How to identify Porphyritic Andesite

To correctly identify Porphyritic Andesite (Andesite (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 light to dark gray, reddish-brown, or black. The color is influenced by the proportion of mafic minerals and the degree of alteration.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Dull to earthy in the groundmass; phenocrysts may exhibit vitreous to pearly luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Porphyritic, with visible phenocrysts (typically 1 mm to several cm) set in a fine-grained to aphanitic groundmass. The groundmass may sometimes be glassy.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Phenocrysts are typically euhedral to subhedral, often prismatic (e.g., plagioclase, hornblende, pyroxene).

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Not a characteristic of the rock as a whole, but individual phenocrysts will exhibit their characteristic cleavage (e.g., plagioclase: two directions at ~90 degrees; pyroxene: two directions at ~90 degrees; hornblende: two directions at ~56 and ~124 degrees).

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Volcanic arcs associated with subduction zones (e.g., Andes Mountains, Pacific Ring of Fire). It forms in stratovolcanoes and lava flows.

Key Facts

Hardness

6-7 (Mohs scale, for constituent minerals like plagioclase and pyroxene; rock as a whole is tough).

Specific Gravity

2.5-2.8 g/cm³.

Crystal System

Not applicable for the rock as a whole; individual phenocrysts have their own crystal systems (e.g., plagioclase: triclinic; pyroxene: monoclinic or orthorhombic; hornblende: monoclinic).

Color

Light to dark gray, reddish-brown, black.

Luster

Dull to earthy (groundmass), vitreous to pearly (phenocrysts).

Transparency

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

Fracture

Conchoidal to irregular.

Cleavage

Not a rock property; phenocrysts exhibit characteristic cleavage.

Composition

Intermediate igneous rock, primarily composed of plagioclase feldspar (andesine to labradorite), pyroxenes (augite, hypersthene), and/or hornblende. Minor amounts of biotite, magnetite, and quartz may be present. Silica content: 52-63 wt% SiO2.

Physical characteristics

• Crystal Habit: Phenocrysts are typically euhedral to subhedral, often prismatic or tabular. Groundmass is aphanitic to cryptocrystalline.
• Cleavage Type: Not applicable for the rock; phenocrysts show distinct cleavage (e.g., plagioclase: perfect in two directions).
• Fracture Type: Conchoidal to irregular, depending on the grain size and mineralogy.
• Tenacity: Brittle.
• Luster Type: Dull to earthy for the groundmass; vitreous to pearly for phenocrysts.

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