Epidote

Description

Epidote is a calcium aluminum iron sorosilicate mineral belonging to the epidote group. It is characterized by its distinctive pistachio-green to yellowish-green color, though it can also be dark green, brownish-green, or black. It commonly forms prismatic crystals, often with striations parallel to the long axis, and can also occur in granular, fibrous, or massive habits. Epidote is a key mineral in many metamorphic rocks, particularly those of the greenschist and amphibolite facies, and is indicative of specific pressure and temperature conditions during metamorphism.

How to Identify

Color

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

Luster

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

Texture

Can be crystalline (prismatic, often striated), granular, fibrous, or massive.

Crystal Form

Prismatic crystals, often elongated and tabular, with characteristic striations parallel to the length. Crystals are typically monoclinic. Can also be found as radiating aggregates or granular masses.

Cleavage

Perfect cleavage on {001} and good cleavage on {100}, often visible as parallel lines or steps on crystal faces.

Geological Environment

Common in regionally metamorphosed rocks (e.g., greenschist, amphibolite facies), contact metamorphic rocks (skarns), hydrothermally altered igneous rocks, and veins.

Key Facts

• Hardness: 6 - 7 on the Mohs scale
• Specific Gravity: 3.3 - 3.5 (relatively dense)
• Crystal System: Monoclinic
• Color: Pistachio-green, yellowish-green, dark 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 yellowish-green
• Luster: Vitreous to resinous
• Streak: White to grayish-white

Physical Characteristics

• Crystal Habit: Prismatic, often elongated and tabular, commonly striated parallel to the length; also granular, fibrous, massive, or radiating aggregates.
• Cleavage Type: Perfect basal cleavage on {001} and good prismatic cleavage on {100}. Cleavage planes are often visible.
• Fracture Type: Uneven to conchoidal. Breaks with irregular, shell-like surfaces.
• Tenacity: Brittle. Easily breaks or crumbles under pressure.
• Luster Type: Vitreous (glassy) to resinous on crystal faces; can be dull in massive forms.

Formation

Epidote is a common rock-forming mineral that typically forms during regional and contact metamorphism of intermediate to basic igneous rocks (e.g., gabbro, basalt, andesite) and certain sedimentary rocks (e.g., marl, impure limestone). It is also a product of hydrothermal alteration, particularly in greenschist facies metamorphic environments, where it replaces plagioclase feldspar, pyroxenes, and amphiboles. It can also form in veins and skarns.

Usage

Epidote is primarily a mineral of scientific and academic interest, serving as an indicator mineral for metamorphic conditions. Well-formed crystals are sought after by mineral collectors. Occasionally, attractive, dense masses of epidote are cut and polished for ornamental purposes or as cabochons, sometimes marketed as 'epidote in quartz' or 'epidote rock'. It has no significant industrial uses.

Age Distribution

Epidote forms under a wide range of metamorphic conditions and can be found in rocks of various ages, from Precambrian to Cenozoic, wherever appropriate metamorphic or hydrothermal alteration has occurred.

Where to Find

Finding Tips

Similar Rocks

Scientific Classification

Mineral Class

Sorosilicates

Group

Epidote Group

Crystal System

Monoclinic

Chemical Formula

Ca2(Al,Fe3+)3(SiO4)(Si2O7)O(OH)

Composition

Calcium aluminum iron sorosilicate. The ratio of Al to Fe3+ can vary, leading to a solid solution series with clinozoisite (Fe-poor epidote) and pistacite (Fe-rich epidote).

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