How to identify Epidote

To correctly identify Epidote (Ca2(Al,Fe)3(SiO4)3(OH)), 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 to yellowish-green, olive-green, or blackish-green. Less commonly brown or colorless. The intensity of the green color generally increases with higher iron content.

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. Often found as radiating aggregates.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Monoclinic, typically forming elongated, prismatic crystals, often with one direction of perfect cleavage. Crystals are commonly striated parallel to their length. Can also occur as granular aggregates or massive forms.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Perfect cleavage on {001}, and good cleavage on {100}. This results in a splintery or step-like fracture along cleavage planes.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Metamorphic rocks (greenschist, amphibolite facies), hydrothermally altered igneous rocks (e.g., porphyry deposits), veins, skarns, and as a secondary alteration product of plagioclase.

Key Facts

Hardness

6 - 7 on the Mohs scale

Specific Gravity

3.3 - 3.5 (varies with Fe content)

Crystal System

Monoclinic

Color

Pistachio-green, yellowish-green, olive-green, blackish-green, brown, rarely colorless

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 hydroxide

Physical characteristics

• Crystal Habit: Prismatic, elongated, often striated crystals; also massive, granular, fibrous, or radiating aggregates.
• Cleavage Type: Perfect in one direction ({001}), good in another ({100}).
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous (glassy) to resinous.

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