How to identify Actinolite

To correctly identify Actinolite (Ca2(Mg,Fe)5Si8O22(OH)2), 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 light to dark green, sometimes grayish-green, blackish-green, or rarely white.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous to silky (especially in fibrous varieties).

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Can be massive, fibrous, columnar, or radiating. Fibrous varieties can be very fine and hair-like.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Prismatic, acicular (needle-like), or bladed crystals. Often found in radiating aggregates.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect cleavage in two directions intersecting at approximately 56° and 124°.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Metamorphic rocks such as greenschist, amphibolite, serpentinite, and skarns. Also found in altered mafic igneous rocks.

Key Facts

Hardness

5.5-6 on the Mohs scale

Specific Gravity

3.0-3.4 (varies with Fe content)

Crystal System

Monoclinic

Color

Light to dark green, grayish-green, blackish-green, rarely white

Luster

Vitreous to silky

Transparency

Translucent to opaque

Fracture

Uneven to subconchoidal

Cleavage

Perfect on {110} (two directions at ~56° and ~124°)

Composition

Calcium magnesium iron silicate hydroxide

Physical characteristics

• Crystal Habit: Prismatic, acicular, bladed, fibrous, columnar, radiating aggregates, massive.
• Cleavage Type: Perfect in two directions, intersecting at approximately 56° and 124°.
• Fracture Type: Uneven to subconchoidal.
• Tenacity: Brittle (non-fibrous); flexible to elastic (fibrous).
• Luster Type: Vitreous to silky.

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