How to identify Garnetiferous Granite

To correctly identify Garnetiferous Granite (Granite with Almandine Garnet), 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 (white, gray, pink, or reddish) matrix with distinct dark red to reddish-brown, often euhedral, garnet crystals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Overall granular, with vitreous luster from quartz, pearly to vitreous from feldspars, and submetallic to vitreous from micas. Garnets exhibit a vitreous to resinous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Phaneritic (coarse-grained) to porphyritic, with interlocking crystals. Garnets often appear as porphyroblasts or phenocrysts, standing out from the groundmass.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz is anhedral to subhedral, feldspars are subhedral, micas are flaky. Almandine garnets are typically euhedral to subhedral, often forming dodecahedral or trapezohedral crystals.

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. Almandine garnet has no cleavage but exhibits conchoidal to uneven fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Intrusive igneous bodies (plutons, batholiths) within continental crust, often associated with orogenic belts where crustal thickening and partial melting of metasedimentary rocks have occurred. Can also be found in anatectic granites formed by high-grade metamorphism and subsequent melting.

Key Facts

Hardness

Overall Mohs hardness of 6-7 (due to quartz and feldspar). Almandine garnet itself has a hardness of 7-7.5.

Specific Gravity

2.6-2.8 for the granite as a whole. Almandine garnet has a specific gravity of 3.9-4.3.

Crystal System

Granite is a rock, not a single mineral. Its constituent minerals have different crystal systems: Quartz (Trigonal), Feldspars (Monoclinic/Triclinic), Almandine Garnet (Isometric).

Color

Light-colored (white, gray, pink) with dark red to reddish-brown garnets.

Luster

Vitreous to resinous (garnet), vitreous (quartz), pearly to vitreous (feldspar), submetallic to vitreous (mica).

Transparency

Opaque to translucent (rock). Garnets are typically translucent to opaque in hand specimen, but can be transparent in thin sections or gem-quality specimens.

Fracture

Uneven to conchoidal (quartz), uneven (feldspar), conchoidal to uneven (garnet).

Cleavage

Feldspars have good cleavage in two directions. Micas have perfect basal cleavage. Quartz and garnet lack cleavage.

Composition

Predominantly quartz (20-60%), alkali feldspar (35-90% of total feldspar), plagioclase feldspar (10-65% of total feldspar), and mica (biotite and/or muscovite). Contains significant amounts of almandine garnet (Fe3Al2(SiO4)3) as an accessory mineral, typically 1-10% by volume, but can be higher.

Physical characteristics

• Crystal Habit: Granular, interlocking crystals of quartz and feldspar; flaky micas; euhedral to subhedral dodecahedral or trapezohedral crystals of almandine garnet.
• Cleavage Type: Feldspars: 2 directions at ~90 degrees. Micas: 1 perfect basal direction. Garnet: None.
• Fracture Type: Conchoidal (quartz, garnet), uneven (feldspar, overall rock).
• Tenacity: Brittle.
• Luster Type: Vitreous to resinous (garnet), vitreous (quartz), pearly to vitreous (feldspar), submetallic to vitreous (mica).

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