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Geology is the Way

Epidote

Monoclinic

Ca2Al2(Al,Fe3+)[Si2O7][SiO4]O(OH)

Epidote is a calcium aluminium sorosilicate and the main member of the homonymous Epidote group, which comprises other rock-forming minerals like clinozoisite, zoisite, piemontite, and allanite. Epidote is a common product of low-grade metamorphism in rocks of greenschists-, blueschists-, and amphibolite-facies, but can be found also in igneous rocks as a product of alteration or metasomatism and in contact metamorphic rocks. The mineral was named by the French mineralogist René Just Haüy in 1801 from the Greek επιδοσιζ (“epidosis”), meaning “increase”, a reference to its longer side parallel to its intermediate crystal axis (b).

Structure and chemistry
Epidote is characterized both by isolated single [SiO4] tetrahedrons and [Si2O7] double tetrahedral groups that link chains of edge-sharing octahedral sites (M), containing Fe3+ and Al, elongated parallel to the intermediate axis b. Tetrahedral and octahedral sites surround large cation sites (A sites) that accommodate the Ca cations. There are three different octahedral sites: M2 octahedral chains, separated from the other octahedral sites, and M1 – M3 chains, which are attached to one another. The M3 site is distorted and non centrosymmetric. Al has a strong preference for the M2 site, while Fe3+ and other cations (Mn) are concentrated in the larger M3 site.
In terms of chemistry, the chemical variability of epidote is mostly governed by the substitution of Fe3+ by Al, which produces a complete solid solution between clinozoisite [Ca2Al3Si3O12(OH)] and epidote [Ca2Al2Fe3+Si3O12(OH)]. The latter was also known as the pistacite molecule, a term that was discredited by IMA. In addition, Ca can be replaced by some Mg, Mn, and Fe2+ or by rare earth elements (solid solution with allanite).

Field features

epidote crystal morphology

Morphology of an epidote crystal, highlighting crystal and optic axes and cleavage planes. Modified after Deer, Howie, & Zussmann (1992).

epidote crystal

Gem-quality epidote with classic color. Size: 4.7 x 2.5 x 1.4 cm. Val Varaita, Cuneo, Piedmont, Italy. Photo © Robert M. Lavinsky.

Properties
Habit: prismatic, acicular, fibrous
Hardness: 6
Cleavage: {001} perfect
Twinning: {100} lamellar, uncommon
Color: green, yellow, grey, black
Luster: vitreous to resinous
Streak: greyish white
Alteration: talc, diopside, stevensite, calcite
In thin section…
α(α^c = 0-15°): 1.715-1.751
β(//b): 1.725-1.784
γ(γ^c = 25-40°): 1.734-1.797
2Vγ
: 90-116°
Color: pale yellow – green
Pleochroism: α colorless, pale yellow/green β pale yellow to greenish yellow, γ pale yellow/green to yellowish green

Birefringence (δ): 0.015-0.051 (high, Harlequin-like)
Relief: high
Optic sign:
[Mindat]

Epidote is easy to recognize in the field when it displays its typical pistachio-like yellow-green or dark green hues and prismatic habit. However, it can be confused with other silicates, like amphiboles or pyroxenes, when it is grey or black. Differently from pyroxene and amphibole, epidote shows only one set of cleavage planes oriented parallel to the long axis. Unfortunately, in most cases epidote occurs as masses, veins or as tiny crystals where cleavage planes are difficult to recognize at hand sample scale. Many green and blue metamorphic rocks (greenstones, greenschists, blueschists…) commonly contain epidote. However, epidote crystals are commonly hard to recognize in the field, as they are often small and hidden by other colored minerals like amphibole (glaucophane, actinolite) or chlorite.

prismatic epidote

Prismatic epidote crystals. Pampa Blanca, Castrovirreyna Province, Huancavelica Department, Peru. Photo by RRUFF.

epidote crystals in quartz

Prismatic epidote crystals in quartz. Nahant, Massachusetts, USA. Photo by RRUFF.

epidote skarn

Massive epidote (pistachio green) and oxides (black) in a metasomatized dolomite marble. Calamita Mine, Island of Elba, Italy.

epidotite

Masses of epidote crystals, showing the characteristic pistachio-green color. Calamita Mine, Island of Elba, Italy.

