How to identify Garnet Amphibolite

To correctly identify Garnet Amphibolite (Amphibolite 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

   Dark green, black, or greenish-black matrix with distinct reddish-brown to dark red garnet porphyroblasts.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Amphibole crystals exhibit a vitreous to silky luster; plagioclase is vitreous; garnet is vitreous to resinous.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Medium- to coarse-grained, often nematoblastic (aligned amphiboles) or granoblastic. Porphyroblastic texture due to large garnet crystals.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Hornblende typically forms prismatic crystals; plagioclase forms tabular crystals; almandine garnet forms dodecahedral or trapezohedral crystals, often euhedral to subhedral.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Hornblende exhibits two distinct cleavages at approximately 56 and 124. Plagioclase has two cleavages at nearly 90. Garnet typically shows no cleavage but may exhibit parting.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Regional metamorphic terrains, especially in ancient shield areas, mountain belts (orogenic zones), and subduction zones where mafic rocks have undergone medium- to high-grade metamorphism.

Key Facts

Hardness

Hornblende: 5-6; Plagioclase: 6-6.5; Almandine Garnet: 6.5-7.5 (Mohs scale)

Specific Gravity

Hornblende: 3.0-3.4; Plagioclase: 2.6-2.8; Almandine Garnet: 3.9-4.3

Crystal System

Hornblende: Monoclinic; Plagioclase: Triclinic; Almandine Garnet: Isometric

Color

Dark green, black, or greenish-black matrix with reddish-brown to dark red garnets

Luster

Vitreous to silky

Transparency

Opaque to translucent (rock); individual minerals can be translucent to opaque

Fracture

Uneven to conchoidal (garnet); splintery (amphibole)

Cleavage

Hornblende: two distinct cleavages at 56 and 124; Plagioclase: two cleavages at nearly 90; Garnet: none

Composition

Predominantly hornblende (calcium amphibole) and plagioclase feldspar, with significant almandine garnet. Minor constituents may include quartz, biotite, epidote, ilmenite, magnetite, and sphene.

Physical characteristics

• Crystal Habit: Prismatic (hornblende), tabular (plagioclase), dodecahedral/trapezohedral (garnet)
• Cleavage Type: Perfect to good (hornblende, plagioclase); none (garnet)
• Fracture Type: Uneven to subconchoidal
• Tenacity: Brittle
• Luster Type: Vitreous to silky

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