Epidote is a kind of island-structure silicate mineral. Due to different iron content, they are usually green in different shades, or colorless or yellowish-green. The more iron, the darker the color.
If the manganese content in epidote is high, it is called piedmontite. It is the primary ore of precious piedmontite jewelry.
Epidote is produced in metamorphic rocks, mainly in green schists. It is commonly found in contact metasomatic deposits and is the alteration product of skarn minerals formed by hydrothermal action.
It is widely distributed in the basic volcanic rocks altered by hydrothermal. The main producing areas are Italy, Norway, Brazil, Mexico, North Carolina, California, Switzerland, Mozambique, etc.
Basic properties of Epidote
1) Mohs hardness: 6-7.
2) Cleavage: {001} complete, {100} incomplete.
3) Density: 3.37~3.50g/cm.
4) Optical property: inhomogeneous body, biaxial crystal, negative optical property.
5) Refractive index: 1.723~1.797, birefringence: 0.013~0.046.
6) Gloss: glass luster to grease luster.
7) Color: usually green in various colors, but also yellow, brown and black.
8) Pleochroism: strong.
9) Transparency: Transparent to translucent.
10) Origin: it is one of the common metamorphic minerals, which can be formed by hydrothermal alteration, contact metamorphism and regional metamorphism. Due to its stable nature, it also is produced in placer ore.
Physical properties of epidote
The chemical formula of epidote is Ca2 (Al, Fe) Al2 [SiO4] [Si2O7] O (OH), and the composition often contains a small amount of Mg, Mn, Sr, Na and other elements.
Form
Epidote belongs to the monoclinic system. A complete isomorphic series can be formed between epidote and plinite.
The crystals extend along the direction of the B axis. The crystals are columnar or thick plates, and often granular.
Common simplex: parallel double-sided a {100}, C {001}, L {101), r {102}, e {101};
Square columns m {110}, O {011), n {111}.
There are longitudinal lines on the crystal plane of the parallel B-axis crystal belt. The gem mineral also has fine grain and fibrous aggregates, and can form flake twins according to (100).
Physical characteristics
The color ranges from light to dark green, brown, yellow, black, mainly yellow-green, glass luster to grease luster, transparent to translucent, biaxial crystal negative, refractive index Np=1.729, Nm=1.739, Ng=1.768 (+0.012, – 0035), the double refractive index is 0.019~0.045, and the optical axis angle is 70 °~90 °.
The refractive index, birefringence and optical axis angle increase with the increase of iron content.
Strong polychrome, absorbing Nm>Ng>Np, Np is light yellow to light green, Nm is yellow to green yellow, and Ng is green.
The dispersion is 0.030, and it is generally erotic to UV fluorescent long-wave and short-wave light.
The absorption spectrum usually has a strong absorption band at 445 nm, and sometimes a weak absorption band at 475 nm.
There are usually 475 nm absorption bands on brownish yellow and dark green epidote.
A group of parallel {001} complete cleavage and {100} incomplete cleavage result in the mineral being mostly in the form of plate-like, uneven to shell fracture, with the hardness of 6-7 and density of 3.4 (+0.10, -0.15) g/cm, which increases with the increase of Fe content.
Identification
Epidote minerals are mostly yellow-green, and some green and brown epidote gem minerals are too dark, almost like black.
It has a columnar crystal shape, with longitudinal lines on the crystal surface, and obvious cleavage is more than olivine, hornblende, etc.
It is especially pleochroic like tourmaline, but tourmaline has a lower refractive index, density, and dispersion.
It can also be distinguished from vesuvianite according to its birefringence, optical properties and spectrum.
Epidote can be partially dissolved in hydrochloric acid and quickly dissolved in hydrofluoric acid.
Enlarged observation shows gas-liquid two-phase inclusions or solid mineral inclusions. Lightens in color after weathering.
Origin
Epidote is mainly produced in the middle-temperature hydrothermal stage and is widely found in metamorphic rocks, various magmatic rocks, and sedimentary rocks affected by the hydrothermal process.
Most of epidote is weathered into yellowish-green. Epidote is often found in altered rocks of dark minerals such as pyroxene and amphibole, but most epidote crystals (with gem value) are often found in metamorphic limestone deposits.
Good crystals are produced in Australia, Israel and France.
The epidotized granite, composed of quartz, pink feldspar and green epidote, is gorgeous as a decorative material. The main producing areas are Austria, Italy, Norway, etc.
Epidote is widely distributed in China, but few of them have gem value.
