How to identify Epidote

To correctly identify Epidote (Epidote Group), 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 pistachio-green, but can also be yellowish-green, brownish-green, or black. The color is often diagnostic.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) to resinous on crystal faces, sometimes dull on massive forms.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Can be massive, granular, fibrous, or in well-formed prismatic crystals. Crystals are often striated.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic, typically forming elongated, prismatic crystals, often with one direction of elongation. Crystals are commonly twinned. Also found as granular aggregates or massive forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect cleavage on {001}, good cleavage on {100}. This can be observed as distinct flat surfaces when broken.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Common in regionally metamorphosed rocks (e.g., schists, gneisses), contact metamorphic skarns, hydrothermally altered igneous rocks (e.g., basalt, gabbro, granite), and as a secondary mineral in altered plagioclase.

Key Facts

Hardness

6 - 7 on Mohs scale

Specific Gravity

3.3 - 3.5 (relatively dense)

Crystal System

Monoclinic

Color

Pistachio-green, yellowish-green, brownish-green, black

Luster

Vitreous to resinous

Transparency

Transparent to translucent to opaque

Fracture

Uneven to conchoidal

Cleavage

Perfect on {001}, good on {100}

Composition

Calcium aluminum iron sorosilicate

Physical characteristics

• Crystal Habit: Prismatic, elongated, often striated parallel to length; also massive, granular, fibrous.
• Cleavage Type: Perfect basal cleavage on {001}, good prismatic cleavage on {100}.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to resinous.

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