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Epidote


Epidote is a metamorphic mineral and the name of a silicate mineral group.



What is Epidote?



Epidote is a name that is used in two different ways in mineralogy: 1) the "Epidote Group" is the name of a group of silicate minerals that share common structural and compositional characteristics; and, 2) "Epidote" is the name of the most common mineral in the Epidote Group.


What is Epidote (the mineral)?



Epidote is a silicate mineral that is commonly found in regionally metamorphosed rocks of low-to-moderate grade. In these rocks, epidote is often associated with amphiboles, feldspars, quartz and chlorite. It occurs as replacements of mineral grains that have been altered by metamorphism. It is frequently found in veins that cut granite. It occurs as monoclinic crystals in pegmatites. It is also found in massive form and as monoclinic crystals in marbles and schists that were formed or altered through contact metamorphism.

Epidote usually ranges between yellowish green and pistachio green in color. Less often it is brownish green to black. In massive form it is usually translucent with a vitreous luster. Well-formed crystals from marble and pegmatite are often transparent.

Physical Properties of Epidote

Chemical Classification silicate
Color usually yellowish green to pistachio green, sometimes brownish-green to black
Streak colorless
Luster vitreous to resinous
Diaphaneity transparent to translucent to nearly opaque
Cleavage perfect in one direction, imperfect
Mohs Hardness 6 to 7
Specific Gravity 3.3 to 3.5
Diagnostic Properties color, cleavage, specific gravity
Chemical Composition Ca2(Al2,Fe)(SiO4)(Si2O7)O(OH)
Crystal System monoclinic
Uses semiprecious gem

Epidote has a chemical composition of Ca2(Al2,Fe)(SiO4)(Si2O7)O(OH). It is an end member of a solid solution series with clinozoisite. In that series, the iron of epidote is gradually replaced by aluminum to the end member clinozoisite composition of Ca2Al3(SiO4)(Si2O7)O(OH). Clinozoisite is usually lighter in color than epidote because iron is what produces epidote's greenish to brownish color.


What is Epidote (the mineral group)?



Members of the epidote mineral group have a crystal structure that consists of isolated and paired silica tetrahedrons. They share a generalized chemical composition of A2M3(Si2O7)(SiO4)O(OH). "A" is a pairing of calcium, manganese, strontium, lead or sometimes a rare earth element. "M" is usually aluminum pairing with iron, magnesium, manganese or vanadium. Some of the member minerals of the epidote group are listed in the table below with their chemical compositions.

Mineral Chemical Composition
Allanite (CaX)(Al2Fe)(Si2O7)(SiO4)O(OH)
X is one of these rare earths: Ce, La, Nd, Y.
Askagenite (MnNd)(Al2Fe)(Si2O7)(SiO4)O2
Clinozoisite Ca2Al3(SiO4)(Si2O7)O(OH)
Sr sometimes substitutes for one Ca.
Dissakisite (CaX)(Al2Mg)(Si2O7)(SiO4)O(OH)
X can be Ce or La.
Dollaseite (CaCe)(Mg2Al)(Si2O7)(SiO4)(OH)F
Epidote Ca2(Al2Fe)(SiO4)(Si2O7)O(OH)
Pb or Sr can substitute for one of the calcium.
Ferriallanite (CaX)(Fe2Al)(SiO4)(Si2O7)O(OH)
X can be Ce or La.
Hancockite (CaPb)(Al2(Fe,Mn)(SiO4)(Si2O7)O(OH)
Khristovite (CaCe)(MgAlMn)(SiO4)(Si2O7)(OH)F
Manganipiemontite (XY)(Mn2Al)(SiO4)(Si2O7)O(OH)
(XY) can be (MnLa) or (CaSr).
Mukhinite (Ca2)(Al2V)(SiO4)(Si2O7)O(OH)
Piemontite (X)(Al2Mn)(SiO4)(Si2O7)O(OH)
(X) can be (Ca2), (CaPb) or (CaSr).
Uedaite (MnCe)(Al2Fe)(SiO4)(Si2O7)O(OH)
Zoisite Ca2Al3(SiO4)(Si2O7)O(OH)


Epidote in Rocks



Epidote is a rock-forming mineral. Many regionally-metamorphosed rocks contain small amounts of epidote. Two rock types that contain significant amounts of epidote are epidosite and unakite. Locations where these rocks can be found are rare, but at those locations significant amounts of these rocks can be present.

Epidosite is a metamorphic rock composed mainly of epidote with small amounts of quartz. It forms when basalts in sheeted dikes and ophiolites are transformed by hydrothermal activity or metasomatism.

Unakite is a rock that forms from the metamorphism of granite. Less resistant minerals in the granite are altered to epidote or replaced by epidote, with the orthoclase and quartz remaining. It is an interesting pink and green colored rock that was first discovered in the Unakas Mountains of North Carolina, from which its name was derived.


Uses of Epidote



Epidote has no significant use as an industrial mineral and has only minor use as a gemstone. High-quality transparent crystals are sometimes cut into faceted stones. These have never attracted much interest in the commercial jewelry market, probably because their colors are not customer favorites. Most of the faceted stones produced are purchased by gem and mineral collectors.

Unakite is a popular rock used by lapidaries to make beads, ornamental objects, and cut into cabochons. The bright pink and pistachio green colors are very unusual and attract attention. Unakite is popular as a tumbled stone. A small amount of epidosite is also cut into cabochons.


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Epidote
Epidote from Rockbridge County, Virginia. This specimen is approximately 4 inches (10 centimeters) across.




Epidote
Epidote from Rockbridge County, Virginia. This specimen is approximately 4 inches (10 centimeters) across.


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Epidote in unakite
Tumbled stones made from unakite, an igneous rock composed mainly of green epidote, pink orthoclase feldspar, and quartz. This unakite was mined in South Africa.


Epidote in unakite
Two cabochons cut from unakite. The cab on the left is about 30 x 19 millimeters in size and is cut from material with a very coarse grain size. The cab on the right is about 39 x 30 millimeters in size and is cut from material with a fine grain size.




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