How to identify Amphibolite

To correctly identify Amphibolite (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

   Typically dark green to black, sometimes with white speckles from plagioclase.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to dull, depending on the mineral components and their grain size.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Medium- to coarse-grained, granoblastic (equigranular, interlocking grains) or nematoblastic (elongate, aligned amphibole crystals). Can be massive or foliated (schistose).

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Amphibole crystals are typically prismatic, often elongated and aligned, contributing to foliation. Plagioclase is anhedral to subhedral.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Amphibole (hornblende) exhibits two distinct cleavages at approximately 56 and 124. Plagioclase has two cleavages at nearly 90.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in regional metamorphic terrains associated with convergent plate boundaries, ancient shield areas, and mountain belts. It can also occur in contact metamorphic aureoles around large igneous intrusions.

Key Facts

Hardness

6-7 (Mohs scale, for hornblende)

Specific Gravity

2.9-3.2 g/cm

Crystal System

Monoclinic (for hornblende and plagioclase)

Color

Dark green, black, sometimes with white or gray patches

Luster

Vitreous to dull

Transparency

Opaque to translucent (for individual mineral grains)

Fracture

Uneven to subconchoidal

Cleavage

Good to perfect in two directions for hornblende (56 and 124); good in two directions for plagioclase (nearly 90)

Composition

Predominantly hornblende and plagioclase feldspar, with variable amounts of quartz, epidote, garnet, biotite, magnetite, ilmenite, and sphene.

Physical characteristics

• Crystal Habit: Prismatic (amphibole), anhedral to subhedral (plagioclase)
• Cleavage Type: Perfect to good in two directions (amphibole, plagioclase)
• Fracture Type: Uneven to subconchoidal
• Tenacity: Brittle
• Luster Type: Vitreous to dull

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