How to identify Porphyritic Granite

To correctly identify Porphyritic Granite (Granite (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-colored, ranging from white, gray, pink, or red, depending on the feldspar composition and presence of mafic minerals. The phenocrysts often have a distinct color from the groundmass.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to sub-vitreous for quartz and feldspar, pearly for mica.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Porphyritic, characterized by large, visible crystals (phenocrysts) embedded in a finer-grained matrix (groundmass). The phenocrysts are typically euhedral to subhedral.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Phenocrysts are often euhedral to subhedral, commonly tabular (feldspar) or prismatic (quartz). Groundmass crystals are anhedral to subhedral.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Feldspars exhibit two distinct cleavages at or near 90 degrees. Micas have perfect basal cleavage. Quartz has no cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Forms in large intrusive bodies (plutons, batholiths, stocks) within continental crust, often associated with convergent plate boundaries and mountain-building events.

Key Facts

Hardness

6-7 on Mohs scale (due to quartz and feldspar)

Specific Gravity

2.6-2.8 g/cm3

Crystal System

Monoclinic (feldspar), Hexagonal (quartz), Monoclinic (mica)

Color

Variable, typically light-colored (white, gray, pink, red) with darker mafic minerals.

Luster

Vitreous to sub-vitreous (quartz, feldspar), pearly (mica)

Transparency

Opaque to translucent (individual crystals can be transparent)

Fracture

Conchoidal (quartz), uneven to splintery (feldspar)

Cleavage

Feldspar: 2 directions at ~90°; Mica: 1 perfect basal; Quartz: none

Composition

Felsic, primarily quartz (>20%), alkali feldspar (orthoclase, microcline), plagioclase feldspar, and micas (biotite, muscovite). Accessory minerals include hornblende, zircon, apatite, magnetite, and tourmaline.

Physical characteristics

• Crystal Habit: Phenocrysts are typically euhedral to subhedral, often tabular (feldspar) or prismatic (quartz). Groundmass crystals are anhedral to subhedral.
• Cleavage Type: Good to perfect in feldspars and micas; absent in quartz.
• Fracture Type: Conchoidal in quartz; uneven to splintery in feldspars.
• Tenacity: Brittle
• Luster Type: Vitreous to sub-vitreous for quartz and feldspar; pearly for mica.

Explore Porphyritic Granite