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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 and accessory minerals like clinozoisite, zoisite, piemontite, and allanite. Epidote is a common product of low-grade metamorphism of mafic and sedimentary rocks, but it occurs 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 elongation parallel to the intermediate crystal axis (b).

Structure and chemistry
The structure of epidote is characterized both by isolated [SiO4] tetrahedrons and [Si2O7] double tetrahedral groups that connect chains of edge-sharing octahedral sites (M) that contain Fe3+ and Al and are elongated parallel to the intermediate axis b. The tetrahedral and octahedral sites surround large cation sites (A sites) that accommodate 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.

epidote crystal structure

3D view of the crystal structure of epidote. Blue tetrahedrons: Si sites. Dark grey octahedrons: Al-sites. Brown octahedrons: Fe3+-sites. Green: Ca atoms. Oxygens and OH anions are not shown for simplicity. Image by Joe Smyth via ruby.colorado.

In terms of chemistry, the compositional variability of epidote is mostly governed by the substitution of Fe3+ by Al which results in a complete solid solution between the clinozoisite [Ca2Al3Si3O12(OH)] and epidote [Ca2Al2Fe3+Si3O12(OH)] end-members. The latter was also known as the pistacite molecule, a term that was useful to distinguish epidote (mineral) from epidote (end-member) but that is now discredited by IMA. Ca in epidote can be partly replaced by minor Mg, Mn, and Fe2+ or by rare earth elements (an epidote with substantial amounts of rare earth elements is known as allanite).

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
Hardness: 6
Cleavage: {001} perfect
Twinning: {100} lamellar, uncommon
Color: green, yellow, grey, black
Luster: vitreous, resinous
Streak: grayish white
Alteration: –
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 to 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]

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 is relatively 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, epidote often occurs as masses, veins or as tiny crystals where cleavage planes are difficult to recognize at hand sample scale. Many green metamorphic rocks (greenstones, greenschists…) commonly contain epidote, often together with other greenish minerals like amphibole (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, showing the characteristic pistachio-green color. Calamita Mine, Island of Elba, Italy.

Epidote in thin section
Epidote is generally characterized by a prismatic habit with six-sided ‘basal’ sections. Epidote has a single set of cleavage planes oriented parallel to the long axis. Under plane polarized light, epidote commonly shows greenish to greenish yellow hues, often appearing very pale. In the latter case, its pleochroism may be subtle or barely detectable, whereas colored epidote varieties exhibit stronger α colorless/pale yellow to β/γ yellow/green pleochroism. Another diagnostic feature of epidote is the high relief (higher than pyroxene and amphibole). At crossed polars, epidote shows very high interference colors that vary greatly along the clinozoisite – epidote series. For this reason, individual epidote crystal often show a wide range of interference colors in function of their internal chemical zoning and are commonly described as appearing Harlequin-like. 

The birefringence of epidote is, indeed, higher close to the Fe3+– rich end member and decreases dramatically towards the clinozoisite end-member. Pure clinozoisite shows first-order grey/yellow interference colors, whereas intermediate epidote compositions typically show second to third-order interference colors. Furthermore, pure clinozoisite is colorless (or show very pale color), 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 and tend to assume Harlequin-like colors at CPL. 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. 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.

Examples of epidote-bearing rocks

Epidote in a metagabbro
Masses of epidote, albite, and quartz in a metagabbro that experienced peak blueschist-facies metamorphism and retrogression to greenschist-facies conditions.
Sample: metagabbro
Assemblage: albite, epidote, chlorite, actinolite, glaucophane, diopside, omphacite, quartz, titanite, rutile, zircon
Locality: Cala Grande, 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.

Epidote in an epidotite
Epidotites are rocks consisting almost exclusively of epidote, produced by hydrothermal alteration of basic or carbonate rocks. This epidotite sample is from a contact aureole and was produced by the interaction between hot mineralizing fluids and carbonate rocks.
Sample: epidotite
Assemblage: epidote, chlorite, calcite, actinolite, quartz, alkali feldspar
Locality: Praticciolo, Calamita, Isola d’Elba, Italy

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, and albite. In blueschist-facies rocks, epidote is stable with chlorite, Na-amphibole (glaucophane), Na-pyroxene (omphacite), and garnet. Epidote may also coexist with hornblende and plagioclase in amphibolites. 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 protoliths are Ca-poor.
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 altering plagioclase, known as saussurite.

References

        

Mineral Properties
Minerals

 

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