How to identify Epidote in Quartz

To correctly identify Epidote in Quartz (Epidote-Quartz), 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 inclusions are typically pistachio green, yellowish-green, dark green, or brownish-green. The quartz matrix is usually colorless, white, or smoky.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) for both quartz and epidote.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Can range from massive to crystalline, with epidote inclusions appearing as fine needles, larger prismatic crystals, or granular aggregates within the quartz.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz typically forms hexagonal prisms with pyramidal terminations, though it can be massive. Epidote often forms elongated prismatic crystals, acicular aggregates, or granular masses within the quartz.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Quartz has no cleavage (conchoidal fracture). Epidote has perfect cleavage on {001} and good cleavage on {100}.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Metamorphic rocks (greenschist, amphibolite facies), hydrothermally altered zones, skarns, and veins associated with mafic igneous rocks or calcium-rich sedimentary rocks.

Key Facts

Hardness

Quartz: 7 on Mohs scale. Epidote: 6-7 on Mohs scale. The aggregate will have a hardness generally between 6 and 7.

Specific Gravity

Quartz: 2.65 g/cm³. Epidote: 3.35-3.47 g/cm³. The aggregate's specific gravity will be between these values, depending on the proportion of each mineral.

Crystal System

Quartz: Trigonal. Epidote: Monoclinic.

Color

Green (epidote) in colorless, white, or smoky (quartz).

Luster

Vitreous.

Transparency

Transparent to translucent.

Fracture

Quartz: Conchoidal. Epidote: Uneven to subconchoidal.

Cleavage

Quartz: None. Epidote: Perfect on {001}, good on {100}.

Composition

Silicon dioxide (SiO₂) for quartz; Ca₂Al₂Fe³⁺(SiO₄)(Si₂O₇)O(OH) for epidote.

Physical characteristics

• Crystal Habit: Quartz: Prismatic, massive, granular. Epidote: Prismatic, acicular, granular, massive.
• Cleavage Type: Quartz: None. Epidote: Perfect basal cleavage, good pinacoidal cleavage.
• Fracture Type: Quartz: Conchoidal. Epidote: Uneven to subconchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous.

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