How to identify Garnetiferous Granite

To correctly identify Garnetiferous Granite (Granite (igneous rock) with Almandine (garnet group) and Quartz (silicate mineral)), 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

   Overall rock color is typically light-colored (white, pink, gray) due to feldspar and quartz, with distinct dark red to reddish-brown spots or crystals of almandine garnet. Quartz is colorless to smoky gray.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Granite has a vitreous to sub-vitreous luster for quartz and feldspar, and a vitreous to resinous luster for almandine garnet.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Phaneritic (coarse-grained) and equigranular to porphyritic. Interlocking crystalline texture. Garnets are typically anhedral to euhedral.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz: Anhedral to subhedral, irregular grains filling spaces. Almandine: Typically euhedral to subhedral dodecahedral or trapezohedral crystals. Feldspar: Subhedral to euhedral, blocky crystals.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Quartz: No cleavage, conchoidal fracture. Almandine: No cleavage, conchoidal to uneven fracture. Feldspar: Two distinct cleavages at or near 90 degrees.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Intrusive igneous bodies (plutons, batholiths) in continental crust, often associated with orogenic belts, continental collision zones, and areas of high-grade regional metamorphism where crustal melting has occurred.

Key Facts

Hardness

Granite (overall): 6-7 on Mohs scale. Quartz: 7. Almandine: 6.5-7.5. Feldspar: 6-6.5.

Specific Gravity

Granite (overall): 2.6-2.7 g/cm³. Quartz: 2.65 g/cm³. Almandine: 3.9-4.3 g/cm³.

Crystal System

Granite (rock): Polycrystalline aggregate. Quartz: Trigonal. Almandine: Isometric.

Color

Granite: Variable (white, pink, gray) with dark red garnets. Quartz: Colorless, white, smoky gray. Almandine: Deep red, reddish-brown, purplish-red.

Luster

Granite: Vitreous to sub-vitreous. Quartz: Vitreous. Almandine: Vitreous to resinous.

Transparency

Granite: Opaque. Quartz: Transparent to translucent. Almandine: Translucent to opaque (rarely transparent).

Fracture

Granite: Irregular to uneven. Quartz: Conchoidal. Almandine: Conchoidal to uneven.

Cleavage

Granite: No overall cleavage (individual minerals have cleavage). Quartz: None. Almandine: None. Feldspar: Two distinct cleavages.

Composition

Granite: Predominantly quartz (20-60%), alkali feldspar (35-90% of total feldspar), plagioclase feldspar (10-65% of total feldspar), micas (biotite, muscovite), and accessory minerals including almandine garnet.

Physical characteristics

• Crystal Habit: Granite: Granular, interlocking crystals. Quartz: Anhedral to subhedral masses. Almandine: Euhedral to subhedral dodecahedra or trapezohedra.
• Cleavage Type: Granite: Not applicable as a rock. Quartz: None. Almandine: None. Feldspar: Perfect to good in two directions.
• Fracture Type: Granite: Irregular to uneven. Quartz: Conchoidal. Almandine: Conchoidal to uneven.
• Tenacity: Granite: Brittle. Quartz: Brittle. Almandine: Brittle.
• Luster Type: Granite: Vitreous to sub-vitreous. Quartz: Vitreous. Almandine: Vitreous to resinous.

Explore Garnetiferous Granite