How to identify Epidote in Basalt

To correctly identify Epidote in Basalt (Epidote in Basalt), 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

   The epidote itself is typically pistachio-green, yellowish-green, or olive-green. The basaltic matrix will be dark gray to black, often with a greenish tint due to associated chlorite.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Epidote crystals exhibit a vitreous to resinous luster. The basaltic matrix will have a dull to sub-vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The basalt matrix is typically fine-grained, aphanitic, or porphyritic. Epidote may occur as fine-grained disseminations, radiating aggregates, prismatic crystals, or granular masses within the basalt, often filling amygdules or veins. The overall texture can be massive or amygdaloidal.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Epidote often forms elongated, prismatic crystals with striations parallel to the length, or granular aggregates. In basalt, it can also be anhedral (without well-formed crystal faces) when disseminated.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Epidote has perfect cleavage on {001} and good cleavage on {100}. The basaltic matrix, being fine-grained, typically does not show macroscopic cleavage.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Hydrothermally altered basaltic rocks, commonly found in oceanic crust, ophiolite complexes, mid-ocean ridge systems, subduction zones, and areas of regional low-grade metamorphism (greenschist facies).

Key Facts

Hardness

6-7 on Mohs scale (epidote), 5-6 (basalt matrix).

Specific Gravity

3.3-3.5 (epidote), 2.8-3.0 (basalt).

Crystal System

Monoclinic (epidote).

Color

Pistachio-green, yellowish-green, olive-green (epidote).

Luster

Vitreous to resinous (epidote).

Transparency

Transparent to translucent (epidote).

Fracture

Uneven to conchoidal (epidote).

Cleavage

Perfect on {001}, good on {100} (epidote).

Composition

Ca2(Al,Fe)3(SiO4)3(OH) (epidote), primarily plagioclase, pyroxene, olivine (basalt).

Physical characteristics

• Crystal Habit: Prismatic, acicular, granular, massive, radiating aggregates (epidote).
• Cleavage Type: Perfect basal cleavage, good pinacoidal cleavage (epidote).
• Fracture Type: Uneven to conchoidal (epidote).
• Tenacity: Brittle (epidote).
• Luster Type: Vitreous to resinous (epidote).

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