How to identify Hornblende

To correctly identify Hornblende (Hornblende), 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, dark brown, or black.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to sub-vitreous, sometimes silky in fibrous varieties.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Prismatic, often elongated crystals. Can be massive or granular.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic system, forming long, slender to stout prismatic crystals with a roughly hexagonal or rhombic cross-section. Often terminated by dome faces.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Two distinct cleavage planes intersecting at approximately 56° and 124° (characteristic of amphiboles). This is a crucial diagnostic feature.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in intermediate to felsic igneous rocks (e.g., diorite, granodiorite, andesite) and a wide range of metamorphic rocks (e.g., amphibolite, hornblende schist, gneiss).

Key Facts

Hardness

5-6 on the Mohs scale

Specific Gravity

3.0-3.4 (variable depending on composition)

Crystal System

Monoclinic

Color

Dark green, dark brown, black

Luster

Vitreous to sub-vitreous

Transparency

Translucent to opaque

Fracture

Uneven to conchoidal

Cleavage

Perfect in two directions, intersecting at 56° and 124°

Composition

Complex calcium, magnesium, iron, aluminum silicate with hydroxyl (OH) groups. The general formula is (Ca,Na)2-3(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2.

Physical characteristics

• Crystal Habit: Prismatic, columnar, acicular, fibrous, massive, granular.
• Cleavage Type: Perfect {110} cleavage, producing two planes at 56° and 124°.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to sub-vitreous, sometimes silky in fibrous varieties.

Explore Hornblende