How to identify Granite

To correctly identify Granite (Granite), 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, pink, or red (due to alkali feldspar) to gray or black (due to biotite and amphibole). The overall color is a mosaic of its constituent minerals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) for quartz and feldspar, pearly to vitreous for micas.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Phaneritic (coarse-grained), meaning individual mineral crystals are visible to the naked eye. The texture is typically equigranular, but porphyritic textures (larger crystals in a finer-grained groundmass) can occur.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Anhedral to subhedral interlocking crystals of quartz, feldspar, and mica. Quartz often fills interstitial spaces, while feldspars may show euhedral to subhedral forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Feldspars exhibit two directions of cleavage at or near 90 degrees. Micas exhibit perfect basal cleavage (one direction). Quartz has no cleavage but displays conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Granite forms in the roots of mountain ranges, within continental crust, often associated with subduction zones and continental collision zones. It is found as large intrusive bodies called batholiths, stocks, and dikes.

Key Facts

Hardness

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

Specific Gravity

2.65 - 2.75 g/cm³

Crystal System

Individual minerals within granite have their own crystal systems (e.g., quartz is trigonal, feldspars are monoclinic or triclinic, micas are monoclinic). The rock itself is a polycrystalline aggregate.

Color

Variable, typically light-colored: white, gray, pink, red, often with dark specks.

Luster

Vitreous (glassy) for quartz and feldspar, pearly to vitreous for micas.

Transparency

Opaque in hand specimen, individual mineral grains can be translucent.

Fracture

Conchoidal (quartz), uneven to subconchoidal (feldspar), irregular.

Cleavage

Good in feldspars (two directions at ~90°), perfect in micas (one basal direction), absent in quartz.

Composition

Predominantly quartz (20-60% by volume), alkali feldspar (35-60%), plagioclase feldspar (10-35%), and micas (biotite and/or muscovite). Accessory minerals may include amphibole, zircon, apatite, magnetite, and sphene.

Physical characteristics

• Crystal Habit: Anhedral to subhedral interlocking grains, forming a granular texture.
• Cleavage Type: Feldspars: two distinct cleavages at or near 90 degrees. Micas: perfect basal cleavage.
• Fracture Type: Quartz: conchoidal. Feldspars: uneven to subconchoidal. Overall rock: irregular.
• Tenacity: Brittle
• Luster Type: Vitreous (glassy) to pearly

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