Epidote

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Epidote is generally well developed crystals composed of Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH), crystallizing inside the monoclinic system, are of frequent prevalence: they’re normally prismatic in dependancy, the course of elongation being perpendicular to the single plane of symmetry. The faces are regularly deeply striated and crystals are regularly twinned. Many of the characters of the mineral range with the quantity of iron present for example, the colour, the optical constants, and the particular gravity. The shade is inexperienced, gray, brown or almost black, however generally a function shade of yellowish-green or pistachio-green. It presentations strong pleochroism, the pleochroic colors being commonly inexperienced, yellow and brown. Clinozoisite is green, white or faded rose-pink group species containing very little iron, therefore having the equal chemical composition as the orthorhombic mineral zoisite.

What is Epidote (the mineral)?

Epidote is a silicate mineral that is typically observed in domestically metamorphosed rocks of low-to-slight grade. In those rocks, epidote is often associated with amphiboles, feldspars, quartz, and chlorite. It occurs as replacements of mineral grains which have been altered by using metamorphism. It is frequently found in veins that cut granite. It takes place as monoclinic crystals in pegmatites. It is likewise discovered in huge form and as monoclinic crystals in marbles and schists that were fashioned or altered thru contact metamorphism.

Crystallography: Monoclinic; prismatic. Crystals are often much elongated parallel to the ortho-axis with a prominent development of the faces of the orthodome zone, giving them a prismatic aspect. Striated parallel to the ortho-axis. Terminated usually only at one end of the ortho-axis and most commonly by the two faces of a pyramid. Some crystals twinned with {100} the twin plane. Usually coarse to fine granular; also fibrous.

Composition: Hydrous calcium aluminum-iron silicate, Ca2(Al,Fe)3- (SiOiMOH). The ratio of aluminum to iron varies from 6 : 1 to 3 : 2

Diagnostic Features: Characterized by its peculiar green color and one perfect cleavage. The slag formed on fusing which is again infusible is diagnostic.

Name: From the Greek meaning increase, since the base of the prism has one side longer than the other

What is Epidote (the mineral group)?

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)

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

Optical Properties of Epidote

Property	Value
Formula	Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH)
Crystal System	monoclinic
Crystal Habit	coarse to fine granular ; also fibrous
Cleavage	{001} perfect, {100} imperfect
Luster	Vitreous, some resinous.
Color/Pleochroism	clinozoisite: pale green to gray. Pleochroism can be strong in transparent forms, appearing green and brown at different angles.
Optic Sign	clinozoisite: Biaxial ( +)
2V	clinozoisite: 2V= 14-19 degrees
Optic Orientation	Y=b O.A.P. = (010)
Refractive Indices alpha = beta = gamma =	clinozoisite 1.670-1.1.715 1.674-1.725 1.690-1.734
Max Birefringence	=0.004 – 0.049
Elongation	Elongate crystals may be either length fast or length slow, since Y is parallel to length.
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.
Sources	Nesse, William D: Introduction to Optical Mineralogy (Oxford University Press, 1986) pp.192-193
Editors	Sarah Hale (’07), Shawn Moore (’13), Tessa Brown (’17)

Epidote Uses

• 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. It is considered to be a semiprecious stone. 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.