How to identify Hornblende Schist

To correctly identify Hornblende Schist (Hornblende Schist), 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, often with a speckled appearance due to the presence of other minerals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Hornblende crystals exhibit a vitreous to sub-vitreous luster. The overall rock may appear dull to somewhat shiny on foliation planes.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Schistose, meaning it has a well-developed foliation (parallel alignment of platy or prismatic minerals). Medium- to coarse-grained, with individual hornblende crystals often visible to the naked eye.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Hornblende crystals are typically prismatic, elongated, and often acicular (needle-like) or bladed. They are aligned parallel to the schistosity.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Hornblende itself has two distinct cleavages at approximately 56 and 124. In the rock, the schistosity represents a plane of easy splitting.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in regional metamorphic terrains associated with convergent plate boundaries, mountain belts, and ancient shield areas. It forms at medium metamorphic grades (amphibolite facies) from mafic protoliths.

Key Facts

Hardness

Variable, generally 5-6 on Mohs scale (due to hornblende and other silicates)

Specific Gravity

2.9-3.4 g/cm (reflecting the density of hornblende and other mafic minerals)

Crystal System

Monoclinic (for hornblende, the dominant mineral)

Color

Dark green, black, often with lighter bands of other minerals

Luster

Vitreous to sub-vitreous (hornblende), overall dull to somewhat shiny

Transparency

Opaque (in hand specimen)

Fracture

Irregular to splintery, but typically splits along schistosity planes

Cleavage

Excellent schistosity (rock cleavage) due to mineral alignment; hornblende itself has two distinct cleavages at 56 and 124

Composition

Primarily hornblende and plagioclase feldspar, with variable amounts of quartz, epidote, garnet, biotite, chlorite, and accessory minerals like magnetite, ilmenite, and sphene.

Physical characteristics

• Crystal Habit: Prismatic, elongated, bladed, or acicular crystals of hornblende, aligned to form schistosity.
• Cleavage Type: Excellent schistosity (rock cleavage) parallel to the alignment of hornblende crystals. Hornblende mineral has two distinct prismatic cleavages.
• Fracture Type: Irregular to splintery across the foliation, but the rock preferentially splits along the schistosity planes.
• Tenacity: Brittle
• Luster Type: Vitreous to sub-vitreous (individual hornblende crystals), overall dull to somewhat shiny on foliation surfaces.

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