The precious gemstones are mainly in Shexian County, Hebei Province, which are produced in the vesuvianite sand-karst type iron ore deposit and coexist with actinolite and plagioclase.
The epidote produced in Anyang, Henan, Wu’an, Hebei and other places is also produced in the skarn type iron ore, and the epidote produced in Anyang, Henan is relatively coarse. Other areas such as Shangluo, Shaanxi and Hunan have also been found.
Epidote types
Oisanite
It generally refers to epidote with good transparency.
hancockite
Epidote containing lead and thought. Chemical formula (Pb, Ca, Sr) 2 (AI, Fe) 3 [SI2O7] [SiO4] O (OH).
- It is green brown to black,
- With refractive index Ng=1.830, Nm=1.810Np=1.788;
- The birefringence is 0.042.
- Pleochroism: red to light yellow-brown.
- The relative density is 4.03.
- The only known origin is Franklin, New Jersey, United States, but the crystal is small.
Ferriepidote
Iron-rich epidote variety, dark-green to green black.
Chromepidote
A chromium-containing dark green (emerald-like) epidote; It shows strong pleochroism: emerald~bright yellow. It is named for the Tumao found in Myanmar. It is also found in Fungwe, Zimbabwe.
Delphinite
A yellowish-green epidote was produced in France.
Whitamite
A rare yellow-red variant of epidote.
Epidote cat’s eye
Epidote with cat’s eye effect due to its parallel fibrous inclusion.
Unakite
It is also called “Unaka Stone.” It is a colorful granite containing green epidote, pink feldspar and milky white quartz.
It is named because it was originally taken from the Unaka Ridge in North Carolina, USA.
Minerals of different colors are distributed in speckles, which is also called “Un-unikite” in the Chinese market.
It is often used for grinding small ornaments and necklaces of plain face type and also for carving.
In addition to the United States, it is also found in Ireland’s Galway Bay, Zimbabwe, and Keimos in the north corner of South Africa, Tanzania and other places.
Piedmont
Manganese-bearing epidote (manganese replaces part of iron and aluminum).
- Red to reddish brown or black;
- Strong pleochroism: bright red to purple to orange-yellow;
- Refractive index Ng=1.762 ～ 1.829,
- Nm=1.750~1.807, Np=1.732~1.794, which increases with the increase of manganese content:
- The birefringence rate is 0.025~0.073;
- The relative density is 3.45 ～ 3.52.
- Origin includes Egypt, the United States, New Zealand, Japan, France, etc.
But the crystal is small and lacks the meaning of gemstone.
However, the Egyptian rhodochrosite containing piedmont can be used for carving and is a famous colored stone.
In addition, piedmonitite jade is composed of piedmont and has been found in Jilin, China.
Piedmonitite jade
A kind of jade mainly composed of piedmont, produced in the middle of Hadaling, Jilin Province, China. After polishing, the stone is fine, smooth and clean, with a hardness of 6-6.5.
Clinozoisite
Epidote group minerals are homomorphic allomorphs of epidote, and they are isomorphic series with epidote, that is, equivalent to epidote without iron or with little iron.
The chemical formula is Ca2AL3[Si2O7][SiO4]O（OH）. Monocline system, crystal columnar or tabular, also commonly granular and radial aggregates; Colorless or gray, light yellow, light green, brown green and other colors;
- Transparent to slightly transparent, glass luster; Biaxial normal crystal,+2V=14 °～ 90 °;
- Refractive index Ng=1.690 ～ 1.734, Nm=1.674 ～ 1.725, and Np=1.670 ～ 1.715;
- Birefringence 0.005 ～ 0.015;
- No pleochroism.
- The hardness is 6~7, and
- The relative density is 3.21~3.38.
- Cleavage parallel (001) complete, parallel (100) incomplete.
It is mainly generated in hydrothermal alteration, also in magmatic period and other metamorphism, and is widely distributed. Gem-grade crystals include greyish green crystals from Kenya, the highest quality crystals from Salzburg, Austria, and large clinoptilolite crystals from scheelite deposits in Baja California, Mexico;
Other producing areas include Canada, Wyoming, Italy, India, etc.
Elinozoisitecat’s eye
Epidote with cat’s eye effect due to the inclusion of parallel fibrous mineral inclusions. Produced in Baja, California, Mexico.
Conclusion:
Epidote is derived from Greek, meaning “increase,” because it develops more crystal planes parallel to the long axis.
Epidote is a widely distributed and common gem mineral. Only a few gem-grade crystals are found in a few areas.
Most epidotes are too dark, which can not be processed into attractive gemstones. But it can be used as small crafts or beads.