Epidote is a
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar ...
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has ...
iron sorosilicate
Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust.
In mineralogy, silica (silicon dioxide, ) is usually co ...
mineral.
Description
Well developed crystals of epidote, Ca
2Al
2(Fe
3+;Al)(SiO
4)(Si
2O
7)O(OH),
crystallizing in the
monoclinic system, 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
Pleochroism (from Greek πλέων, ''pléōn'', "more" and χρῶμα, ''khrôma'', "color") is an optical phenomenon in which a substance has different colors when observed at different angles, especially with polarized light.
Backgroun ...
, the pleochroic colors being usually green, yellow and brown.
Clinozoisite
Clinozoisite is a complex calcium aluminium sorosilicate mineral with formula: Ca2Al3(Si2O7)(SiO4)O(OH). It forms a continuous solid solution series with epidote by substitution of iron(III) in the aluminium (m3 site) and is also called ''alumini ...
is green, white or pale rose-red group species containing very little iron, thus having the same chemical composition as the
orthorhombic mineral
zoisite
Zoisite, first known as saualpite, after its type locality, is a calcium aluminium hydroxy sorosilicate belonging to the epidote group of minerals. Its chemical formula is Ca2 Al3( Si O4)(Si2O7)O(O H).
Zoisite occurs as prismatic, orthorhomb ...
.
The name, due to
Haüy, is derived from the Greek word "epidosis" (ἐπίδοσις) which means "addition" in allusion to one side of the ideal prism being longer than the other.
Epidote is an abundant rock-forming mineral, but one of secondary origin. It occurs in
marble and
schistose rocks of
metamorphic origin. It is also a product of
hydrothermal alteration of various minerals (
feldspars,
micas,
pyroxenes,
amphibole
Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is Am ...
s,
garnets, and others) composing
igneous rocks. A rock composed of
quartz and epidote is known as
epidosite
Epidosite () is a highly altered epidote and quartz bearing rock. It is the result of slow hydrothermal alteration or metasomatism of the basaltic sheeted dike complex and associated plagiogranites that occurs below the massive sulfide ore depo ...
. Well-developed crystals are found at many localities: Knappenwand, near the
Großvenediger
Großvenediger () is the main peak of the Venediger Group within the Hohe Tauern mountain range, on the border of the Austrian state of Tyrol (East Tyrol) with Salzburg. It is generally considered to be Austria's fourth highest mountain (although ...
in the Untersulzbachthal in
Salzburg, as magnificent, dark green crystals of long prismatic habit in cavities in epidote schist, with
asbestos
Asbestos () is a naturally occurring fibrous silicate mineral. There are six types, all of which are composed of long and thin fibrous crystals, each fibre being composed of many microscopic "fibrils" that can be released into the atmosphere ...
,
adularia
Orthoclase, or orthoclase feldspar (endmember formula K Al Si3 O8), is an important tectosilicate mineral which forms igneous rock. The name is from the Ancient Greek for "straight fracture," because its two cleavage planes are at right angles ...
,
calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on the Mohs scale of mineral hardness, based on scratc ...
, and
apatite
Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common ...
; the Ala valley and
Traversella
Traversella is a ''comune'' (municipality) in the Metropolitan City of Turin in the Italian region Piedmont, located Metropolitan Cityabout north of Turin.
Traversella borders the following municipalities: Pontboset, Donnas, Valprato Soana, ...
in
Piedmont; Arendal in
Norway;
Le Bourg-d'Oisans
Le Bourg-d'Oisans () is a commune in the Isère department in southeastern France.
It is located in the Oisans region of the French Alps. Le Bourg-d'Oisans is located in the valley of the Romanche river, on the road from Grenoble to Brianç ...
in
Dauphiné;
Haddam in
Connecticut;
Prince of Wales Island in
Alaska
Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S., ...
, here as large, dark green, tabular crystals with
copper ore
Ore is natural rock or sediment that contains one or more valuable minerals, typically containing metals, that can be mined, treated and sold at a profit.Encyclopædia Britannica. "Ore". Encyclopædia Britannica Online. Retrieved 7 April ...
s in metamorphosed limestone.
The perfectly transparent, dark green crystals from the Knappenwand and from
Brazil have occasionally been cut as
gemstones.
Related species
Belonging to the same isomorphous group with epidote are the
REE-rich allanite
Allanite (also called orthite) is a sorosilicate group of minerals within the broader epidote group that contain a significant amount of rare-earth elements. The mineral occurs mainly in metamorphosed clay-rich sediments and felsic igneous rocks. ...
(containing primarily
lanthanum,
cerium
Cerium is a chemical element with the symbol Ce and atomic number 58. Cerium is a soft, ductile, and silvery-white metal that tarnishes when exposed to air. Cerium is the second element in the lanthanide series, and while it often shows the +3 ...
, and
yttrium
Yttrium is a chemical element with the symbol Y and atomic number 39. It is a silvery-metallic transition metal chemically similar to the lanthanides and has often been classified as a "rare-earth element". Yttrium is almost always found in co ...
), and the
manganese-rich
piemontite.
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 and
dollaseite-(Ce)
Dollaseite-(Ce) is a sorosilicate end-member epidote rare-earth mineral which was discovered by Per Geijer (1927) in the Ostanmossa mine (Östanmossa gruva), Norberg district, Sweden. Dollaseite-(Ce), although not very well known, is part of a b ...
have the same general epidote formula and contain metals of the cerium group. In external appearance allanite 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
Syenite is a coarse-grained intrusive igneous rock with a general composition similar to that of granite, but deficient in quartz, which, if present at all, occurs in relatively small concentrations (< 5%). Some syenites contain larger prop ...
,
rhyolite
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals ( phenocrysts) in an otherwise fine-grained groundmass. The miner ...
,
andesite, and others. It was first found in the granite of east
Greenland and 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
In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal.
Etymology
The term comes from the Greek ''a'' ("wit ...
: for this reason several varieties have been distinguished, and many different names applied. Orthite was the name given by
Jöns Berzelius in 1818 to a hydrated form found as slender prismatic crystals, sometimes a foot in length, at Finbo, near
Falun in
Sweden.
Dollaseite is less common, famous from the Ostanmossa mine in the
Norberg district of
Sweden.
References
External links
The mineral Epidote Mineral Galleries
{{Authority control
Calcium minerals
Aluminium minerals
Iron(III) minerals
Epidote group
Monoclinic minerals
Minerals in space group 11