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Photomicrograph of epidote in plain polarized light and cross polarized light. Note the high order colors in crossed polars. |
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| 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 | |
| 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
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clinozoisite |
epidote |
| 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. | |
| 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) Data Source: Nesse, William D: Introduction to Optical Mineralogy ( Oxford University Press, 1986) pp.192-193 | |
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Photomicrograph of epidote in plain polarized light and cross polarized light. Note the high order colors in crossed polars. |