How to identify Epidote in Quartz

To correctly identify Epidote in Quartz (Epidote (Ca2(Al,Fe)3(SiO4)3(OH)) in Quartz (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 quartz matrix can be colorless, white (milky), or smoky. The epidote inclusions are typically pistachio green, yellowish-green, dark green, or brownish-green. The overall appearance is often a green-speckled or green-streaked quartz.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Vitreous (glassy) for both quartz and epidote, though epidote can sometimes have a slightly resinous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Typically granular to crystalline. Epidote crystals can be acicular (needle-like), prismatic, or granular within the quartz matrix. The overall texture can be massive or show distinct crystal forms.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Quartz typically forms hexagonal prisms with pyramidal terminations, though in massive aggregates it can be anhedral. Epidote forms monoclinic crystals, often prismatic, acicular, or granular. Inclusions within quartz may show these forms or appear as irregular grains.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Quartz has no true cleavage, exhibiting conchoidal fracture. Epidote has one perfect cleavage plane {001} and one good cleavage plane {100}.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Found in regionally metamorphosed rocks (e.g., greenschist facies), contact metamorphic aureoles (skarns), and hydrothermally altered igneous rocks (e.g., granodiorites, basalts). It is common in areas that have undergone significant tectonic activity and associated metamorphism.

Key Facts

Hardness

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

Specific Gravity

Quartz: 2.65 g/cm³; Epidote: 3.3-3.5 g/cm³. The aggregate's specific gravity will be intermediate, 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 (quartz), translucent to opaque (epidote)

Fracture

Conchoidal (quartz), uneven to subconchoidal (epidote)

Cleavage

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

Composition

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

Physical characteristics

• Crystal Habit: Quartz: Prismatic, massive, granular; Epidote: Prismatic, acicular, granular, massive
• Cleavage Type: Quartz: None; Epidote: Perfect in one direction, good in another
• Fracture Type: Quartz: Conchoidal; Epidote: Uneven to subconchoidal
• Tenacity: Brittle
• Luster Type: Vitreous

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