Dictionary:

epidote

  (ĕp'ĭ-dōt')

A lustrous yellow, green, or black mineral, Ca₂(Al, Fe)₃(SiO₄)₃OH, commonly found in metamorphic rock.

[French épidote, from Greek epididonai, to give in addition, increase (from the greater length of its crystals compared with those of related minerals) : epi-, epi- + didonai, to give.]

The group name for a family of minerals of general composition Ca₂(Fe³⁺, Al, Mn³⁺)Al₂O[SiO₄][Si₂O](OH) that occur widely in metamorphic and igneous rocks. Epidote [octahedral ferric iron (Fe³⁺) dominant] and clinozoisite [aluminum (Al) dominant] represent the most common compositions among the epidote group; a third composition, piemontite [manganese (Mn³⁺) dominant], is less abundant. Allanite refers to compositions displaying significant rare-earth (such as lanthanum or cerium) substitution for calcium (Ca²⁺), with corresponding replacement of Fe³⁺ by ferrous iron (Fe²⁺). A fifth member, zoisite, is equivalent to clinozoisite, but it has a different crystalline system. Rare epidote-clinozoisites abundant in chromium (Cr), vanadium (V), and lead (Pb) and allanites rich in fluorine (F), beryllium (Be), and phosphorus (P) also exist. See also Crystal structure; Solid solution.

Epidote group minerals, particularly epidote and clinozoisite, are common and widespread in regional- and contact-metamorphic rocks, both as primary and secondary (that is, alteration) minerals. They occur together as individual grains, as intergrowths, or as zoned crystals. Epidote and (clino)zoisite are found in aluminous limestones with grossularite, anorthite, microcline, quartz, and calcite; in mafic schists and gneisses with hornblende, albite, and chloritoid; in actinolite greenschists with chlorite, sphene, albite, quartz, calcite, and magnetite; in hornfels with diopside, actinolite, grossularite, and albite; in glaucophane schists; in quartzites; and in slates. Approximate depth-temperature conditions of their formation range from 5–25 km (3–15 mi) and 300–500°C (570–1020°F; low-grade) to 5–25 km (3–15 mi) and 450–650°C (840–1200°F; medium-grade). However, their stabilities are also sensitive to the pressure of oxygen in the rock during metamorphism. See also Metamorphic rocks.

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Environment

Metamorphic rocks, contact-metamorphosed limestones, altered igneous rocks, pegmatites, and in traprock amygdules with zeolites. Tiny pistachio green crystals common on shrinkage seams in granite, formed from the last gases or solutions to escape, often accompanied by albite and fluorite.

Often crystallized in green-black surfaces or as long, slender, grooved prisms, crystals actually stretched out along a horizontal ( b -axis) direction and giving an impression that the side faces are slanting, if we follow our normal inclination to stand the crystals upright. Also in very thin crusts of small paler green crystals, and in greenish films thickening to crusts of massive or fine-grained "pistacite" (suggestive of the color).

Pistachio green, green, blackish green, brown, light yellow-brown. Luster glassy (pearly on cleavage); hardness 6-7; specific gravity 3.4-3.5; fracture uneven; cleavage perfect basal (remember the base is usually a face paralleling the elongation of the crystal). Transparent to translucent; strongly pleochroic, two different colors coming through as a translucent prism is rotated, usually green and dark brown.

Alkaline calcium iron silicate (averaging about 23.5% CaO, 11.5% Fe ₂ O ₃ , 25.0% Al ₂ O ₃ , 38.0% SiO ₂ , and just under 2% H ₂ O).

Fuses with bubbling to a usually magnetic dull black scoriaceous glass. Since it is insoluble in dilute hydrochloric acid, epidote walling calcite veins is readily exposed by an acid soaking (dilute hydrochloric acid) of the specimen.

The color and the general appearance of epidote are so characteristic that tests are rarely necessary. Splintery actinolite, the green amphibole, has two cleavages, and single crystals do not show any pronounced color change as a prism is rotated. Viewed from the side, tourmaline, with no cleavage, also shows no color change.

This mineral is extremely common. Notable localities include Sulzer, Prince of Wales I., Alaska, with crystals remarkable for their size (to 3 in., 7-8 cm), and a short prismatic, almost tablet, shape. Slender prisms are found stretched across pegmatite feldspar in the Mitchell Co. area of North Carolina. Epidote and garnet are abundant at several localities in California, where in one locality they sometimes form alternating bands shaped by the garnet crystal outline.

The world's leading epidote locality is Untersulzbachtal, in the Austrian Tyrol, where magnificent, dark, lustrous crystals up to a foot long (30 cm) and an inch (3 cm) or more across were found in a notable pocket in a chlorite-actinolite schist, with colorless apatite crystals and byssolite bundles. Small sprays of crystals and large singles and crusts have been found in Baja California and on Guadalupe I. in the Gulf. Some very black crystals come from Guerrero, Mexico. Larger sprays, up to 3 in. (7-8 cm) long, have been found in Minas Gerais, Brazil, in pegmatites. Most recently crystals of Austrian quality have turned up in Namibia and in Afghanistan, though from neither land have we seen the numbers that came in Untersulzbachtal.

Epidote from Slovakia

Epidote crystals

Epidote is a calcium aluminium iron sorosilicate mineral, Ca₂(Al, Fe)₃(SiO₄)₃(OH), crystallizing in the monoclinic system. 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 (3.33). The hardness is 6.5. The color is green, grey, brown or nearly black, but usually a characteristic shade of yellowish-green or pistachio-green. The pleochroism is strong, the pleochroic colors being usually green, yellow and brown. Clinozoisite is white or pale rose-red variety containing very little iron, thus having the same chemical composition as the orthorhombic mineral zoisite.

Epidote is an abundant rock-forming mineral, but one of secondary origin. It occurs in crystalline limestones and schistose rocks of metamorphic origin. It is also a product of hydrothermal alteration of various minerals (feldspars, micas, pyroxenes, amphiboles, garnets, and others) composing igneous rocks. A rock composed of quartz and epidote is known as epidosite. Well-developed crystals are found at many localities: Knappenwand, near the Großvenediger in 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'Oisans in 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 Brazil have occasionally been cut as gemstones.

Belonging to the same isomorphous group with epidote are the species piemontite and allanite, which may be described as manganese and cerium epidotes 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 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: 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. hardness: 6-7 density:3.25-3.45 luster: vitreous color:pistachio green to yellowish-green streak: white

References

• This article incorporates text from the Encyclopædia Britannica Eleventh Edition, a publication now in the public domain.

External link

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