How to identify Epidote in Quartz

To correctly identify Epidote in Quartz (Ca2(Al,Fe)3(SiO4)3(OH) in SiO2), 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, or dark green. The quartz matrix can be colorless, white, milky, or smoky.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Epidote exhibits a vitreous to resinous luster. Quartz has a vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   The texture is typically granular to crystalline, with distinct epidote crystals or aggregates embedded within the quartz. The quartz itself can be massive or crystalline.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Epidote often forms prismatic, elongated crystals, sometimes acicular, or granular aggregates. Quartz forms hexagonal prisms with pyramidal terminations, or massive anhedral grains.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Epidote has one perfect cleavage parallel to {001}. Quartz has no true cleavage but exhibits conchoidal fracture.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in regionally metamorphosed rocks (greenschist to amphibolite facies), contact metamorphic skarns, and hydrothermally altered igneous rocks, particularly in veins and replacement bodies.

Key Facts

Hardness

Epidote: 6-7 (Mohs); Quartz: 7 (Mohs). The aggregate will scratch glass.

Specific Gravity

Epidote: 3.3-3.5; Quartz: 2.65. The overall specific gravity will be an average depending on the proportions.

Crystal System

Epidote: Monoclinic; Quartz: Trigonal.

Color

Green (epidote) in colorless/white/milky (quartz).

Luster

Vitreous to resinous (epidote), Vitreous (quartz).

Transparency

Epidote: Translucent to opaque; Quartz: Transparent to translucent.

Fracture

Epidote: Uneven to conchoidal; Quartz: Conchoidal.

Cleavage

Epidote: Perfect on {001}; Quartz: None.

Composition

Epidote: Calcium aluminum iron sorosilicate hydroxide (Ca2(Al,Fe)3(SiO4)3(OH)); Quartz: Silicon dioxide (SiO2).

Physical characteristics

• Crystal Habit: Epidote: Prismatic, acicular, granular, massive. Quartz: Prismatic, massive, granular.
• Cleavage Type: Epidote: Perfect, one direction; Quartz: Absent.
• Fracture Type: Epidote: Uneven to conchoidal; Quartz: Conchoidal.
• Tenacity: Epidote: Brittle; Quartz: Brittle.
• Luster Type: Vitreous to resinous (epidote), Vitreous (quartz).

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