Epidote

This fairly dense, fragile mineral has distinct cleavage and it is rarely cut like a gemstone. Crystals are yellow, green, or dark brown columnar prisms, with faces finely striated parallel towards the crystal’s length. Pleochroism is strong, showing either yellow, green, or brown. Rock composed mainly of epidote might be polished or tumbled and sold as “unakite”.
Green crystals occur within the Austrian and French Alps. Epidote can also be found within the former USSR, Italy, the area of Elba (Italy), Mozambique and Mexico.

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.