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 blackish-green, brown, or rarely colorless. It commonly forms prismatic crystals, often with striations parallel to the crystal length. Epidote is a widespread rock-forming mineral, particularly in metamorphic and hydrothermally altered rocks. Its chemical formula, Ca2(Al,Fe)3(SiO4)3(OH), indicates a solid solution series between clinozoisite (iron-poor end-member) and epidote (iron-rich end-member). The iron content (Fe3+) is responsible for its characteristic green color.

How to Identify

Color

Typically pistachio-green to yellowish-green, olive-green, or blackish-green. Less commonly brown or colorless. The intensity of the green color generally increases with higher iron content.

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. Often found as radiating aggregates.

Crystal Form

Monoclinic, typically forming elongated, prismatic crystals, often with one direction of perfect cleavage. Crystals are commonly striated parallel to their length. Can also occur as granular aggregates or massive forms.

Cleavage

Perfect cleavage on {001}, and good cleavage on {100}. This results in a splintery or step-like fracture along cleavage planes.

Geological Environment

Metamorphic rocks (greenschist, amphibolite facies), hydrothermally altered igneous rocks (e.g., porphyry deposits), veins, skarns, and as a secondary alteration product of plagioclase.

Key Facts

• Hardness: 6 - 7 on the Mohs scale
• Specific Gravity: 3.3 - 3.5 (varies with Fe content)
• Crystal System: Monoclinic
• Color: Pistachio-green, yellowish-green, olive-green, blackish-green, brown, rarely colorless
• 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 hydroxide

Quick Check

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

Physical Characteristics

• Crystal Habit: Prismatic, elongated, often striated crystals; also massive, granular, fibrous, or radiating aggregates.
• Cleavage Type: Perfect in one direction ({001}), good in another ({100}).
• Fracture Type: Uneven to conchoidal.
• Tenacity: Brittle.
• Luster Type: Vitreous (glassy) to resinous.

Formation

Epidote typically forms during regional or contact metamorphism of intermediate to basic igneous rocks (e.g., basalt, gabbro, andesite) and certain sedimentary rocks (e.g., marls, impure limestones). It is a common product of greenschist facies metamorphism and can also form in hydrothermal alteration zones, particularly in porphyry copper deposits, and in veins associated with granitic intrusions. It can also occur as a secondary mineral in altered plagioclase feldspar.

Usage

Epidote is primarily a mineral of scientific and educational interest. Well-formed, transparent crystals are occasionally cut as gemstones, though their relative softness (Mohs 6-7) limits their durability. It is also used as an ornamental stone when massive and attractive. Its presence is a key indicator of specific metamorphic conditions and hydrothermal alteration, making it valuable to geologists for understanding geological 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)3(OH)

Composition

Calcium aluminum iron silicate hydroxide

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