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Epidote


Properties
Chemical Composition: 
Calcium Aluminium Silicate with Iron - Ca2(Fe,Al)3(SiO4)3(OH)
Classification / Type: 
The epidote group is an isomorphous group comprising the following minerals:
  1. Zoisite
  2. Clinozoisite
  3. Epidote
  4. Piedmontite
  5. Hancockite
  6. Allanite
Colors / Varieties: 
  • Dark green, brownish green, black (darker colors with increasing iron content). Transparency: Transparent to Opaque.
  • Chrome rich green: Tawmawite
Crystal System / Forms: 
Monoclinic System / Prismatic crystals with vertical striations.
Hardness: 
6.5
Specific Gravity: 
3.40
Cleavage / Fracture: 
1 directional prismatic cleavage / Conchoidal fracture
Optic Character: 
Anisotropic, D.R.; Biaxial negative (may show pseudo-uniaxial)
Lustre: 
Vitreous
Refractive Index / Birefringence: 
1.729 - 1.768 / 0.019 - 0.045
Pleochroism: 
Strong (green, brownish red, yellow)
Dispersion: 
0.030
Magnification: 
Parallel needle inclusions, strong doubling of facet edges and inclusions, color zoning.
U.V. Fluorescence: 
Inert
Spectrum: 
455nm, 475nm (Directional)
Cause of Color: 
Iron
Treatment (Enhancement): 
-
Specific Tests & Remarks: 
The strong reddish brown pleochroic color is often mistaken for a color change phenomena.
Synthesis: 
-
Simulants (with key separation tests): 
Tourmaline (R.I., S.G., spectrum), Kornerupine (R.I., S.G., spectrum), Sillimanite (R.I., S.G., spectrum, pleochroism)
Geological Occurrence: 
Generally in low to medium grade metamorphic rocks and igneous rocks.
Sources: 
U.S.A., Sri Lanka, India, Myanmar, Madagascar, Austria, Norway, Switzerland.
Cuts & Uses: 
Facetted, cabochons, etc.

The epidote group is an isomorphous group comprising the following minerals:

  1. Zoisite
  2. Clinozoisite
  3. Epidote
  4. Piedmontite
  5. Hancockite
  6. Allanite

Of these, zoisite, clinozoisite and epidote are used as gemstones, while the others are mainly collector's stones. The properties of Epidote and Zoisite have been discussed.

  • Clinozoisite is an epidote with less Iron content. As a result the color is a lighter green and its properties are S.G. 3.37; R.I. 1.724 - 1.734; Birefringence: 0.010; Anisotropic - Biaxial Positive, almost nil pleochroism and crystalline in the monoclinic system.
  • Unakite: This is basically an epidote rock, granite consisting of pink feldspar and green epidote and is cut as cabochons or beads.
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