Epidote

Description

Epidote is a common calcium aluminum iron sorosilicate mineral. It is the most prevalent member of the Epidote Group, which includes other minerals like clinozoisite (iron-poor epidote) and zoisite (orthorhombic polymorph). Epidote typically forms prismatic crystals, often with striations parallel to their length, and can also occur in massive, granular, or fibrous habits. Its characteristic pistachio-green color is a key identifying feature, though it can range from yellowish-green to brownish-green and even black.

How to Identify

Color

Typically pistachio-green, but can also be yellowish-green, brownish-green, or black. The color is often diagnostic.

Luster

Vitreous (glassy) to resinous on crystal faces, sometimes dull on massive forms.

Texture

Can be massive, granular, fibrous, or in well-formed prismatic crystals. Crystals are often striated.

Crystal Form

Monoclinic, typically forming elongated, prismatic crystals, often with one direction of elongation. Crystals are commonly twinned. Also found as granular aggregates or massive forms.

Cleavage

Perfect cleavage on {001}, good cleavage on {100}. This can be observed as distinct flat surfaces when broken.

Geological Environment

Common in regionally metamorphosed rocks (e.g., schists, gneisses), contact metamorphic skarns, hydrothermally altered igneous rocks (e.g., basalt, gabbro, granite), and as a secondary mineral in altered plagioclase.

Key Facts

• Hardness: 6 - 7 on Mohs scale
• Specific Gravity: 3.3 - 3.5 (relatively dense)
• Crystal System: Monoclinic
• Color: Pistachio-green, yellowish-green, brownish-green, black
• Luster: Vitreous to resinous
• Transparency: Transparent to translucent to opaque
• Fracture: Uneven to conchoidal
• Cleavage: Perfect on {001}, good on {100}
• Composition: Calcium aluminum iron sorosilicate

Quick Check

• Color: Pistachio-green to dark green
• Luster: Vitreous to resinous
• Streak: White to grayish-white

Physical Characteristics

• Crystal Habit: Prismatic, elongated, often striated parallel to length; also massive, granular, fibrous.
• Cleavage Type: Perfect basal cleavage on {001}, good prismatic cleavage on {100}.
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous to resinous.

Formation

Epidote forms primarily under regional and contact metamorphic conditions, particularly in greenschist to amphibolite facies. It is also a common product of hydrothermal alteration of mafic igneous rocks (e.g., gabbro, basalt) and some felsic rocks (e.g., granite, granodiorite). It can also occur as a secondary mineral in altered plagioclase feldspar.

Usage

Epidote is not a major industrial mineral. However, attractive, well-formed crystals are highly prized by mineral collectors. Transparent varieties can be cut as gemstones, though they are relatively soft for jewelry. It is also used as an indicator mineral in geological studies to understand metamorphic conditions and alteration processes.

Age Distribution

Found in rocks of various ages, from Precambrian to Cenozoic, wherever appropriate metamorphic or hydrothermal conditions have occurred.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Sorosilicates

Group

Epidote Group

Crystal System

Monoclinic

Chemical Formula

Ca2(Al,Fe)3(SiO4)(Si2O7)O(OH)

Composition

Calcium aluminum iron sorosilicate, with varying proportions of Al and Fe3+.

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