How to identify Garnet Amphibolite

To correctly identify Garnet Amphibolite (Garnet Amphibolite), 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

   Dark green to black matrix with prominent reddish-brown to purplish-red garnets.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to sub-vitreous for garnets, vitreous to silky for amphiboles, dull to vitreous for plagioclase.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Medium to coarse-grained, typically foliated or lineated, often porphyroblastic with large garnet crystals.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Garnets typically form euhedral to subhedral dodecahedral or trapezohedral crystals. Amphiboles are prismatic to acicular. Plagioclase is anhedral to subhedral.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Amphiboles exhibit two distinct cleavages at approximately 56° and 124°. Garnets have no cleavage. Plagioclase has two cleavages at nearly 90°.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Regional metamorphic terrains, often associated with convergent plate boundaries, within medium to high-grade metamorphic zones (amphibolite facies).

Key Facts

Hardness

6.5-7.5 (garnet), 5-6 (amphibole), 6-6.5 (plagioclase)

Specific Gravity

3.1-4.3 (garnet), 2.9-3.4 (amphibole), 2.6-2.8 (plagioclase)

Crystal System

Isometric (garnet), Monoclinic (amphibole, plagioclase)

Color

Dark green to black matrix with reddish-brown to purplish-red garnets

Luster

Vitreous to sub-vitreous (garnet), vitreous to silky (amphibole), dull to vitreous (plagioclase)

Transparency

Translucent to opaque (garnet), translucent to opaque (amphibole), translucent (plagioclase)

Fracture

Conchoidal to uneven (garnet), uneven to splintery (amphibole), uneven (plagioclase)

Cleavage

Garnet: None; Amphibole: Two distinct cleavages at ~56° and ~124°; Plagioclase: Two cleavages at nearly 90°

Composition

Primarily hornblende (amphibole), garnet (almandine-pyrope series common), and plagioclase feldspar. Minor constituents may include quartz, biotite, epidote, ilmenite, magnetite, and rutile.

Physical characteristics

• Crystal Habit: Garnets are typically euhedral to subhedral dodecahedra or trapezohedra. Amphiboles are prismatic to acicular. Plagioclase is anhedral to subhedral granular.
• Cleavage Type: Amphiboles exhibit perfect prismatic cleavage. Garnets have no cleavage. Plagioclase has perfect basal and good pinacoidal cleavage.
• Fracture Type: Garnets show conchoidal to uneven fracture. Amphiboles and plagioclase typically have uneven to splintery fracture.
• Tenacity: Brittle for all major constituent minerals.
• Luster Type: Vitreous to sub-vitreous for garnets, vitreous to silky for amphiboles, dull to vitreous for plagioclase.

Explore Garnet Amphibolite