How to identify Epidote in Epidosite

To correctly identify Epidote in Epidosite (Epidote (Ca2(Al,Fe)3(SiO4)3(OH)) in Epidosite (metamorphic rock composed primarily of epidote and 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

   Epidosite is typically pistachio-green to yellowish-green, often with a mottled or speckled appearance due to the epidote and quartz. Epidote crystals within the rock are usually green, yellowish-green, or brownish-green.

2. 2

   Luster

   How the surface reflects light: metallic, vitreous, dull

   Epidote exhibits a vitreous (glassy) to resinous luster. Epidosite as a rock will have an overall duller, more earthy luster on fresh surfaces, but individual epidote grains will show vitreous luster.

3. 3

   Texture

   Grain size and surface feel: coarse, fine, glassy

   Epidosite is typically fine-grained to medium-grained, often granoblastic (equigranular, interlocking grains) or nematoblastic (with elongated minerals like epidote showing preferred orientation). Epidote crystals can be prismatic, acicular, or granular.

4. 4

   Crystal Form

   Shape of visible crystals or crystal faces

   Epidote commonly forms prismatic crystals, often elongated and striated parallel to the length. It can also occur as granular aggregates or massive forms within epidosite.

5. 5

   Cleavage

   How it breaks: flat cleavage planes vs irregular fracture

   Epidote has one perfect cleavage parallel to {001}. This cleavage may be visible in larger epidote crystals within the epidosite.

6. 6

   Geological Environment

   The rock formation and setting where it occurs

   Epidosite is found in metamorphic terrains, particularly in areas that have undergone greenschist to lower amphibolite facies metamorphism. It is commonly associated with altered mafic igneous rocks (basalts, gabbros, diabases) and sometimes with altered calcareous sediments. It can be found in subduction zones, mid-ocean ridges (hydrothermal alteration), and regional metamorphic belts.

Key Facts

Hardness

6 - 7 (Mohs scale for epidote)

Specific Gravity

3.3 - 3.5 (for epidote)

Crystal System

Monoclinic (for epidote)

Color

Pistachio-green, yellowish-green, brownish-green (for epidote and epidosite)

Luster

Vitreous to resinous (for epidote), dull to earthy (for epidosite)

Transparency

Transparent to translucent to opaque (for epidote)

Fracture

Uneven to conchoidal (for epidote)

Cleavage

Perfect on {001} (for epidote)

Composition

Epidosite: Primarily epidote (Ca2(Al,Fe)3(SiO4)3(OH)) and quartz (SiO2), with minor chlorite, actinolite, plagioclase. Epidote: Calcium aluminum iron sorosilicate hydroxide.

Physical characteristics

• Crystal Habit: Prismatic, acicular, granular, massive (for epidote)
• Cleavage Type: Perfect on one direction ({001}) (for epidote)
• Fracture Type: Uneven to conchoidal (for epidote)
• Tenacity: Brittle (for epidote)
• Luster Type: Vitreous to resinous (for epidote)

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