type: Dull to sub-vitreous for the groundmass; phenocrysts can be vitreous to pearly.

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