How to identify Epidote in Quartz

To correctly identify Epidote in Quartz (Epidote-Quartz rock), 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 rock will exhibit a combination of the characteristic green to yellowish-green of epidote and the white, gray, or colorless translucence of quartz. The green color is often patchy or streaky due to the distribution of epidote crystals.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Epidote typically has a vitreous to resinous luster, while quartz has a vitreous luster. The overall rock will display a vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Granular to crystalline, with individual epidote crystals (often prismatic or acicular) visible within the quartz matrix. Can be massive or show a schistose texture if subjected to directed pressure during metamorphism.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Epidote crystals are typically prismatic, often with striations parallel to their length, or granular. Quartz crystals can be anhedral (irregular) filling spaces, or euhedral (well-formed) if grown in open cavities.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Epidote has one perfect cleavage plane ({001}) and one imperfect cleavage plane ({100}). Quartz has no cleavage, exhibiting conchoidal fracture. The rock as a whole will show the cleavage of epidote where present, and the fracture of quartz.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Commonly found in regionally metamorphosed terrains, especially in greenschist to amphibolite facies rocks derived from mafic igneous rocks or impure calcareous sediments. Also occurs in contact metamorphic aureoles and hydrothermal alteration zones.

Key Facts

Hardness

Epidote: 6-7; Quartz: 7 (Mohs scale)

Specific Gravity

Epidote: 3.35-3.45; Quartz: 2.65

Crystal System

Epidote: Monoclinic; Quartz: Trigonal

Color

Epidote: Pistachio green, yellowish-green, dark green, brownish-green; Quartz: Colorless, white, gray, translucent to opaque.

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}, imperfect on {100}; Quartz: None

Composition

Epidote: Ca2(Al,Fe3+)3(SiO4)3(OH); Quartz: SiO2

Physical characteristics

• Crystal Habit: Epidote: Prismatic, acicular, granular, massive; Quartz: Massive, granular, sometimes euhedral hexagonal prisms.
• Cleavage Type: Epidote: Perfect basal cleavage; Quartz: None
• Fracture Type: Epidote: Uneven to conchoidal; Quartz: Conchoidal
• Tenacity: Epidote: Brittle; Quartz: Brittle
• Luster Type: Vitreous to resinous

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