How to identify Porphyritic Andesite

To correctly identify Porphyritic Andesite (Porphyritic Andesite), 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 medium to dark gray, greenish-gray, or reddish-brown. The color is influenced by the proportion of mafic minerals (darker) versus plagioclase (lighter).

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   The groundmass can appear dull to earthy, while phenocrysts, especially plagioclase, may exhibit a vitreous (glassy) luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Porphyritic texture is the defining characteristic, with visible phenocrysts (typically 1 mm to several cm) set in an aphanitic (fine-grained) or glassy groundmass. The phenocrysts are often euhedral to subhedral.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Phenocrysts often show well-developed crystal forms, such as tabular plagioclase, prismatic hornblende, or blocky pyroxene.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Cleavage is observed in the phenocrysts (e.g., two directions at nearly 90 degrees in pyroxene, two directions at 56 and 124 degrees in hornblende, and two directions at 90 degrees in plagioclase). The fine-grained groundmass typically does not show macroscopic cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Forms in volcanic arcs above subduction zones, associated with stratovolcanoes. It is a common rock type in convergent plate boundaries.

Key Facts

Hardness

6-7 on Mohs scale (for individual phenocrysts, overall rock is tough).

Specific Gravity

2.5-2.8 g/cm³.

Crystal System

Monoclinic (hornblende, pyroxene), Triclinic (plagioclase).

Color

Medium to dark gray, greenish-gray, reddish-brown.

Luster

Vitreous (phenocrysts), dull to earthy (groundmass).

Transparency

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

Fracture

Conchoidal to uneven.

Cleavage

Present in phenocrysts (e.g., plagioclase, hornblende, pyroxene).

Composition

Intermediate igneous rock, typically 52-63 wt% SiO₂. Dominant minerals include plagioclase feldspar (andesine-labradorite), pyroxene (augite, hypersthene), hornblende, and sometimes biotite or quartz.

Physical characteristics

• Crystal Habit: Phenocrysts are typically euhedral to subhedral, often tabular (plagioclase) or prismatic (hornblende, pyroxene). Groundmass is microcrystalline or glassy.
• Cleavage Type: Good to perfect in phenocrysts (e.g., plagioclase: two directions at 90 degrees; hornblende: two directions at 56 and 124 degrees; pyroxene: two directions at nearly 90 degrees).
• Fracture Type: Conchoidal to uneven, depending on the grain size and presence of glass.
• Tenacity: Brittle.
• Luster Type: Vitreous to dull.

Explore Porphyritic Andesite