name = Epidote
caption = Epidote crystals
formula = Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH)
color = Pistachio-green, yellow-green, greenish black
habit = Prismatic with striations, fibrous, massive
twinning = On 
cleavage = Perfect on 
fracture = Flat regular to uneven
mohs = 7
luster = Vitreous to resinous
refractive = nα = 1.715 - 1.751 nβ = 1.725 - 1.784 nγ = 1.734 - 1.797
opticalprop = Biaxial (-)
birefringence = δ = 0.019 - 0.046
pleochroism = Strong
streak = Greyish white
gravity = 3.3 - 3.6
diaphaneity = Transparent to nearly opaque
references = [http://rruff.geo.arizona.edu/doclib/hom/epidote.pdf Handbook of Mineralogy] [http://www.mindat.org/min-1389.html Mindat] [http://webmineral.com/data/Epidote.shtml Webmineral]
Epidote is a
calcium aluminium ironsorosilicate mineral, Ca2Al2(Fe3+;Al)(SiO4)(Si2O7)O(OH), crystallizing in the monoclinicsystem. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. The faces are often deeply striated and crystals are often twinned. Many of the characters of the mineral vary with the amount of iron present for instance, the color, the optical constants, and the specific gravity. The color is green, grey, brown or nearly black, but usually a characteristic shade of yellowish-green or pistachio-green. It displays strong pleochroism, the pleochroic colors being usually green, yellow and brown. Clinozoisiteis white or pale rose-red variety containing very little iron, thus having the same chemical composition as the orthorhombicmineral zoisite.
Epidote is an abundant rock-forming mineral, but one of secondary origin. It occurs in
marbleand schistose rocks of metamorphic origin. It is also a product of hydrothermalalteration of various minerals ( feldspars, micas, pyroxenes, amphiboles, garnets, and others) composing igneous rocks. A rock composed of quartzand epidote is known as epidosite. Well-developed crystals are found at many localities: Knappenwand, near the Großvenedigerin the Untersulzbachthal in Salzburg, as magnificent, dark green crystals of long prismatic habit in cavities in epidote schist, with asbestos, adularia, calcite, and apatite; the Ala valley and Traversella in Piedmont; Arendal in Norway; Le Bourg-d'Oisansin Dauphiné; Haddam in Connecticut; Prince of Wales Island in Alaska, here as large, dark green, tabular crystals with copper ores in metamorphosed limestone.
The perfectly transparent, dark green crystals from the Knappenwand and from
Brazilhave occasionally been cut as gemstones.
Belonging to the same isomorphous group with epidote are the species
piemontiteand allanite, which may be described as manganeseand ceriumepidotes respectively.
Piemontite occurs as small, reddish-black, monoclinic crystals in the manganese mines at San Marcel, near Ivrea in Piedmont, and in crystalline schists at several places in
Japan. The purple color of the Egyptian porfido rosso antico is due to the presence of this mineral.
Allanite has the same general epidote formula and contains metals of the cerium group. In external appearance it differs widely from epidote, being black or dark brown in color, pitchy in lustre, and opaque in the mass; further, there is little or no cleavage, and well-developed crystals are rare. The crystallographic and optical characters are similar to those of epidote; the pleochroism is strong with reddish-, yellowish-, and greenish-brown colors. Although not a common mineral, allanite is of fairly wide distribution as a primary accessory constituent of many crystalline rocks,
gneiss, granite, syenite, rhyolite, andesite, and others. It was first found in the granite of east Greenlandand described by Thomas Allan in 1808, after whom the species was named. Allanite is a mineral readily altered by hydration, becoming optically isotropic and amorphous: for this reason several varieties have been distinguished, and many different names applied. Orthite was the name given by Jöns Berzeliusin 1818 to a hydrated form found as slender prismatic crystals, sometimes a foot in length, at Finbo, near Falun in Sweden..
* [http://mineral.galleries.com/minerals/silicate/epidote/epidote.htm The mineral Epidote Mineral Galleries]
Wikimedia Foundation. 2010.