Epidote

Physical Properties
Chemical formula Ca2(Al,Fe)3Si3O12(OH)
Class Sorosilicate
Paired tetrahedra groups
Crystal system Monoclinic
Habit Prismatic
Fibrous
Acicular
Granular
Massive
Fibrous aggregates
Color Pistachio green
Yellow-green to black
Hardness 6 to 7
Specific gravity 3.4 to 3.5
Cleavage Perfect (001), poor (100)
Fracture Uneven
Luster Vitreous
Transparency Transparent to translucent
Streak White
Optical Properties
PPL Usually colorless; may be light
green or pink, depending on
composition
Typically non-pleochroic
High relief
XPL Up to 3rd order colors
with increasing Fe content
Football-shaped
Concentric color rings
δ 0.01
after Perkins, 350

Epidote in Hand Sample

Epidote in unakite

Epidote in a different unakite

Epidote in Catoctin Formation metabasalt

Epidote-family mineral zoisite, with black hornblende and red corundum (ruby)

Epidote in Thin Section

Thin Section GigaPans

Epidote in epidosite, plane polars

Epidote in epidosite, crossed polars

Epidote in unakite, plane polars

Epidote in unakite, crossed polars

Epidote in epidosite, PPL

Epidote in epidosite, XPL

The preceding two pictures and videos are of epidote in epidosite, where it’s quite easy to spot. You’re more likely to encounter epidote as a tiny, easily overlooked accessory mineral; the following two pictures videos are of a tiny, easily overlooked accessory epidote.

Epidote grain, PPL

Epidote grain, XPL

Epidote in unakite, plane polars
Epidote in unakite, plane polars

Epidote in unakite, crossed polars
Epidote in unakite, crossed polars

Chlorite with epidote grains,  PPL

Chlorite with epidote grains, XPL

Further Reading

Epidote at webmineral.com

Epidote at mindat.org