Smith College - Geology 222b - Petrology
Petrographic Data File
Epidote
Property
Value
Comments
Formula Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH) Complete solid solution from clinozoisite (Al: Fe 3+ = 3:0) to epidote (Al:Fe 3+ = 2:1)
Crystal System monoclinic  
Crystal Habit coarse to fine granular ; also fibrous Found in aggregates of elongate prismatic crystals with pseudohexagonal cross sections
Cleavage {001} perfect, {100} imperfect perfect cleavage in one direction
Color/Pleochroism

clinozoisite: pale green to gray

epidote: pistachio-green to yellowish-green to black
Optic Sign clinozoisite: Biaxial ( +)

epidote: Biaxial ( -)

2V clinozoisite: 2V= 14-19 degrees

epidote: 2V = 64-90 degrees

Optic Orientation Y=b
O.A.P. = (010)		 Varies strongly with composition.

Refractive Indices
alpha = 
beta = 
gamma = 

clinozoisite
1.670-1.1.715
1.674-1.725
1.690-1.734

epidote
1.715-1.751
1.725-1.784
1.734-1.797
Max Birefringence =0.004 - 0.049 Refractive indices and birefringence increase with iron content.
Elongation Elongate crystals may be either length fast or length slow, since Y is parallel to length. Zoned crystals may show adjacent zones with different sign of elongation.
Extinction Parallel to length of elongate crystals and to the trace of cleavage. Oblique to cleavage in pseudo-hexagonal sections, otherwise straight on cleavage in prismatic section
Dispersion Optic axis dispersion is usually strong with v > r (clinozoisite) or r > v (epidote.)  
Distinguishing Features Epidote is characterized by its green color and one perfect cleavage. H= 6-7. G = 3.25 to 4.45. Streak is white to gray. Clinozoisite and epidote are distinguised from eachother by optic sign, birefringence, and color.
Occurrence Occurs in areas of regional metamorphism; forms during retrograde metamorphism and forms as a reaction product of plagioclase, pyroxene, and amphibole. Common in metamorphosed limestones with calcium rich garnets, diopside, vesuvianite, and calcite.
Editors

Sarah Hale (07), Shawn Moore (13)
Data Source: Nesse, William D: Introduction to Optical Mineralogy (Oxford University Press, 1986) pp.192-193


 
Photomicrograph of epidote in plain polarized light and cross polarized light. Note the high order colors in crossed polars.
Epidote Photomicrograph of epidote in plain polarized light showing pleochroism in rollover image. The width of the image is 2.28 mm (Sample PE-27).
Return to Petrography Index