Epidote in thin section
Epidote is characterized by a prismatic or acicular (more rarely fibrous) habit with pseudo-hexagonal ‘basal’ sections. It shows only one set of cleavage planes parallel to the long axis, which allows to distinguish it from amphibole and pyroxene. At N// epidote ranges is normally colored in greenish to greenish yellow hues, which can be even very pale. When it shows pale colors, the pleochroism can be very subtle but more colored varieties commonly show α colorless/pale yellow to β/γ yellow/green. Another diagnostic feature of epidote is the high relief (higher than pyroxene and amphibole). At NX, epidote shows very high interference colors which are very sensitive to chemical variations within individual epidote crystals. Indeed, epidote is often said to appear Harlequin-like. The birefringence is higher close to the Fe3+– rich end member and decreases dramatically along the epidote-clinozoisite series. Pure clinozoisite is first-order grey/yellow, while intermediate epidote compositions typically show second to third-order interference colors. In addition, pure clinozoisite is colorless (or show very pale yellow/green colors), generally lacks pleochroism, and it is optically positive.


The pleochroism of epidote is generally subtle and characterized by changes in hues of pale green/yellow. Ferric compositions are normally more distinctly pleochroic. Width: 1 mm. Epidotite. Calamita, Island of Elba, Italy.

CPL
CPL
CPL
PPL
PPL

Epidote crystals with high relief and very high interference colors. Interference colors are higher close to (010) (plane perpendicular to elongation), which has a quasi-hexagonal outline. Width: 1 mm. Metagabbro. Monte Argentario, Italy.

CPL
CPL
CPL
PPL
PPL

Optical properties of epidote vary greatly depending on composition. Epidote crystals (which are frequently zoned) tend to assume Harlequin-like colors. Width: 1 mm. Epidotite. Calamita, Island of Elba, Italy.

CPL
CPL
CPL
PPL
PPL

Prismatic crystals of epidote with high relief and high interference colors, surrounded by albite. Epidote has straight extinction, but the optic axis oriented parallel to the long axis is the intermediate axis (β). Consequently epidote crystals may be length slow or length fast (see below). Width: 1 mm. Metagabbro. Monte Argentario, Italy.

epidote in albite

Prismatic epidote crystals included in albite. CPL + λ. Width: 1 mm. Metagabbro. Cala Grande, Monte Argentario, Italy.

Gallery 1 – Epidote metagabbro
Masses of epidote, albite, and quartz in blueschist-facies metagabbro retrogressed to greenschist-facies conditions from the Argentario Promontory, Southern Tuscany, Italy.
Sample courtesy Federico Rossetti.

epidote metagabbro

Prismatic epidote crystals included in albite, crosscut by quartz veins. CPL. Width: 3 mm. Metagabbro. Cala Grande, Monte Argentario, Italy.

Gallery 2 – Epidotite from Elba
Epidotites are rocks consisting almost exclusively of epidote. This epidotite from the Calamita (Island of Elba, Italy) in places contains also calcite, chlorite, and actinolite, the latter commonly included in large epidote crystals.

epidotite

Large epidote crystals with actinolite inclusions. CPL. Width: 3 mm. Epidotite. Calamita, Island of Elba, Italy.

Occurrence
Epidote occurs predominantly in low-grade mafic metamorphic rocks and it is a primary mineral in greenschist-, epidote blueschist-, and epidote amphibolite-facies. Epidote is stable even at lower grade, in rocks of zeolite- and prehnite-pumpellyite-facies. At greenschist-facies, epidote is commonly found in association with Ca-amphibole (tremolite-actinolite), chlorite, albite, and quartz. In blueschist-facies rocks, epidote is stable with chlorite, Na-amphibole (glaucophane), Na-pyroxene (omphacite), and garnet. In amphibolite-facies rocks, epidote may coexist with hornblende and plagioclase. Less commonly, epidote is found along with white mica, biotite, and stilpnomelane. Epidote can be found also in low-grade metasedimentary rocks (schists, phyllites), normally as an accessory minerals since these rocks are less Ca-rich.
Epidote is an important constituent of contact-aureole rocks, where it may form along with other calcsilicates during the metamorphism or metasomatism of impure marbles. Metasomatism and hydrothermal alteration may produce skarn containing epidote or even rocks consisting almost exclusively of epidote (epidotites). Hydrothermally-altered igneous rocks commonly contain epidote or a mixture of epidote and other fine-grained minerals (saussurite) altering plagioclase.

References

        

Petrography websites:
Alexstrekeisen.it
Virtual Microscope

it_IT Italiano
Mineral Properties
Minerals

 

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